README

 # SPDX-License-Identifier: GPL-2.0+
#
# (C) Copyright 2000 - 2013
# Wolfgang Denk, DENX Software Engineering, wd@denx.de.

Summary:
========

This directory contains the source code for U-Boot, a boot loader for
Embedded boards based on PowerPC, ARM, MIPS and several other
processors, which can be installed in a boot ROM and used to
initialize and test the hardware or to download and run application
code.

The development of U-Boot is closely related to Linux: some parts of
the source code originate in the Linux source tree, we have some
header files in common, and special provision has been made to
support booting of Linux images.

Some attention has been paid to make this software easily
configurable and extendable. For instance, all monitor commands are
implemented with the same call interface, so that it's very easy to
add new commands. Also, instead of permanently adding rarely used
code (for instance hardware test utilities) to the monitor, you can
load and run it dynamically.


Status:
=======

In general, all boards for which a default configuration file exists in the
configs/ directory have been tested to some extent and can be considered
"working". In fact, many of them are used in production systems.

In case of problems you can use

     scripts/get_maintainer.pl <path>

to identify the people or companies responsible for various boards and
subsystems. Or have a look at the git log.


Where to get help:
==================

In case you have questions about, problems with or contributions for
U-Boot, you should send a message to the U-Boot mailing list at
<u-boot@lists.u-boot-project.org>. There is also an archive of previous traffic
on the mailing list - please search the archive before asking FAQ's.
Please see https://lists.u-boot-project.org/pipermail/u-boot and
https://marc.info/?l=u-boot

Where to get source code:
=========================

The U-Boot source code is maintained in the Git repository at
https://git.u-boot-project.org/u-boot/u-boot.git ; you can browse it online at
https://git.u-boot-project.org/u-boot/u-boot

The "Tags" links on this page allow you to download tarballs of
any version you might be interested in. Official releases are also
available from the DENX file server through HTTPS or FTP.
https://ftp.denx.de/pub/u-boot/
ftp://ftp.denx.de/pub/u-boot/


Where we come from:
===================

- start from 8xxrom sources
- create PPCBoot project (https://sourceforge.net/projects/ppcboot)
- clean up code
- make it easier to add custom boards
- make it possible to add other [PowerPC] CPUs
- extend functions, especially:
  * Provide extended interface to Linux boot loader
  * S-Record download
  * network boot
  * ATA disk / SCSI ... boot
- create ARMBoot project (https://sourceforge.net/projects/armboot)
- add other CPU families (starting with ARM)
- create U-Boot project (https://sourceforge.net/projects/u-boot)
- current project page: see https://www.denx.de/wiki/U-Boot


Names and Spelling:
===================

The "official" name of this project is "Das U-Boot". The spelling
"U-Boot" shall be used in all written text (documentation, comments
in source files etc.). Example:

	This is the README file for the U-Boot project.

File names etc. shall be based on the string "u-boot". Examples:

	include/asm-ppc/u-boot.h

	#include <asm/u-boot.h>

Variable names, preprocessor constants etc. shall be either based on
the string "u_boot" or on "U_BOOT". Example:

	U_BOOT_VERSION		u_boot_logo
	IH_OS_U_BOOT		u_boot_hush_start


Software Configuration:
=======================

Selection of Processor Architecture and Board Type:
---------------------------------------------------

For all supported boards there are ready-to-use default
configurations available; just type "make <board_name>_defconfig".

Example: For a TQM823L module type:

	cd u-boot
	make TQM823L_defconfig

Note: If you're looking for the default configuration file for a board
you're sure used to be there but is now missing, check the file
doc/README.scrapyard for a list of no longer supported boards.

Sandbox Environment:
--------------------

U-Boot can be built natively to run on a Linux host using the 'sandbox'
board. This allows feature development which is not board- or architecture-
specific to be undertaken on a native platform. The sandbox is also used to
run some of U-Boot's tests.

See doc/arch/sandbox/sandbox.rst for more details.

The following options need to be configured:

- CPU Type:	Define exactly one, e.g. CONFIG_MPC85XX.

- Board Type:	Define exactly one, e.g. CONFIG_MPC8540ADS.

- 85xx CPU Options:
		CONFIG_SYS_PPC64

		Specifies that the core is a 64-bit PowerPC implementation (implements
		the "64" category of the Power ISA). This is necessary for ePAPR
		compliance, among other possible reasons.

		CONFIG_SYS_FSL_ERRATUM_A004510

		Enables a workaround for erratum A004510.  If set,
		then CONFIG_SYS_FSL_ERRATUM_A004510_SVR_REV and
		CFG_SYS_FSL_CORENET_SNOOPVEC_COREONLY must be set.

		CONFIG_SYS_FSL_ERRATUM_A004510_SVR_REV
		CONFIG_SYS_FSL_ERRATUM_A004510_SVR_REV2 (optional)

		Defines one or two SoC revisions (low 8 bits of SVR)
		for which the A004510 workaround should be applied.

		The rest of SVR is either not relevant to the decision
		of whether the erratum is present (e.g. p2040 versus
		p2041) or is implied by the build target, which controls
		whether CONFIG_SYS_FSL_ERRATUM_A004510 is set.

		See Freescale App Note 4493 for more information about
		this erratum.

		CFG_SYS_FSL_CORENET_SNOOPVEC_COREONLY

		This is the value to write into CCSR offset 0x18600
		according to the A004510 workaround.

		CONFIG_SYS_FSL_SINGLE_SOURCE_CLK
		Single Source Clock is clocking mode present in some of FSL SoC's.
		In this mode, a single differential clock is used to supply
		clocks to the sysclock, ddrclock and usbclock.

- Generic CPU options:

		CONFIG_SYS_FSL_DDR
		Freescale DDR driver in use. This type of DDR controller is
		found in mpc83xx, mpc85xx as well as some ARM core SoCs.

		CFG_SYS_FSL_DDR_ADDR
		Freescale DDR memory-mapped register base.

		CONFIG_SYS_FSL_IFC_CLK_DIV
		Defines divider of platform clock(clock input to IFC controller).

		CONFIG_SYS_FSL_LBC_CLK_DIV
		Defines divider of platform clock(clock input to eLBC controller).

		CFG_SYS_FSL_DDR_SDRAM_BASE_PHY
		Physical address from the view of DDR controllers. It is the
		same as CFG_SYS_DDR_SDRAM_BASE for  all Power SoCs. But
		it could be different for ARM SoCs.

- ARM options:
		CFG_SYS_EXCEPTION_VECTORS_HIGH

		Select high exception vectors of the ARM core, e.g., do not
		clear the V bit of the c1 register of CP15.

		COUNTER_FREQUENCY
		Generic timer clock source frequency.

		COUNTER_FREQUENCY_REAL
		Generic timer clock source frequency if the real clock is
		different from COUNTER_FREQUENCY, and can only be determined
		at run time.

- Linux Kernel Interface:
		CONFIG_OF_LIBFDT

		New kernel versions are expecting firmware settings to be
		passed using flattened device trees (based on open firmware
		concepts).

		CONFIG_OF_LIBFDT
		 * New libfdt-based support
		 * Adds the "fdt" command
		 * The bootm command automatically updates the fdt

		OF_TBCLK - The timebase frequency.

		boards with QUICC Engines require OF_QE to set UCC MAC
		addresses

		CONFIG_OF_IDE_FIXUP

		U-Boot can detect if an IDE device is present or not.
		If not, and this new config option is activated, U-Boot
		removes the ATA node from the DTS before booting Linux,
		so the Linux IDE driver does not probe the device and
		crash. This is needed for buggy hardware (uc101) where
		no pull down resistor is connected to the signal IDE5V_DD7.

- vxWorks boot parameters:

		bootvx constructs a valid bootline using the following
		environments variables: bootdev, bootfile, ipaddr, netmask,
		serverip, gatewayip, hostname, othbootargs.
		It loads the vxWorks image pointed bootfile.

		Note: If a "bootargs" environment is defined, it will override
		the defaults discussed just above.

- Cache Configuration for ARM:
		CFG_SYS_PL310_BASE - Physical base address of PL310
					controller register space

- Serial Ports:
		CFG_PL011_CLOCK

		If you have Amba PrimeCell PL011 UARTs, set this variable to
		the clock speed of the UARTs.

		CFG_PL01x_PORTS

		If you have Amba PrimeCell PL010 or PL011 UARTs on your board,
		define this to a list of base addresses for each (supported)
		port. See e.g. include/configs/versatile.h

		CONFIG_SERIAL_HW_FLOW_CONTROL

		Define this variable to enable hw flow control in serial driver.
		Current user of this option is drivers/serial/nsl16550.c driver

- Removal of commands
		If no commands are needed to boot, you can disable
		CONFIG_CMDLINE to remove them. In this case, the command line
		will not be available, and when U-Boot wants to execute the
		boot command (on start-up) it will call board_run_command()
		instead. This can reduce image size significantly for very
		simple boot procedures.

- Regular expression support:
		CONFIG_REGEX
		If this variable is defined, U-Boot is linked against
		the SLRE (Super Light Regular Expression) library,
		which adds regex support to some commands, as for
		example "env grep" and "setexpr".

- Watchdog:
		CFG_SYS_WATCHDOG_FREQ
		Some platforms automatically call WATCHDOG_RESET()
		from the timer interrupt handler every
		CFG_SYS_WATCHDOG_FREQ interrupts. If not set by the
		board configuration file, a default of CONFIG_SYS_HZ/2
		(i.e. 500) is used. Setting CFG_SYS_WATCHDOG_FREQ
		to 0 disables calling WATCHDOG_RESET() from the timer
		interrupt.

