ENGR00285897 mx6 busfreq: fix system crash while enter low bus with 'maxcpus=1'

Use for_each_online_cpu instead of for_each_present_cpu to take this case,
otherwise system will crash as below when go into low bus with 'maxcpus=1'
setting in command line.

Unable to handle kernel NULL pointer dereference at virtual address 00000000
pgd = 80004000
[00000000] *pgd=00000000
Internal error: Oops: 817 [#1] SMP ARM
Modules linked in:
CPU: 0 PID: 68 Comm: kworker/0:2 Not tainted 3.10.17-16647-g0868f35 #27
Workqueue: events reduce_bus_freq_handler
task: ac156d80 ti: ac2a2000 task.ti: ac2a2000
PC is at update_ddr_freq+0x98/0x2d0
LR is at 0x0
pc : [<80021928>]    lr : [<00000000>]    psr: 400f0013
sp : ac2a3e98  ip : 00000000  fp : 814db740
r10: 016e3600  r9 : 00000000  r8 : 00000000
r7 : 814de900  r6 : 80c60cc0  r5 : 0000000f  r4 : 80c60dc0
r3 : 00000000  r2 : 80c60dc0  r1 : 80c60d34  r0 : 00000000
Flags: nZcv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment kernel
Control: 10c53c7d  Table: 3c49404a  DAC: 00000015
Process kworker/0:2 (pid: 68, stack limit = 0xac2a2238)
Stack: (0xac2a3e98 to 0xac2a4000)
3e80:                                                       00000000 00000000
3ea0: 00000000 00000000 00000001 80c60cc0 80c603a4 80c60cc0 814de900 00000000
3ec0: 00000000 ac2a2038 814db740 80020154 00000064 ac02f6c0 00000004 80c2103c
3ee0: 80c60d38 814db740 814de900 80020628 ac135780 8003d7ac 00000001 ac083eb8
3f00: 00000000 00000000 00000003 ac135780 814db754 ac135798 ac2a2000 ac2a2030
3f20: 00000001 ac2a2000 814db740 8003e4b8 8003e380 00000000 00000000 80c5fcc1
3f40: ac2a3f64 ac083ea0 00000000 ac135780 8003e380 00000000 00000000 00000000
3f60: 00000000 800437e0 fd7efff9 00000000 7faf7bfd ac135780 00000000 00000000
3f80: ac2a3f80 ac2a3f80 00000000 00000000 ac2a3f90 ac2a3f90 ac2a3fac ac083ea0
3fa0: 8004372c 00000000 00000000 8000e018 00000000 00000000 00000000 00000000
3fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
3fe0: 00000000 00000000 00000000 00000000 00000013 00000000 fffc7fae d4cadbdb
[<80021928>] (update_ddr_freq+0x98/0x2d0) from [<80020154>] (reduce_bus_freq+
0x58/0x518)
[<80020154>] (reduce_bus_freq+0x58/0x518) from [<80020628>] (reduce_bus_freq_
handler+0x14/0x24)
[<80020628>] (reduce_bus_freq_handler+0x14/0x24) from [<8003d7ac>] (process_one
_work+0x10c/0x374)
[<8003d7ac>] (process_one_work+0x10c/0x374) from [<8003e4b8>] (worker_thread+
0x138/0x3fc)
[<8003e4b8>] (worker_thread+0x138/0x3fc) from [<800437e0>] (kthread+0xb4/0xb8)
[<800437e0>] (kthread+0xb4/0xb8) from [<8000e018>] (ret_from_fork+0x14/0x3c)
Code: e5940014 e3002dc0 e594e018 e34820c6 (e5835000)
---[ end trace 206df98575045d04 ]---
Unable to handle kernel paging request at virtual address ffffffec
pgd = 80004000
[ffffffec] *pgd=3ff7e821, *pte=00000000, *ppte=00000000

Signed-off-by: Robin Gong <[email protected]>

Author: Robin Gong

arch/arm/mach-imx/busfreq_ddr3.c

@@ -343,7 +343,7 @@ int init_mmdc_ddr3_settings(struct platform_device *busfreq_pdev)
 	irqs_used = devm_kzalloc(dev, sizeof(u32) * num_present_cpus(),
 					GFP_KERNEL);
 
-	for_each_present_cpu(cpu) {
+	for_each_online_cpu(cpu) {
 		int irq;
 
 		/*