Linux Sysfs Driver

1. Introduction

Sysfs is a virtual filesystem in Linux that exposes:

Kernel objects
Devices
Drivers
Bus information
Runtime attributes

to user space through files and directories.

Sysfs is mounted at:

/sys

It is one of the most important interfaces used in Linux device driver development.

Unlike procfs, sysfs is:

Structured
Device-oriented
Stable
Recommended for modern Linux drivers

2. What is a Sysfs Driver?

A Sysfs Driver is a Linux kernel module that creates entries inside:

/sys

These entries allow:

Reading kernel/device data
Writing configuration values
Runtime control of drivers

User space interacts using:

cat
echo

commands.

3. Why Do We Use Sysfs?

Sysfs provides a clean interface between:

Kernel Space ↔ User Space

It is mainly used for:

Usage	Purpose
Device configuration	Runtime settings
Driver control	Enable/disable features
Monitoring	View device status
Hardware abstraction	Standard Linux interface
Debugging	Device diagnostics

4. Real-Time Examples

Sysfs Path	Purpose
/sys/class/gpio	GPIO control
/sys/class/leds	LED control
/sys/class/pwm	PWM management
/sys/block	Block devices
/sys/bus	Bus information

Driver examples:

Sensor value reading
LED brightness control
GPIO direction setup
Device enable/disable
Runtime tuning

5. Sysfs Architecture

+-------------------------------+
| User Space Application        |
|-------------------------------|
| cat                           |
| echo                          |
+---------------+---------------+
                |
                v
+-------------------------------+
| Sysfs Virtual Filesystem      |
+---------------+---------------+
                |
                v
+-------------------------------+
| Kernel Object (kobject)       |
+---------------+---------------+
                |
                v
+-------------------------------+
| Sysfs Driver                  |
|-------------------------------|
| show()                        |
| store()                       |
+---------------+---------------+
                |
                v
+-------------------------------+
| Device / Kernel Data          |
+-------------------------------+

6. Sysfs vs Procfs

Feature	Sysfs	Procfs
Purpose	Device model	Process/kernel info
Structure	Structured	Free-form
Recommended	Yes	Limited
Device Integration	Excellent	Weak

7. Important Sysfs Concepts

kobject

Represents kernel object.

Sysfs is built around:

struct kobject

Attribute

Represents sysfs file.

Each attribute has:

show() → Read
store() → Write

kset

Collection of kobjects.

8. Important Header Files

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/kobject.h>
#include <linux/sysfs.h>
#include <linux/string.h>

9. Sysfs Driver Flow

Step 1 – Module Loaded

Driver creates kobject.

Step 2 – Sysfs File Created

Attribute file added.

Step 3 – User Reads File

Kernel calls:

show()

Step 4 – User Writes File

Kernel calls:

store()

Step 5 – Module Removed

Sysfs files removed.

10. Full Sysfs Driver Example

sysfs_driver.c

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/kobject.h>
#include <linux/sysfs.h>

static struct kobject *my_kobject;
static int value = 0;

static ssize_t value_show(struct kobject *kobj,
                          struct kobj_attribute *attr,
                          char *buf)
{
    return sprintf(buf, "%d\n", value);
}

static ssize_t value_store(struct kobject *kobj,
                           struct kobj_attribute *attr,
                           const char *buf,
                           size_t count)
{
    sscanf(buf, "%d", &value);

    printk(KERN_INFO "Sysfs Value = %d\n", value);

    return count;
}

static struct kobj_attribute value_attribute =
    __ATTR(value, 0664, value_show, value_store);

static int __init sysfs_driver_init(void)
{
    int ret;

    my_kobject = kobject_create_and_add("my_sysfs", kernel_kobj);

    if (!my_kobject)
        return -ENOMEM;

    ret = sysfs_create_file(my_kobject,
                            &value_attribute.attr);

    if (ret) {
        printk(KERN_ERR "Failed to create sysfs file\n");
        kobject_put(my_kobject);
        return ret;
    }

    printk(KERN_INFO "Sysfs Driver Loaded\n");

    return 0;
}

static void __exit sysfs_driver_exit(void)
{
    kobject_put(my_kobject);

    printk(KERN_INFO "Sysfs Driver Removed\n");
}

module_init(sysfs_driver_init);
module_exit(sysfs_driver_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("Simple Sysfs Driver");

11. Makefile

obj-m += sysfs_driver.o

KDIR := /lib/modules/$(shell uname -r)/build
PWD  := $(shell pwd)

all:
	make -C $(KDIR) M=$(PWD) modules

clean:
	make -C $(KDIR) M=$(PWD) clean

12. Compile Driver

make

Output:

sysfs_driver.ko

13. Insert Driver

sudo insmod sysfs_driver.ko

14. Verify Sysfs Entry

ls /sys/kernel/my_sysfs/

Expected:

value

15. Read Sysfs File

cat /sys/kernel/my_sysfs/value

Expected Output:

16. Write Sysfs File

echo 100 > /sys/kernel/my_sysfs/value

17. Check Kernel Logs

dmesg | tail

Expected:

Sysfs Value = 100

18. Important Sysfs APIs

kobject_create_and_add()

Creates sysfs directory.