- GPIO Support:
		The CFG_SYS_I2C_PCA953X_WIDTH option specifies a list of
		chip-ngpio pairs that tell the PCA953X driver the number of
		pins supported by a particular chip.

		Note that if the GPIO device uses I2C, then the I2C interface
		must also be configured. See I2C Support, below.

- Timestamp Support:

		When CONFIG_TIMESTAMP is selected, the timestamp
		(date and time) of an image is printed by image
		commands like bootm or iminfo. This option is
		automatically enabled when you select CONFIG_CMD_DATE .

- Partition Labels (disklabels) Supported:
		Zero or more of the following:
		CONFIG_MAC_PARTITION   Apple's MacOS partition table.
		CONFIG_ISO_PARTITION   ISO partition table, used on CDROM etc.
		CONFIG_EFI_PARTITION   GPT partition table, common when EFI is the
				       bootloader.  Note 2TB partition limit; see
				       disk/part_efi.c
		CONFIG_SCSI) you must configure support for at
		least one non-MTD partition type as well.

- NETWORK Support (PCI):
		CONFIG_E1000_SPI
		Utility code for direct access to the SPI bus on Intel 8257x.
		This does not do anything useful unless you set at least one
		of CONFIG_CMD_E1000 or CONFIG_E1000_SPI_GENERIC.

		CONFIG_NATSEMI
		Support for National dp83815 chips.

		CONFIG_NS8382X
		Support for National dp8382[01] gigabit chips.

- NETWORK Support (other):
		CONFIG_CALXEDA_XGMAC
		Support for the Calxeda XGMAC device

		CONFIG_LAN91C96
		Support for SMSC's LAN91C96 chips.

			CONFIG_LAN91C96_USE_32_BIT
			Define this to enable 32 bit addressing

			CFG_SYS_DAVINCI_EMAC_PHY_COUNT
			Define this if you have more then 3 PHYs.

		CONFIG_FTGMAC100
		Support for Faraday's FTGMAC100 Gigabit SoC Ethernet

			CONFIG_FTGMAC100_EGIGA
			Define this to use GE link update with gigabit PHY.
			Define this if FTGMAC100 is connected to gigabit PHY.
			If your system has 10/100 PHY only, it might not occur
			wrong behavior. Because PHY usually return timeout or
			useless data when polling gigabit status and gigabit
			control registers. This behavior won't affect the
			correctnessof 10/100 link speed update.

		CONFIG_SH_ETHER
		Support for Renesas on-chip Ethernet controller

- TPM Support:
		CONFIG_TPM
		Support TPM devices.

		CONFIG_TPM_TIS_INFINEON
		Support for Infineon i2c bus TPM devices. Only one device
		per system is supported at this time.

			CONFIG_TPM_TIS_I2C_BURST_LIMITATION
			Define the burst count bytes upper limit

		CONFIG_TPM_ATMEL_TWI
		Support for Atmel TWI TPM device. Requires I2C support.

		CONFIG_TPM_TIS_LPC
		Support for generic parallel port TPM devices. Only one device
		per system is supported at this time.

		CONFIG_TPM
		Define this to enable the TPM support library which provides
		functional interfaces to some TPM commands.
		Requires support for a TPM device.

		CONFIG_TPM_AUTH_SESSIONS
		Define this to enable authorized functions in the TPM library.
		Requires CONFIG_TPM and CONFIG_SHA1.

- USB Support:
		At the moment only the UHCI host controller is
		supported (PIP405, MIP405); define
		CONFIG_USB_UHCI to enable it.
		define CONFIG_USB_KEYBOARD to enable the USB Keyboard
		and define CONFIG_USB_STORAGE to enable the USB
		storage devices.
		Note:
		Supported are USB Keyboards and USB Floppy drives
		(TEAC FD-05PUB).

		CONFIG_USB_DWC2_REG_ADDR the physical CPU address of the DWC2
		HW module registers.

- USB Device:
		Define the below if you wish to use the USB console.
		Once firmware is rebuilt from a serial console issue the
		command "env set stdin usbtty; env set stdout usbtty" and
		attach your USB cable. The Unix command "dmesg" should print
		it has found a new device. The environment variable usbtty
		can be set to gserial or cdc_acm to enable your device to
		appear to a USB host as a Linux gserial device or a
		Common Device Class Abstract Control Model serial device.
		If you select usbtty = gserial you should be able to enumerate
		a Linux host by
		# modprobe usbserial vendor=0xVendorID product=0xProductID
		else if using cdc_acm, simply setting the environment
		variable usbtty to be cdc_acm should suffice. The following
		might be defined in YourBoardName.h

		If you have a USB-IF assigned VendorID then you may wish to
		define your own vendor specific values either in BoardName.h
		or directly in usbd_vendor_info.h. If you don't define
		CONFIG_USBD_MANUFACTURER, CONFIG_USBD_PRODUCT_NAME,
		CONFIG_USBD_VENDORID and CONFIG_USBD_PRODUCTID, then U-Boot
		should pretend to be a Linux device to it's target host.

			CONFIG_USBD_MANUFACTURER
			Define this string as the name of your company for
			- CONFIG_USBD_MANUFACTURER "my company"

			CONFIG_USBD_PRODUCT_NAME
			Define this string as the name of your product
			- CONFIG_USBD_PRODUCT_NAME "acme usb device"

			CONFIG_USBD_VENDORID
			Define this as your assigned Vendor ID from the USB
			Implementors Forum. This *must* be a genuine Vendor ID
			to avoid polluting the USB namespace.
			- CONFIG_USBD_VENDORID 0xFFFF

			CONFIG_USBD_PRODUCTID
			Define this as the unique Product ID
			for your device
			- CONFIG_USBD_PRODUCTID 0xFFFF

- MMC Support:
		CONFIG_SH_MMCIF
		Support for Renesas on-chip MMCIF controller

			CONFIG_SH_MMCIF_ADDR
			Define the base address of MMCIF registers

			CONFIG_SH_MMCIF_CLK
			Define the clock frequency for MMCIF

- USB Device Firmware Update (DFU) class support:
		CONFIG_DFU_OVER_USB
		This enables the USB portion of the DFU USB class

		CONFIG_DFU_NAND
		This enables support for exposing NAND devices via DFU.

		CONFIG_DFU_RAM
		This enables support for exposing RAM via DFU.
		Note: DFU spec refer to non-volatile memory usage, but
		allow usages beyond the scope of spec - here RAM usage,
		one that would help mostly the developer.

		CONFIG_SYS_DFU_DATA_BUF_SIZE
		Dfu transfer uses a buffer before writing data to the
		raw storage device. Make the size (in bytes) of this buffer
		configurable. The size of this buffer is also configurable
		through the "dfu_bufsiz" environment variable.

		CONFIG_SYS_DFU_MAX_FILE_SIZE
		When updating files rather than the raw storage device,
		we use a static buffer to copy the file into and then write
		the buffer once we've been given the whole file.  Define
		this to the maximum filesize (in bytes) for the buffer.
		Default is 4 MiB if undefined.

		DFU_DEFAULT_POLL_TIMEOUT
		Poll timeout [ms], is the timeout a device can send to the
		host. The host must wait for this timeout before sending
		a subsequent DFU_GET_STATUS request to the device.

		DFU_MANIFEST_POLL_TIMEOUT
		Poll timeout [ms], which the device sends to the host when
		entering dfuMANIFEST state. Host waits this timeout, before
		sending again an USB request to the device.

- Keyboard Support:
		See Kconfig help for available keyboard drivers.

- MII/PHY support:
		CONFIG_PHY_CLOCK_FREQ (ppc4xx)

		The clock frequency of the MII bus

		CONFIG_PHY_CMD_DELAY (ppc4xx)

		Some PHY like Intel LXT971A need extra delay after
		command issued before MII status register can be read

- BOOTP Recovery Mode:
		CONFIG_BOOTP_RANDOM_DELAY

		If you have many targets in a network that try to
		boot using BOOTP, you may want to avoid that all
		systems send out BOOTP requests at precisely the same
		moment (which would happen for instance at recovery
		from a power failure, when all systems will try to
		boot, thus flooding the BOOTP server. Defining
		CONFIG_BOOTP_RANDOM_DELAY causes a random delay to be
		inserted before sending out BOOTP requests. The
		following delays are inserted then:

		1st BOOTP request:	delay 0 ... 1 sec
		2nd BOOTP request:	delay 0 ... 2 sec
		3rd BOOTP request:	delay 0 ... 4 sec
		4th and following
		BOOTP requests:		delay 0 ... 8 sec

		CFG_BOOTP_ID_CACHE_SIZE

		BOOTP packets are uniquely identified using a 32-bit ID. The
		server will copy the ID from client requests to responses and
		U-Boot will use this to determine if it is the destination of
		an incoming response. Some servers will check that addresses
		aren't in use before handing them out (usually using an ARP
		ping) and therefore take up to a few hundred milliseconds to
		respond. Network congestion may also influence the time it
		takes for a response to make it back to the client. If that
		time is too long, U-Boot will retransmit requests. In order
		to allow earlier responses to still be accepted after these
		retransmissions, U-Boot's BOOTP client keeps a small cache of
		IDs. The CFG_BOOTP_ID_CACHE_SIZE controls the size of this
		cache. The default is to keep IDs for up to four outstanding
		requests. Increasing this will allow U-Boot to accept offers
		from a BOOTP client in networks with unusually high latency.

- DHCP Advanced Options:

 - Link-local IP address negotiation:
		Negotiate with other link-local clients on the local network
		for an address that doesn't require explicit configuration.
		This is especially useful if a DHCP server cannot be guaranteed
		to exist in all environments that the device must operate.

		See doc/README.link-local for more information.

 - MAC address from environment variables

		FDT_SEQ_MACADDR_FROM_ENV

		Fix-up device tree with MAC addresses fetched sequentially from
		environment variables. This config work on assumption that
		non-usable ethernet node of device-tree are either not present
		or their status has been marked as "disabled".

 - CDP Options:
		CONFIG_CDP_DEVICE_ID

		The device id used in CDP trigger frames.

		CONFIG_CDP_DEVICE_ID_PREFIX

		A two character string which is prefixed to the MAC address
		of the device.

		CONFIG_CDP_PORT_ID

		A printf format string which contains the ascii name of
		the port. Normally is set to "eth%d" which sets
		eth0 for the first Ethernet, eth1 for the second etc.

		CONFIG_CDP_CAPABILITIES

		A 32bit integer which indicates the device capabilities;
		0x00000010 for a normal host which does not forwards.

		CONFIG_CDP_VERSION

		An ascii string containing the version of the software.

		CONFIG_CDP_PLATFORM

		An ascii string containing the name of the platform.

		CONFIG_CDP_TRIGGER

		A 32bit integer sent on the trigger.

		CONFIG_CDP_POWER_CONSUMPTION

		A 16bit integer containing the power consumption of the
		device in .1 of milliwatts.

		CONFIG_CDP_APPLIANCE_VLAN_TYPE

		A byte containing the id of the VLAN.

- I2C Support:
		CFG_SYS_NUM_I2C_BUSES
		Hold the number of i2c buses you want to use.

		CFG_SYS_I2C_BUSES
		hold a list of buses you want to use

		 CFG_SYS_I2C_BUSES	{{0, {I2C_NULL_HOP}}, \
					{0, {{I2C_MUX_PCA9547, 0x70, 1}}}, \
					{0, {{I2C_MUX_PCA9547, 0x70, 2}}}, \
					{0, {{I2C_MUX_PCA9547, 0x70, 3}}}, \
					{0, {{I2C_MUX_PCA9547, 0x70, 4}}}, \
					{0, {{I2C_MUX_PCA9547, 0x70, 5}}}, \
					{1, {I2C_NULL_HOP}}, \
					{1, {{I2C_MUX_PCA9544, 0x72, 1}}}, \
					{1, {{I2C_MUX_PCA9544, 0x72, 2}}}, \
					}

		which defines
			bus 0 on adapter 0 without a mux
			bus 1 on adapter 0 with a PCA9547 on address 0x70 port 1
			bus 2 on adapter 0 with a PCA9547 on address 0x70 port 2
			bus 3 on adapter 0 with a PCA9547 on address 0x70 port 3
			bus 4 on adapter 0 with a PCA9547 on address 0x70 port 4
			bus 5 on adapter 0 with a PCA9547 on address 0x70 port 5
			bus 6 on adapter 1 without a mux
			bus 7 on adapter 1 with a PCA9544 on address 0x72 port 1
			bus 8 on adapter 1 with a PCA9544 on address 0x72 port 2

		If you do not have i2c muxes on your board, omit this define.

- SPI Support:	CONFIG_SPI

		Enables SPI driver (so far only tested with
		SPI EEPROM, also an instance works with Crystal A/D and
		D/As on the SACSng board)

		CFG_SYS_SPI_MXC_WAIT
		Timeout for waiting until spi transfer completed.
		default: (CONFIG_SYS_HZ/100)     /* 10 ms */

- FPGA Support: CONFIG_FPGA

		Enables FPGA subsystem.

		CONFIG_FPGA_<vendor>

		Enables support for specific chip vendors.
		(ALTERA, XILINX)

		CONFIG_FPGA_<family>

		Enables support for FPGA family.
		(SPARTAN2, SPARTAN3, VIRTEX2, CYCLONE2, ACEX1K, ACEX)

		CONFIG_SYS_FPGA_CHECK_BUSY

		Enable checks on FPGA configuration interface busy
		status by the configuration function. This option
		will require a board or device specific function to
		be written.

		CFG_FPGA_DELAY

		If defined, a function that provides delays in the FPGA
		configuration driver.

		CFG_SYS_FPGA_CHECK_ERROR

		Check for configuration errors during FPGA bitfile
		loading. For example, abort during Virtex II
		configuration if the INIT_B line goes low (which
		indicated a CRC error).

		CFG_SYS_FPGA_WAIT_INIT

		Maximum time to wait for the INIT_B line to de-assert
		after PROB_B has been de-asserted during a Virtex II
		FPGA configuration sequence. The default time is 500
		ms.

		CFG_SYS_FPGA_WAIT_BUSY

		Maximum time to wait for BUSY to de-assert during
		Virtex II FPGA configuration. The default is 5 ms.

		CFG_SYS_FPGA_WAIT_CONFIG

		Time to wait after FPGA configuration. The default is
		200 ms.

- Vendor Parameter Protection:

		U-Boot considers the values of the environment
		variables "serial#" (Board Serial Number) and
		"ethaddr" (Ethernet Address) to be parameters that
		are set once by the board vendor / manufacturer, and
		protects these variables from casual modification by
		the user. Once set, these variables are read-only,
		and write or delete attempts are rejected. You can
		change this behaviour:

		If CONFIG_ENV_OVERWRITE is #defined in your config
		file, the write protection for vendor parameters is
		completely disabled. Anybody can change or delete
		these parameters.

		The same can be accomplished in a more flexible way
		for any variable by configuring the type of access
		to allow for those variables in the ".flags" variable
		or by setting CONFIG_ENV_FLAGS_LIST_STATIC.

- Protected RAM:
		CFG_PRAM

		Define this variable to enable the reservation of
		"protected RAM", i. e. RAM which is not overwritten
		by U-Boot. Define CFG_PRAM to hold the number of
		kB you want to reserve for pRAM. You can overwrite
		this default value by defining an environment
		variable "pram" to the number of kB you want to
		reserve. Note that the board info structure will
		still show the full amount of RAM. If pRAM is
		reserved, a new environment variable "mem" will
		automatically be defined to hold the amount of
		remaining RAM in a form that can be passed as boot
		argument to Linux, for instance like that:

			env set bootargs ... mem=\${mem}
			saveenv

		This way you can tell Linux not to use this memory,
		either, which results in a memory region that will
		not be affected by reboots.

		*WARNING* If your board configuration uses automatic
		detection of the RAM size, you must make sure that
		this memory test is non-destructive. So far, the
		following board configurations are known to be
		"pRAM-clean":

			IVMS8, IVML24, SPD8xx,
			HERMES, IP860, RPXlite, LWMON,
			FLAGADM

- Error Recovery:
	Note:

		In the current implementation, the local variables
		space and global environment variables space are
		separated. Local variables are those you define by
		simply typing `name=value'. To access a local
		variable later on, you have write `$name' or
		`${name}'; to execute the contents of a variable
		directly type `$name' at the command prompt.

		Global environment variables are those you use "env set"
		or "env print" to work with. To run a command stored
		in such a variable, you need to use the run command,
		and you must not use the '$' sign to access them.

		To store commands and special characters in a
		variable, please use double quotation marks
		surrounding the whole text of the variable, instead
		of the backslashes before semicolons and special
		symbols.

- Default Environment:
		CFG_EXTRA_ENV_SETTINGS

		Define this to contain any number of null terminated
		strings (variable = value pairs) that will be part of
		the default environment compiled into the boot image.

		For example, place something like this in your
		board's config file:

		#define CFG_EXTRA_ENV_SETTINGS \
			"myvar1=value1\0" \
			"myvar2=value2\0"

		Warning: This method is based on knowledge about the
		internal format how the environment is stored by the
		U-Boot code. This is NOT an official, exported
		interface! Although it is unlikely that this format
		will change soon, there is no guarantee either.
		You better know what you are doing here.

		Note: overly (ab)use of the default environment is
		discouraged. Make sure to check other ways to preset
		the environment like the "source" command or the
		boot command first.

- Automatic software updates via TFTP server
		CONFIG_UPDATE_TFTP
		CONFIG_UPDATE_TFTP_CNT_MAX
		CONFIG_UPDATE_TFTP_MSEC_MAX

		These options enable and control the auto-update feature;
		for a more detailed description refer to doc/README.update.

- MTD Support (mtdparts command, UBI support)
		CONFIG_MTD_UBI_WL_THRESHOLD
		This parameter defines the maximum difference between the highest
		erase counter value and the lowest erase counter value of eraseblocks
		of UBI devices. When this threshold is exceeded, UBI starts performing
		wear leveling by means of moving data from eraseblock with low erase
		counter to eraseblocks with high erase counter.

		The default value should be OK for SLC NAND flashes, NOR flashes and
		other flashes which have eraseblock life-cycle 100000 or more.
		However, in case of MLC NAND flashes which typically have eraseblock
		life-cycle less than 10000, the threshold should be lessened (e.g.,
		to 128 or 256, although it does not have to be power of 2).

		default: 4096

		CONFIG_MTD_UBI_BEB_LIMIT
		This option specifies the maximum bad physical eraseblocks UBI
		expects on the MTD device (per 1024 eraseblocks). If the
		underlying flash does not admit of bad eraseblocks (e.g. NOR
		flash), this value is ignored.

		NAND datasheets often specify the minimum and maximum NVM
		(Number of Valid Blocks) for the flashes' endurance lifetime.
		The maximum expected bad eraseblocks per 1024 eraseblocks
		then can be calculated as "1024 * (1 - MinNVB / MaxNVB)",
		which gives 20 for most NANDs (MaxNVB is basically the total
		count of eraseblocks on the chip).