Example:

kobject_create_and_add(name,
                       parent);

sysfs_create_file()

Creates sysfs file.

Example:

sysfs_create_file(kobj,
                  &attr);

kobject_put()

Releases kobject.

Example:

kobject_put(kobj);

19. __ATTR Macro

Creates sysfs attribute.

Syntax:

__ATTR(name,
       permission,
       show,
       store)

20. Sysfs Permissions

Example:

Meaning:

Value	Permission
6	Read + Write
4	Read Only

21. show() Function

Used when user reads sysfs file.

Example:

cat /sys/kernel/my_sysfs/value

calls:

value_show()

22. store() Function

Used when user writes sysfs file.

Example:

echo 10 > value

calls:

value_store()

23. Advantages of Sysfs

Advantage	Description
Structured Interface	Clean device representation
Device-Oriented	Integrated with driver model
Standard Linux Method	Widely used
Easy User Access	cat/echo support
Runtime Configuration	Dynamic control

24. Disadvantages of Sysfs

Disadvantage	Description
Text Only	No binary support
Limited Complexity	Simple interfaces only
Permission Management	Security considerations
Not for Large Data	Small attributes preferred

25. Sysfs vs Procfs vs Debugfs

Feature	Sysfs	Procfs	Debugfs
Purpose	Device model	Process/kernel info	Debugging
Stable ABI	Yes	Partial	No
Recommended for Drivers	Yes	Limited	Debug only
Location	/sys	/proc	/sys/kernel/debug

26. Common Interview Questions

Q1. What is Sysfs?

A virtual filesystem exposing kernel objects and device attributes to user space.

Q2. Why Use Sysfs?

For:

Runtime configuration
Device management
Monitoring

Q3. Difference Between show() and store()?

Function	Purpose
show()	Read operation
store()	Write operation

Q4. What is kobject?

Core Linux kernel object used in sysfs hierarchy.

Q5. Why is Sysfs Preferred Over Procfs?

Sysfs provides structured device representation.

27. Common Errors

Error: Sysfs File Not Created

Cause:

sysfs_create_file() failure

Fix:

Verify return values

Error: Permission Denied

Cause:

Wrong attribute permissions

Fix:

Verify mode bits

Error: Kernel Crash

Cause:

Invalid buffer handling

Fix:

Validate inputs

28. Sysfs Debugging Techniques

Check Sysfs Entries

ls /sys/kernel/

Read Sysfs File

cat /sys/kernel/my_sysfs/value

Kernel Logs

dmesg | tail

29. Advanced Sysfs Topics

After learning basic sysfs drivers, move to:

Device attributes
Attribute groups
Binary sysfs attributes
Driver model integration
Bus subsystem
Power management
Runtime PM

30. Sysfs Attribute Groups

Used to create multiple attributes together.

Example:

sysfs_create_group()

31. Best Practices

Keep Sysfs Files Simple

One value per file.

Validate User Input

Always validate:

store()

input.

Use Sysfs for Configuration

Preferred over procfs for modern drivers.

Avoid Large Data

Sysfs is designed for small text attributes.

32. Real Hardware Platforms

Sysfs is heavily used on:

Raspberry Pi 5
BeagleBone Black
NVIDIA Jetson Nano
STM32MP157

33. Real Linux Sysfs Examples

Path	Purpose
/sys/class/gpio	GPIO control
/sys/class/leds	LED management
/sys/block	Storage devices
/sys/bus	Bus subsystem
/sys/devices	Device hierarchy

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Linux Sysfs Driver

1. Introduction

2. What is a Sysfs Driver?

3. Why Do We Use Sysfs?

4. Real-Time Examples

5. Sysfs Architecture

6. Sysfs vs Procfs

7. Important Sysfs Concepts

kobject

Attribute

kset

8. Important Header Files

9. Sysfs Driver Flow

Step 1 – Module Loaded

Step 2 – Sysfs File Created

Step 3 – User Reads File

Step 4 – User Writes File

Step 5 – Module Removed

10. Full Sysfs Driver Example

sysfs_driver.c

11. Makefile

12. Compile Driver

13. Insert Driver

14. Verify Sysfs Entry

15. Read Sysfs File

16. Write Sysfs File

17. Check Kernel Logs

18. Important Sysfs APIs

kobject_create_and_add()

sysfs_create_file()

kobject_put()

19. __ATTR Macro

20. Sysfs Permissions

21. show() Function

22. store() Function

23. Advantages of Sysfs

24. Disadvantages of Sysfs

25. Sysfs vs Procfs vs Debugfs

26. Common Interview Questions

Q1. What is Sysfs?

Q2. Why Use Sysfs?

Q3. Difference Between show() and store()?

Q4. What is kobject?

Q5. Why is Sysfs Preferred Over Procfs?

27. Common Errors

Error: Sysfs File Not Created

Error: Permission Denied

Error: Kernel Crash

28. Sysfs Debugging Techniques

Check Sysfs Entries

Read Sysfs File

Kernel Logs

29. Advanced Sysfs Topics

30. Sysfs Attribute Groups

31. Best Practices

Keep Sysfs Files Simple

Validate User Input

Use Sysfs for Configuration

Avoid Large Data

32. Real Hardware Platforms

33. Real Linux Sysfs Examples