		To put it differently, if this value is 20, UBI will try to
		reserve about 1.9% of physical eraseblocks for bad blocks
		handling. And that will be 1.9% of eraseblocks on the entire
		NAND chip, not just the MTD partition UBI attaches. This means
		that if you have, say, a NAND flash chip admits maximum 40 bad
		eraseblocks, and it is split on two MTD partitions of the same
		size, UBI will reserve 40 eraseblocks when attaching a
		partition.

		default: 20

		CONFIG_MTD_UBI_FASTMAP
		Fastmap is a mechanism which allows attaching an UBI device
		in nearly constant time. Instead of scanning the whole MTD device it
		only has to locate a checkpoint (called fastmap) on the device.
		The on-flash fastmap contains all information needed to attach
		the device. Using fastmap makes only sense on large devices where
		attaching by scanning takes long. UBI will not automatically install
		a fastmap on old images, but you can set the UBI parameter
		CONFIG_MTD_UBI_FASTMAP_AUTOCONVERT to 1 if you want so. Please note
		that fastmap-enabled images are still usable with UBI implementations
		without	fastmap support. On typical flash devices the whole fastmap
		fits into one PEB. UBI will reserve PEBs to hold two fastmaps.

		CONFIG_MTD_UBI_FASTMAP_AUTOCONVERT
		Set this parameter to enable fastmap automatically on images
		without a fastmap.
		default: 0

		CONFIG_MTD_UBI_FM_DEBUG
		Enable UBI fastmap debug
		default: 0

- SPL framework
		CONFIG_SPL
		Enable building of SPL globally.

		CONFIG_SPL_PANIC_ON_RAW_IMAGE
		When defined, SPL will panic() if the image it has
		loaded does not have a signature.
		Defining this is useful when code which loads images
		in SPL cannot guarantee that absolutely all read errors
		will be caught.
		An example is the LPC32XX MLC NAND driver, which will
		consider that a completely unreadable NAND block is bad,
		and thus should be skipped silently.

		CONFIG_SPL_DISPLAY_PRINT
		For ARM, enable an optional function to print more information
		about the running system.

		CONFIG_SPL_MPC83XX_WAIT_FOR_NAND
		Set this for NAND SPL on PPC mpc83xx targets, so that
		start.S waits for the rest of the SPL to load before
		continuing (the hardware starts execution after just
		loading the first page rather than the full 4K).

		CONFIG_SPL_UBI
		Support for a lightweight UBI (fastmap) scanner and
		loader

		CONFIG_SYS_NAND_5_ADDR_CYCLE, CONFIG_SYS_NAND_PAGE_SIZE,
		CONFIG_SYS_NAND_OOBSIZE, CONFIG_SYS_NAND_BLOCK_SIZE,
		CONFIG_SYS_NAND_BAD_BLOCK_POS, CFG_SYS_NAND_ECCPOS,
		CFG_SYS_NAND_ECCSIZE, CFG_SYS_NAND_ECCBYTES
		Defines the size and behavior of the NAND that SPL uses
		to read U-Boot

		CFG_SYS_NAND_U_BOOT_DST
		Location in memory to load U-Boot to

		CFG_SYS_NAND_U_BOOT_SIZE
		Size of image to load

		CFG_SYS_NAND_U_BOOT_START
		Entry point in loaded image to jump to

		CONFIG_SPL_RAM_DEVICE
		Support for running image already present in ram, in SPL binary

		CONFIG_SPL_FIT_PRINT
		Printing information about a FIT image adds quite a bit of
		code to SPL. So this is normally disabled in SPL. Use this
		option to re-enable it. This will affect the output of the
		bootm command when booting a FIT image.

- Interrupt support (PPC):

		There are common interrupt_init() and timer_interrupt()
		for all PPC archs. interrupt_init() calls interrupt_init_cpu()
		for CPU specific initialization. interrupt_init_cpu()
		should set decrementer_count to appropriate value. If
		CPU resets decrementer automatically after interrupt
		(ppc4xx) it should set decrementer_count to zero.
		timer_interrupt() calls timer_interrupt_cpu() for CPU
		specific handling. If board has watchdog / status_led
		/ other_activity_monitor it works automatically from
		general timer_interrupt().


Board initialization settings:
------------------------------

During Initialization u-boot calls a number of board specific functions
to allow the preparation of board specific prerequisites, e.g. pin setup
before drivers are initialized. To enable these callbacks the
following configuration macros have to be defined. Currently this is
architecture specific, so please check arch/your_architecture/lib/board.c
typically in board_init_f() and board_init_r().

- CONFIG_BOARD_EARLY_INIT_F: Call board_early_init_f()
- CONFIG_BOARD_EARLY_INIT_R: Call board_early_init_r()
- CONFIG_BOARD_LATE_INIT: Call board_late_init()

Low Level (hardware related) configuration options:
---------------------------------------------------

- CONFIG_SYS_CACHELINE_SIZE:
		Cache Line Size of the CPU.

- CONFIG_SYS_CCSRBAR_DEFAULT:
		Default (power-on reset) physical address of CCSR on Freescale
		PowerPC SOCs.

- CFG_SYS_CCSRBAR:
		Virtual address of CCSR.  On a 32-bit build, this is typically
		the same value as CONFIG_SYS_CCSRBAR_DEFAULT.

- CFG_SYS_CCSRBAR_PHYS:
		Physical address of CCSR.  CCSR can be relocated to a new
		physical address, if desired.  In this case, this macro should
		be set to that address.	 Otherwise, it should be set to the
		same value as CONFIG_SYS_CCSRBAR_DEFAULT.  For example, CCSR
		is typically relocated on 36-bit builds.  It is recommended
		that this macro be defined via the _HIGH and _LOW macros:

		#define CFG_SYS_CCSRBAR_PHYS ((CFG_SYS_CCSRBAR_PHYS_HIGH
			* 1ull) << 32 | CFG_SYS_CCSRBAR_PHYS_LOW)

- CFG_SYS_CCSRBAR_PHYS_HIGH:
		Bits 33-36 of CFG_SYS_CCSRBAR_PHYS.	This value is typically
		either 0 (32-bit build) or 0xF (36-bit build).	This macro is
		used in assembly code, so it must not contain typecasts or
		integer size suffixes (e.g. "ULL").

- CFG_SYS_CCSRBAR_PHYS_LOW:
		Lower 32-bits of CFG_SYS_CCSRBAR_PHYS.  This macro is
		used in assembly code, so it must not contain typecasts or
		integer size suffixes (e.g. "ULL").

- CONFIG_SYS_IMMR:	Physical address of the Internal Memory.
		DO NOT CHANGE unless you know exactly what you're
		doing! (11-4) [MPC8xx systems only]

- CFG_SYS_INIT_RAM_ADDR:

		Start address of memory area that can be used for
		initial data and stack; please note that this must be
		writable memory that is working WITHOUT special
		initialization, i. e. you CANNOT use normal RAM which
		will become available only after programming the
		memory controller and running certain initialization
		sequences.

		U-Boot uses the following memory types:
		- MPC8xx: IMMR (internal memory of the CPU)

- CONFIG_SYS_SCCR:	System Clock and reset Control Register (15-27)

- CONFIG_SYS_OR_TIMING_SDRAM:
		SDRAM timing

- CONFIG_SYS_SRIOn_MEM_VIRT:
		Virtual Address of SRIO port 'n' memory region

- CONFIG_SYS_SRIOn_MEM_PHYxS:
		Physical Address of SRIO port 'n' memory region

- CONFIG_SYS_SRIOn_MEM_SIZE:
		Size of SRIO port 'n' memory region

- CONFIG_SYS_NAND_BUSWIDTH_16BIT
		Defined to tell the NAND controller that the NAND chip is using
		a 16 bit bus.
		Not all NAND drivers use this symbol.
		Example of drivers that use it:
		- drivers/mtd/nand/raw/ndfc.c
		- drivers/mtd/nand/raw/mxc_nand.c

- CONFIG_SYS_NDFC_EBC0_CFG
		Sets the EBC0_CFG register for the NDFC. If not defined
		a default value will be used.

- CONFIG_SYS_SPD_BUS_NUM
		If SPD EEPROM is on an I2C bus other than the first
		one, specify here. Note that the value must resolve
		to something your driver can deal with.

- CONFIG_FSL_DDR_INTERACTIVE
		Enable interactive DDR debugging. See doc/README.fsl-ddr.

- CONFIG_FSL_DDR_SYNC_REFRESH
		Enable sync of refresh for multiple controllers.

- CONFIG_FSL_DDR_BIST
		Enable built-in memory test for Freescale DDR controllers.

- CONFIG_RMII
		Enable RMII mode for all FECs.
		Note that this is a global option, we can't
		have one FEC in standard MII mode and another in RMII mode.

- CONFIG_CRC32_VERIFY
		Add a verify option to the crc32 command.
		The syntax is:

		=> crc32 -v <address> <count> <crc32>

		Where address/count indicate a memory area
		and crc32 is the correct crc32 which the
		area should have.

- CONFIG_LOOPW
		Add the "loopw" memory command. This only takes effect if
		the memory commands are activated globally (CONFIG_CMD_MEMORY).

- CONFIG_CMD_MX_CYCLIC
		Add the "mdc" and "mwc" memory commands. These are cyclic
		"md/mw" commands.
		Examples:

		=> mdc.b 10 4 500
		This command will print 4 bytes (10,11,12,13) each 500 ms.

		=> mwc.l 100 12345678 10
		This command will write 12345678 to address 100 all 10 ms.

		This only takes effect if the memory commands are activated
		globally (CONFIG_CMD_MEMORY).

- CONFIG_XPL_BUILD
		Set when the currently running compilation is for an artifact
		that will end up in one of the 'xPL' builds, i.e. SPL, TPL or
		VPL. Code that needs phase-specific behaviour can check this,
		or (where possible) use xpl_phase() instead.

- CONFIG_TPL_BUILD
		Set when the currently running compilation is for an artifact
		that will end up in the TPL build (as opposed to SPL, VPL or
		U-Boot proper). Code that needs phase-specific behaviour can
		check this, or (where possible) use xpl_phase() instead.

- CONFIG_VPL_BUILD
		Set when the currently running compilation is for an artifact
		that will end up in the VPL build (as opposed to the SPL, TPL
		or U-Boot proper). Code that needs phase-specific behaviour can
		check this, or (where possible) use xpl_phase() instead.

- CONFIG_ARCH_MAP_SYSMEM
		Generally U-Boot (and in particular the md command) uses
		effective address. It is therefore not necessary to regard
		U-Boot address as virtual addresses that need to be translated
		to physical addresses. However, sandbox requires this, since
		it maintains its own little RAM buffer which contains all
		addressable memory. This option causes some memory accesses
		to be mapped through map_sysmem() / unmap_sysmem().

- CONFIG_X86_RESET_VECTOR
		If defined, the x86 reset vector code is included. This is not
		needed when U-Boot is running from Coreboot.

Freescale QE/FMAN Firmware Support:
-----------------------------------

The Freescale QUICCEngine (QE) and Frame Manager (FMAN) both support the
loading of "firmware", which is encoded in the QE firmware binary format.
This firmware often needs to be loaded during U-Boot booting, so macros
are used to identify the storage device (NOR flash, SPI, etc) and the address
within that device.

- CONFIG_SYS_FMAN_FW_ADDR
	The address in the storage device where the FMAN microcode is located.  The
	meaning of this address depends on which CONFIG_SYS_QE_FMAN_FW_IN_xxx macro
	is also specified.

- CONFIG_SYS_QE_FW_ADDR
	The address in the storage device where the QE microcode is located.  The
	meaning of this address depends on which CONFIG_SYS_QE_FMAN_FW_IN_xxx macro
	is also specified.

- CONFIG_SYS_QE_FMAN_FW_LENGTH
	The maximum possible size of the firmware.  The firmware binary format
	has a field that specifies the actual size of the firmware, but it
	might not be possible to read any part of the firmware unless some
	local storage is allocated to hold the entire firmware first.

- CONFIG_SYS_QE_FMAN_FW_IN_NOR
	Specifies that QE/FMAN firmware is located in NOR flash, mapped as
	normal addressable memory via the LBC.  CONFIG_SYS_FMAN_FW_ADDR is the
	virtual address in NOR flash.

- CONFIG_SYS_QE_FMAN_FW_IN_NAND
	Specifies that QE/FMAN firmware is located in NAND flash.
	CONFIG_SYS_FMAN_FW_ADDR is the offset within NAND flash.

- CONFIG_SYS_QE_FMAN_FW_IN_MMC
	Specifies that QE/FMAN firmware is located on the primary SD/MMC
	device.  CONFIG_SYS_FMAN_FW_ADDR is the byte offset on that device.

- CONFIG_SYS_QE_FMAN_FW_IN_REMOTE
	Specifies that QE/FMAN firmware is located in the remote (master)
	memory space.	CONFIG_SYS_FMAN_FW_ADDR is a virtual address which
	can be mapped from slave TLB->slave LAW->slave SRIO or PCIE outbound
	window->master inbound window->master LAW->the ucode address in
	master's memory space.

Freescale Layerscape Management Complex Firmware Support:
---------------------------------------------------------
The Freescale Layerscape Management Complex (MC) supports the loading of
"firmware".
This firmware often needs to be loaded during U-Boot booting, so macros
are used to identify the storage device (NOR flash, SPI, etc) and the address
within that device.

- CONFIG_FSL_MC_ENET
	Enable the MC driver for Layerscape SoCs.

Freescale Layerscape Debug Server Support:
-------------------------------------------
The Freescale Layerscape Debug Server Support supports the loading of
"Debug Server firmware" and triggering SP boot-rom.
This firmware often needs to be loaded during U-Boot booting.

- CONFIG_SYS_MC_RSV_MEM_ALIGN
	Define alignment of reserved memory MC requires


Building the Software:
======================

Building U-Boot has been tested in several native build environments
and in many different cross environments. Of course we cannot support
all possibly existing versions of cross development tools in all
(potentially obsolete) versions. In case of tool chain problems we
recommend to use the ELDK (see https://www.denx.de/wiki/DULG/ELDK)
which is extensively used to build and test U-Boot.

If you are not using a native environment, it is assumed that you
have GNU cross compiling tools available in your path. In this case,
you must set the environment variable CROSS_COMPILE in your shell.
Note that no changes to the Makefile or any other source files are
necessary. For example using the ELDK on a 4xx CPU, please enter:

	$ CROSS_COMPILE=ppc_4xx-
	$ export CROSS_COMPILE

U-Boot is intended to be simple to build. After installing the
sources you must configure U-Boot for one specific board type. This
is done by typing:

	make NAME_defconfig

where "NAME_defconfig" is the name of one of the existing configu-
rations; see configs/*_defconfig for supported names.

Note: for some boards special configuration names may exist; check if
      additional information is available from the board vendor; for
      instance, the TQM823L systems are available without (standard)
      or with LCD support. You can select such additional "features"
      when choosing the configuration, i. e.

      make TQM823L_defconfig
	- will configure for a plain TQM823L, i. e. no LCD support

      make TQM823L_LCD_defconfig
	- will configure for a TQM823L with U-Boot console on LCD

      etc.


Finally, type "make all", and you should get some working U-Boot
images ready for download to / installation on your system:

- "u-boot.bin" is a raw binary image
- "u-boot" is an image in ELF binary format
- "u-boot.srec" is in Motorola S-Record format

User specific CPPFLAGS, AFLAGS and CFLAGS can be passed to the compiler by
setting the according environment variables KCPPFLAGS, KAFLAGS and KCFLAGS.
For example to treat all compiler warnings as errors:

	make KCFLAGS=-Werror

Please be aware that the Makefiles assume you are using GNU make, so
for instance on NetBSD you might need to use "gmake" instead of
native "make".


If the system board that you have is not listed, then you will need
to port U-Boot to your hardware platform. To do this, follow these
steps:

1.  Create a new directory to hold your board specific code. Add any
    files you need. In your board directory, you will need at least
    the "Makefile" and a "<board>.c".
2.  Create a new configuration file "include/configs/<board>.h" for
    your board.
3.  If you're porting U-Boot to a new CPU, then also create a new
    directory to hold your CPU specific code. Add any files you need.
4.  Run "make <board>_defconfig" with your new name.
5.  Type "make", and you should get a working "u-boot.srec" file
    to be installed on your target system.
6.  Debug and solve any problems that might arise.
    [Of course, this last step is much harder than it sounds.]


Testing of U-Boot Modifications, Ports to New Hardware, etc.:
==============================================================

If you have modified U-Boot sources (for instance added a new board
or support for new devices, a new CPU, etc.) you are expected to
provide feedback to the other developers. The feedback normally takes
the form of a "patch", i.e. a context diff against a certain (latest
official or latest in the git repository) version of U-Boot sources.

But before you submit such a patch, please verify that your modifi-
cation did not break existing code. At least make sure that *ALL* of
the supported boards compile WITHOUT ANY compiler warnings. To do so,
just run the buildman script (tools/buildman/buildman), which will
configure and build U-Boot for ALL supported system. Be warned, this
will take a while. Please see the buildman README, or run 'buildman -H'
for documentation.


See also "U-Boot Porting Guide" below.


Monitor Commands - Overview:
============================

go	- start application at address 'addr'
run	- run commands in an environment variable
bootm	- boot application image from memory
bootp	- boot image via network using BootP/TFTP protocol
bootz   - boot zImage from memory
tftpboot- boot image via network using TFTP protocol
	       and env variables "ipaddr" and "serverip"
	       (and eventually "gatewayip")
tftpput - upload a file via network using TFTP protocol
rarpboot- boot image via network using RARP/TFTP protocol
diskboot- boot from IDE devicebootd   - boot default, i.e., run 'bootcmd'
loads	- load S-Record file over serial line
loadb	- load binary file over serial line (kermit mode)
loadm   - load binary blob from source address to destination address
md	- memory display
mm	- memory modify (auto-incrementing)
nm	- memory modify (constant address)
mw	- memory write (fill)
ms	- memory search
cp	- memory copy
cmp	- memory compare
crc32	- checksum calculation
i2c	- I2C sub-system
sspi	- SPI utility commands
base	- print or set address offset
printenv- print environment variables
pwm	- control pwm channels
seama   - load SEAMA NAND image
setenv	- set environment variables
saveenv - save environment variables to persistent storage
protect - enable or disable FLASH write protection
erase	- erase FLASH memory
flinfo	- print FLASH memory information
nand	- NAND memory operations (see doc/README.nand)
bdinfo	- print Board Info structure
iminfo	- print header information for application image
coninfo - print console devices and informations
ide	- IDE sub-system
loop	- infinite loop on address range
loopw	- infinite write loop on address range
mtest	- simple RAM test
icache	- enable or disable instruction cache
dcache	- enable or disable data cache
reset	- Perform RESET of the CPU
echo	- echo args to console
version - print monitor version
help	- print online help
?	- alias for 'help'


Monitor Commands - Detailed Description:
========================================

TODO.

For now: just type "help <command>".


Note for Redundant Ethernet Interfaces:
=======================================

Some boards come with redundant Ethernet interfaces; U-Boot supports
such configurations and is capable of automatic selection of a
"working" interface when needed. MAC assignment works as follows:

Network interfaces are numbered eth0, eth1, eth2, ... Corresponding
MAC addresses can be stored in the environment as "ethaddr" (=>eth0),
"eth1addr" (=>eth1), "eth2addr", ...

If the network interface stores some valid MAC address (for instance
in SROM), this is used as default address if there is NO correspon-
ding setting in the environment; if the corresponding environment
variable is set, this overrides the settings in the card; that means:

o If the SROM has a valid MAC address, and there is no address in the
  environment, the SROM's address is used.

o If there is no valid address in the SROM, and a definition in the
  environment exists, then the value from the environment variable is
  used.

o If both the SROM and the environment contain a MAC address, and
  both addresses are the same, this MAC address is used.

o If both the SROM and the environment contain a MAC address, and the
  addresses differ, the value from the environment is used and a
  warning is printed.

o If neither SROM nor the environment contain a MAC address, an error
  is raised. If CONFIG_NET_RANDOM_ETHADDR is defined, then in this case
  a random, locally-assigned MAC is used.

If Ethernet drivers implement the 'write_hwaddr' function, valid MAC addresses
will be programmed into hardware as part of the initialization process.	 This
may be skipped by setting the appropriate 'ethmacskip' environment variable.
The naming convention is as follows:
"ethmacskip" (=>eth0), "eth1macskip" (=>eth1) etc.

Image Formats:
==============

U-Boot is capable of booting (and performing other auxiliary operations on)
images in two formats:

New uImage format (FIT)
-----------------------

Flexible and powerful format based on Flattened Image Tree -- FIT (similar
to Flattened Device Tree). It allows the use of images with multiple
components (several kernels, ramdisks, etc.), with contents protected by
SHA1, MD5 or CRC32. More details are found in the doc/usage/fit directory.


Old uImage format
-----------------

Old image format is based on binary files which can be basically anything,
preceded by a special header; see the definitions in include/image.h for
details; basically, the header defines the following image properties:

* Target Operating System (Provisions for OpenBSD, NetBSD, FreeBSD,
  4.4BSD, Linux, SVR4, Esix, Solaris, Irix, SCO, Dell, NCR, VxWorks,
  LynxOS, pSOS, QNX, RTEMS, INTEGRITY;
  Currently supported: Linux, NetBSD, VxWorks, QNX, RTEMS, INTEGRITY).
* Target CPU Architecture (Provisions for Alpha, ARM, Intel x86,
  IA64, MIPS, Nios II, PowerPC, IBM S390, SuperH, Sparc, Sparc 64 Bit;
  Currently supported: ARM, Intel x86, MIPS, Nios II, PowerPC).
* Compression Type (uncompressed, gzip, bzip2)
* Load Address
* Entry Point
* Image Name
* Image Timestamp

The header is marked by a special Magic Number, and both the header
and the data portions of the image are secured against corruption by
CRC32 checksums.


Linux Support:
==============

Although U-Boot should support any OS or standalone application
easily, the main focus has always been on Linux during the design of
U-Boot.

U-Boot includes many features that so far have been part of some
special "boot loader" code within the Linux kernel. Also, any
"initrd" images to be used are no longer part of one big Linux image;
instead, kernel and "initrd" are separate images. This implementation
serves several purposes:

- the same features can be used for other OS or standalone
  applications (for instance: using compressed images to reduce the
  Flash memory footprint)

- it becomes much easier to port new Linux kernel versions because
  lots of low-level, hardware dependent stuff are done by U-Boot

- the same Linux kernel image can now be used with different "initrd"
  images; of course this also means that different kernel images can
  be run with the same "initrd". This makes testing easier (you don't
  have to build a new "zImage.initrd" Linux image when you just
  change a file in your "initrd"). Also, a field-upgrade of the
  software is easier now.


Linux HOWTO:
============

Porting Linux to U-Boot based systems:
---------------------------------------

U-Boot cannot save you from doing all the necessary modifications to
configure the Linux device drivers for use with your target hardware
(no, we don't intend to provide a full virtual machine interface to
Linux :-).

But now you can ignore ALL boot loader code (in arch/powerpc/mbxboot).

Just make sure your machine specific header file (for instance
include/asm-ppc/tqm8xx.h) includes the same definition of the Board
Information structure as we define in include/asm-<arch>/u-boot.h,
and make sure that your definition of IMAP_ADDR uses the same value
as your U-Boot configuration in CONFIG_SYS_IMMR.

Note that U-Boot now has a driver model, a unified model for drivers.
If you are adding a new driver, plumb it into driver model. If there
is no uclass available, you are encouraged to create one. See
doc/driver-model.


Configuring the Linux kernel:
-----------------------------

No specific requirements for U-Boot. Make sure you have some root
device (initial ramdisk, NFS) for your target system.


Building a Linux Image:
-----------------------

With U-Boot, "normal" build targets like "zImage" or "bzImage" are
not used. If you use recent kernel source, a new build target
"uImage" will exist which automatically builds an image usable by
U-Boot. Most older kernels also have support for a "pImage" target,
which was introduced for our predecessor project PPCBoot and uses a
100% compatible format.

Example:

	make TQM850L_defconfig
	make oldconfig
	make dep
	make uImage

The "uImage" build target uses a special tool (in 'tools/mkimage') to
encapsulate a compressed Linux kernel image with header	 information,
CRC32 checksum etc. for use with U-Boot. This is what we are doing:

* build a standard "vmlinux" kernel image (in ELF binary format):

* convert the kernel into a raw binary image:

	${CROSS_COMPILE}-objcopy -O binary \
				 -R .note -R .comment \
				 -S vmlinux linux.bin

* compress the binary image:

	gzip -9 linux.bin

* package compressed binary image for U-Boot:

	mkimage -A ppc -O linux -T kernel -C gzip \
		-a 0 -e 0 -n "Linux Kernel Image" \
		-d linux.bin.gz uImage


The "mkimage" tool can also be used to create ramdisk images for use
with U-Boot, either separated from the Linux kernel image, or
combined into one file. "mkimage" encapsulates the images with a 64
byte header containing information about target architecture,
operating system, image type, compression method, entry points, time
stamp, CRC32 checksums, etc.

"mkimage" can be called in two ways: to verify existing images and
print the header information, or to build new images.

In the first form (with "-l" option) mkimage lists the information
contained in the header of an existing U-Boot image; this includes
checksum verification:

	tools/mkimage -l image
	  -l ==> list image header information

The second form (with "-d" option) is used to build a U-Boot image
from a "data file" which is used as image payload:

	tools/mkimage -A arch -O os -T type -C comp -a addr -e ep \
		      -n name -d data_file image
	  -A ==> set architecture to 'arch'
	  -O ==> set operating system to 'os'
	  -T ==> set image type to 'type'
	  -C ==> set compression type 'comp'
	  -a ==> set load address to 'addr' (hex)
	  -e ==> set entry point to 'ep' (hex)
	  -n ==> set image name to 'name'
	  -d ==> use image data from 'datafile'

Right now, all Linux kernels for PowerPC systems use the same load
address (0x00000000), but the entry point address depends on the
kernel version:

- 2.2.x kernels have the entry point at 0x0000000C,
- 2.3.x and later kernels have the entry point at 0x00000000.

So a typical call to build a U-Boot image would read:

	-> tools/mkimage -n '2.4.4 kernel for TQM850L' \
	> -A ppc -O linux -T kernel -C gzip -a 0 -e 0 \
	> -d /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/powerpc/coffboot/vmlinux.gz \
	> examples/uImage.TQM850L
	Image Name:   2.4.4 kernel for TQM850L
	Created:      Wed Jul 19 02:34:59 2000
	Image Type:   PowerPC Linux Kernel Image (gzip compressed)
	Data Size:    335725 Bytes = 327.86 kB = 0.32 MB
	Load Address: 0x00000000
	Entry Point:  0x00000000

To verify the contents of the image (or check for corruption):

	-> tools/mkimage -l examples/uImage.TQM850L
	Image Name:   2.4.4 kernel for TQM850L
	Created:      Wed Jul 19 02:34:59 2000
	Image Type:   PowerPC Linux Kernel Image (gzip compressed)
	Data Size:    335725 Bytes = 327.86 kB = 0.32 MB
	Load Address: 0x00000000
	Entry Point:  0x00000000

NOTE: for embedded systems where boot time is critical you can trade
speed for memory and install an UNCOMPRESSED image instead: this
needs more space in Flash, but boots much faster since it does not
need to be uncompressed:

	-> gunzip /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/powerpc/coffboot/vmlinux.gz
	-> tools/mkimage -n '2.4.4 kernel for TQM850L' \
	> -A ppc -O linux -T kernel -C none -a 0 -e 0 \
	> -d /opt/elsk/ppc_8xx/usr/src/linux-2.4.4/arch/powerpc/coffboot/vmlinux \
	> examples/uImage.TQM850L-uncompressed
	Image Name:   2.4.4 kernel for TQM850L
	Created:      Wed Jul 19 02:34:59 2000
	Image Type:   PowerPC Linux Kernel Image (uncompressed)
	Data Size:    792160 Bytes = 773.59 kB = 0.76 MB
	Load Address: 0x00000000
	Entry Point:  0x00000000


Similar you can build U-Boot images from a 'ramdisk.image.gz' file
when your kernel is intended to use an initial ramdisk:

	-> tools/mkimage -n 'Simple Ramdisk Image' \
	> -A ppc -O linux -T ramdisk -C gzip \
	> -d /LinuxPPC/images/SIMPLE-ramdisk.image.gz examples/simple-initrd
	Image Name:   Simple Ramdisk Image
	Created:      Wed Jan 12 14:01:50 2000
	Image Type:   PowerPC Linux RAMDisk Image (gzip compressed)
	Data Size:    566530 Bytes = 553.25 kB = 0.54 MB
	Load Address: 0x00000000
	Entry Point:  0x00000000

The "dumpimage" tool can be used to disassemble or list the contents of images
built by mkimage. See dumpimage's help output (-h) for details.

Installing a Linux Image:
-------------------------

To downloading a U-Boot image over the serial (console) interface,
you must convert the image to S-Record format:

	objcopy -I binary -O srec examples/image examples/image.srec

The 'objcopy' does not understand the information in the U-Boot
image header, so the resulting S-Record file will be relative to
address 0x00000000. To load it to a given address, you need to
specify the target address as 'offset' parameter with the 'loads'
command.

Example: install the image to address 0x40100000 (which on the
TQM8xxL is in the first Flash bank):

	=> erase 40100000 401FFFFF

	.......... done
	Erased 8 sectors

	=> loads 40100000
	## Ready for S-Record download ...
	~>examples/image.srec
	1 2 3 4 5 6 7 8 9 10 11 12 13 ...
	...
	15989 15990 15991 15992
	[file transfer complete]
	[connected]
	## Start Addr = 0x00000000


You can check the success of the download using the 'iminfo' command;
this includes a checksum verification so you can be sure no data
corruption happened:

	=> imi 40100000

	## Checking Image at 40100000 ...
	   Image Name:	 2.2.13 for initrd on TQM850L
	   Image Type:	 PowerPC Linux Kernel Image (gzip compressed)
	   Data Size:	 335725 Bytes = 327 kB = 0 MB
	   Load Address: 00000000
	   Entry Point:	 0000000c
	   Verifying Checksum ... OK


Boot Linux:
-----------

The "bootm" command is used to boot an application that is stored in
memory (RAM or Flash). In case of a Linux kernel image, the contents
of the "bootargs" environment variable is passed to the kernel as
parameters. You can check and modify this variable using the
"env print" and "env set" commands:


	=> env print bootargs
	bootargs=root=/dev/ram

	=> env set bootargs root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2

	=> env print bootargs
	bootargs=root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2

	=> bootm 40020000
	## Booting Linux kernel at 40020000 ...
	   Image Name:	 2.2.13 for NFS on TQM850L
	   Image Type:	 PowerPC Linux Kernel Image (gzip compressed)
	   Data Size:	 381681 Bytes = 372 kB = 0 MB
	   Load Address: 00000000
	   Entry Point:	 0000000c
	   Verifying Checksum ... OK
	   Uncompressing Kernel Image ... OK
	Linux version 2.2.13 (wd@denx.local.net) (gcc version 2.95.2 19991024 (release)) #1 Wed Jul 19 02:35:17 MEST 2000
	Boot arguments: root=/dev/nfs rw nfsroot=10.0.0.2:/LinuxPPC nfsaddrs=10.0.0.99:10.0.0.2
	time_init: decrementer frequency = 187500000/60
	Calibrating delay loop... 49.77 BogoMIPS
	Memory: 15208k available (700k kernel code, 444k data, 32k init) [c0000000,c1000000]
	...

If you want to boot a Linux kernel with initial RAM disk, you pass
the memory addresses of both the kernel and the initrd image (PPBCOOT
format!) to the "bootm" command:

	=> imi 40100000 40200000

	## Checking Image at 40100000 ...
	   Image Name:	 2.2.13 for initrd on TQM850L
	   Image Type:	 PowerPC Linux Kernel Image (gzip compressed)
	   Data Size:	 335725 Bytes = 327 kB = 0 MB
	   Load Address: 00000000
	   Entry Point:	 0000000c
	   Verifying Checksum ... OK

	## Checking Image at 40200000 ...
	   Image Name:	 Simple Ramdisk Image
	   Image Type:	 PowerPC Linux RAMDisk Image (gzip compressed)
	   Data Size:	 566530 Bytes = 553 kB = 0 MB
	   Load Address: 00000000
	   Entry Point:	 00000000
	   Verifying Checksum ... OK

	=> bootm 40100000 40200000
	## Booting Linux kernel at 40100000 ...
	   Image Name:	 2.2.13 for initrd on TQM850L
	   Image Type:	 PowerPC Linux Kernel Image (gzip compressed)
	   Data Size:	 335725 Bytes = 327 kB = 0 MB
	   Load Address: 00000000
	   Entry Point:	 0000000c
	   Verifying Checksum ... OK
	   Uncompressing Kernel Image ... OK
	## Loading RAMDisk Image at 40200000 ...
	   Image Name:	 Simple Ramdisk Image
	   Image Type:	 PowerPC Linux RAMDisk Image (gzip compressed)
	   Data Size:	 566530 Bytes = 553 kB = 0 MB
	   Load Address: 00000000
	   Entry Point:	 00000000
	   Verifying Checksum ... OK
	   Loading Ramdisk ... OK
	Linux version 2.2.13 (wd@denx.local.net) (gcc version 2.95.2 19991024 (release)) #1 Wed Jul 19 02:32:08 MEST 2000
	Boot arguments: root=/dev/ram
	time_init: decrementer frequency = 187500000/60
	Calibrating delay loop... 49.77 BogoMIPS
	...
	RAMDISK: Compressed image found at block 0
	VFS: Mounted root (ext2 filesystem).

	bash#

Boot Linux and pass a flat device tree:
-----------

First, U-Boot must be compiled with the appropriate defines. See the section
titled "Linux Kernel Interface" above for a more in depth explanation. The
following is an example of how to start a kernel and pass an updated
flat device tree:

=> print oftaddr
oftaddr=0x300000
=> print oft
oft=oftrees/mpc8540ads.dtb
=> tftp $oftaddr $oft
Speed: 1000, full duplex
Using TSEC0 device
TFTP from server 192.168.1.1; our IP address is 192.168.1.101
Filename 'oftrees/mpc8540ads.dtb'.
Load address: 0x300000
Loading: #
done
Bytes transferred = 4106 (100a hex)
=> tftp $loadaddr $bootfile
Speed: 1000, full duplex
Using TSEC0 device
TFTP from server 192.168.1.1; our IP address is 192.168.1.2
Filename 'uImage'.
Load address: 0x200000
Loading:############
done
Bytes transferred = 1029407 (fb51f hex)
=> print loadaddr
loadaddr=200000
=> print oftaddr
oftaddr=0x300000
=> bootm $loadaddr - $oftaddr
## Booting image at 00200000 ...
   Image Name:	 Linux-2.6.17-dirty
   Image Type:	 PowerPC Linux Kernel Image (gzip compressed)
   Data Size:	 1029343 Bytes = 1005.2 kB
   Load Address: 00000000
   Entry Point:	 00000000
   Verifying Checksum ... OK
   Uncompressing Kernel Image ... OK
Booting using flat device tree at 0x300000
Using MPC85xx ADS machine description
Memory CAM mapping: CAM0=256Mb, CAM1=256Mb, CAM2=0Mb residual: 0Mb
[snip]


More About U-Boot Image Types:
------------------------------

U-Boot supports the following image types:

   "Standalone Programs" are directly runnable in the environment
	provided by U-Boot; it is expected that (if they behave
	well) you can continue to work in U-Boot after return from
	the Standalone Program.
   "OS Kernel Images" are usually images of some Embedded OS which
	will take over control completely. Usually these programs
	will install their own set of exception handlers, device
	drivers, set up the MMU, etc. - this means, that you cannot
	expect to re-enter U-Boot except by resetting the CPU.
   "RAMDisk Images" are more or less just data blocks, and their
	parameters (address, size) are passed to an OS kernel that is
	being started.
   "Multi-File Images" contain several images, typically an OS
	(Linux) kernel image and one or more data images like
	RAMDisks. This construct is useful for instance when you want
	to boot over the network using BOOTP etc., where the boot
	server provides just a single image file, but you want to get
	for instance an OS kernel and a RAMDisk image.

	"Multi-File Images" start with a list of image sizes, each
	image size (in bytes) specified by an "uint32_t" in network
	byte order. This list is terminated by an "(uint32_t)0".
	Immediately after the terminating 0 follow the images, one by
	one, all aligned on "uint32_t" boundaries (size rounded up to
	a multiple of 4 bytes).

   "Firmware Images" are binary images containing firmware (like
	U-Boot or FPGA images) which usually will be programmed to
	flash memory.

   "Script files" are command sequences that will be executed by
	U-Boot's command interpreter; this feature is especially
	useful when you configure U-Boot to use a real shell (hush)
	as command interpreter.

Booting the Linux zImage:
-------------------------

On some platforms, it's possible to boot Linux zImage. This is done
using the "bootz" command. The syntax of "bootz" command is the same
as the syntax of "bootm" command.

Note, defining the CONFIG_SUPPORT_RAW_INITRD allows user to supply
kernel with raw initrd images. The syntax is slightly different, the
address of the initrd must be augmented by it's size, in the following
format: "<initrd addres>:<initrd size>".


Standalone HOWTO:
=================

One of the features of U-Boot is that you can dynamically load and
run "standalone" applications, which can use some resources of
U-Boot like console I/O functions or interrupt services.

Two simple examples are included with the sources:

"Hello World" Demo:
-------------------

'examples/hello_world.c' contains a small "Hello World" Demo
application; it is automatically compiled when you build U-Boot.
It's configured to run at address 0x00040004, so you can play with it
like that:

	=> loads
	## Ready for S-Record download ...
	~>examples/hello_world.srec
	1 2 3 4 5 6 7 8 9 10 11 ...
	[file transfer complete]
	[connected]
	## Start Addr = 0x00040004

	=> go 40004 Hello World! This is a test.
	## Starting application at 0x00040004 ...
	Hello World
	argc = 7
	argv[0] = "40004"
	argv[1] = "Hello"
	argv[2] = "World!"
	argv[3] = "This"
	argv[4] = "is"
	argv[5] = "a"
	argv[6] = "test."
	argv[7] = "<NULL>"
	Hit any key to exit ...

	## Application terminated, rc = 0x0

Another example, which demonstrates how to register a CPM interrupt
handler with the U-Boot code, can be found in 'examples/timer.c'.
Here, a CPM timer is set up to generate an interrupt every second.
The interrupt service routine is trivial, just printing a '.'
character, but this is just a demo program. The application can be
controlled by the following keys:

	? - print current values og the CPM Timer registers
	b - enable interrupts and start timer
	e - stop timer and disable interrupts
	q - quit application

	=> loads
	## Ready for S-Record download ...
	~>examples/timer.srec
	1 2 3 4 5 6 7 8 9 10 11 ...
	[file transfer complete]
	[connected]
	## Start Addr = 0x00040004

	=> go 40004
	## Starting application at 0x00040004 ...
	TIMERS=0xfff00980
	Using timer 1
	  tgcr @ 0xfff00980, tmr @ 0xfff00990, trr @ 0xfff00994, tcr @ 0xfff00998, tcn @ 0xfff0099c, ter @ 0xfff009b0

Hit 'b':
	[q, b, e, ?] Set interval 1000000 us
	Enabling timer
Hit '?':
	[q, b, e, ?] ........
	tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0xef6, ter=0x0
Hit '?':
	[q, b, e, ?] .
	tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0x2ad4, ter=0x0
Hit '?':
	[q, b, e, ?] .
	tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0x1efc, ter=0x0
Hit '?':
	[q, b, e, ?] .
	tgcr=0x1, tmr=0xff1c, trr=0x3d09, tcr=0x0, tcn=0x169d, ter=0x0
Hit 'e':
	[q, b, e, ?] ...Stopping timer
Hit 'q':
	[q, b, e, ?] ## Application terminated, rc = 0x0


Implementation Internals:
=========================

The following is not intended to be a complete description of every
implementation detail. However, it should help to understand the
inner workings of U-Boot and make it easier to port it to custom
hardware.


Initial Stack, Global Data:
---------------------------

The implementation of U-Boot is complicated by the fact that U-Boot
starts running out of ROM (flash memory), usually without access to
system RAM (because the memory controller is not initialized yet).
This means that we don't have writable Data or BSS segments, and BSS
is not initialized as zero. To be able to get a C environment working
at all, we have to allocate at least a minimal stack. Implementation
options for this are defined and restricted by the CPU used: Some CPU
models provide on-chip memory (like the IMMR area on MPC8xx and
MPC826x processors), on others (parts of) the data cache can be
locked as (mis-) used as memory, etc.

	Chris Hallinan posted a good summary of these issues to the
	U-Boot mailing list:

	Subject: RE: [U-Boot-Users] RE: More On Memory Bank x (nothingness)?
	From: "Chris Hallinan" <clh@net1plus.com>
	Date: Mon, 10 Feb 2003 16:43:46 -0500 (22:43 MET)
	...

	Correct me if I'm wrong, folks, but the way I understand it
	is this: Using DCACHE as initial RAM for Stack, etc, does not
	require any physical RAM backing up the cache. The cleverness
	is that the cache is being used as a temporary supply of
	necessary storage before the SDRAM controller is setup. It's
	beyond the scope of this list to explain the details, but you
	can see how this works by studying the cache architecture and
	operation in the architecture and processor-specific manuals.

	OCM is On Chip Memory, which I believe the 405GP has 4K. It
	is another option for the system designer to use as an
	initial stack/RAM area prior to SDRAM being available. Either
	option should work for you. Using CS 4 should be fine if your
	board designers haven't used it for something that would
	cause you grief during the initial boot! It is frequently not
	used.

	CFG_SYS_INIT_RAM_ADDR should be somewhere that won't interfere
	with your processor/board/system design. The default value
	you will find in any recent u-boot distribution in
	walnut.h should work for you. I'd set it to a value larger
	than your SDRAM module. If you have a 64MB SDRAM module, set
	it above 400_0000. Just make sure your board has no resources
	that are supposed to respond to that address! That code in
	start.S has been around a while and should work as is when
	you get the config right.

	-Chris Hallinan
	DS4.COM, Inc.

It is essential to remember this, since it has some impact on the C
code for the initialization procedures:

* Initialized global data (data segment) is read-only. Do not attempt
  to write it.

* Do not use any uninitialized global data (or implicitly initialized
  as zero data - BSS segment) at all - this is undefined, initiali-
  zation is performed later (when relocating to RAM).

* Stack space is very limited. Avoid big data buffers or things like
  that.

Having only the stack as writable memory limits means we cannot use
normal global data to share information between the code. But it
turned out that the implementation of U-Boot can be greatly
simplified by making a global data structure (gd_t) available to all
functions. We could pass a pointer to this data as argument to _all_
functions, but this would bloat the code. Instead we use a feature of
the GCC compiler (Global Register Variables) to share the data: we
place a pointer (gd) to the global data into a register which we
reserve for this purpose.

When choosing a register for such a purpose we are restricted by the
relevant  (E)ABI  specifications for the current architecture, and by
GCC's implementation.

For PowerPC, the following registers have specific use:
	R1:	stack pointer
	R2:	reserved for system use
	R3-R4:	parameter passing and return values
	R5-R10: parameter passing
	R13:	small data area pointer
	R30:	GOT pointer
	R31:	frame pointer

	(U-Boot also uses R12 as internal GOT pointer. r12
	is a volatile register so r12 needs to be reset when
	going back and forth between asm and C)

    ==> U-Boot will use R2 to hold a pointer to the global data

    Note: on PPC, we could use a static initializer (since the
    address of the global data structure is known at compile time),
    but it turned out that reserving a register results in somewhat
    smaller code - although the code savings are not that big (on
    average for all boards 752 bytes for the whole U-Boot image,
    624 text + 127 data).

On ARM, the following registers are used:

	R0:	function argument word/integer result
	R1-R3:	function argument word
	R9:	platform specific
	R10:	stack limit (used only if stack checking is enabled)
	R11:	argument (frame) pointer
	R12:	temporary workspace
	R13:	stack pointer
	R14:	link register
	R15:	program counter

    ==> U-Boot will use R9 to hold a pointer to the global data

    Note: on ARM, only R_ARM_RELATIVE relocations are supported.

On Nios II, the ABI is documented here:
	https://www.altera.com/literature/hb/nios2/n2cpu_nii51016.pdf

    ==> U-Boot will use gp to hold a pointer to the global data

    Note: on Nios II, we give "-G0" option to gcc and don't use gp
    to access small data sections, so gp is free.

On RISC-V, the following registers are used:

	x0: hard-wired zero (zero)
	x1: return address (ra)
	x2:	stack pointer (sp)
	x3:	global pointer (gp)
	x4:	thread pointer (tp)
	x5:	link register (t0)
	x8:	frame pointer (fp)
	x10-x11:	arguments/return values (a0-1)
	x12-x17:	arguments (a2-7)
	x28-31:	 temporaries (t3-6)
	pc:	program counter (pc)

    ==> U-Boot will use gp to hold a pointer to the global data

System Initialization:
----------------------

In the reset configuration, U-Boot starts at the reset entry point
(on most PowerPC systems at address 0x00000100). Because of the reset
configuration for CS0# this is a mirror of the on board Flash memory.
To be able to re-map memory U-Boot then jumps to its link address.
To be able to implement the initialization code in C, a (small!)
initial stack is set up in the internal Dual Ported RAM (in case CPUs
which provide such a feature like), or in a locked part of the data
cache. After that, U-Boot initializes the CPU core, the caches and
the SIU.

Next, all (potentially) available memory banks are mapped using a
preliminary mapping. For example, we put them on 512 MB boundaries
(multiples of 0x20000000: SDRAM on 0x00000000 and 0x20000000, Flash
on 0x40000000 and 0x60000000, SRAM on 0x80000000). Then UPM A is
programmed for SDRAM access. Using the temporary configuration, a
simple memory test is run that determines the size of the SDRAM
banks.

When there is more than one SDRAM bank, and the banks are of
different size, the largest is mapped first. For equal size, the first
bank (CS2#) is mapped first. The first mapping is always for address
0x00000000, with any additional banks following immediately to create
contiguous memory starting from 0.

Then, the monitor installs itself at the upper end of the SDRAM area
and allocates memory for use by malloc() and for the global Board
Info data; also, the exception vector code is copied to the low RAM
pages, and the final stack is set up.

Only after this relocation will you have a "normal" C environment;
until that you are restricted in several ways, mostly because you are
running from ROM, and because the code will have to be relocated to a
new address in RAM.


Contributing
============

The U-Boot projects depends on contributions from the user community.
If you want to participate, please, have a look at the 'General'
section of https://docs.u-boot-project.org/en/latest/develop/index.html
where we describe coding standards and the patch submission process.