Subject: imx6q restart is broken

From: [email-masked] (Hui Wang)
Date: Thu, 9 Aug 2012 11:18:47 +0800

Dirk Behme wrote:
> On 08.08.2012 12:18, Shawn Guo wrote:
>> Thanks Dirk for reporting that imx6q restart (reboot command) is broken.
>>
>> I tracked down the issue a little bit and found imx6q_restart hangs
>> on the of_iomap/ioremap call. The following change, moving the call
>> somewhere else than imx6q_restart, will just fix the problem.
>>
>> Does that mean ioremap call is not allowed in platform restart hook?
>> I'm not sure about that, because I found it works just fine if I build
>> imx_v6_v7_defconfig with V6 (imx3) platforms excluded (IOW, build a V7
>> only kernel - imx5 and imx6), which is the case how I tested imx6q
>> restart feature when I was adding it.
>>
>> To summarize, the imx6q_restart hangs at ioremap call on a V6 + V7
>> kernel, while it works fine on a V7 only image. I need some help to
>> understand that.
>
> Some additional information from my debugging:
>
> a) Having a JTAG debugger attached to the i.MX6 SabreLite board I use 
> (kernel built with imx_v6_v7_defconfig) the reboot does work. No hang. 
> This does mean I can't debug the reboot hang with a JTAG debugger. 
> Therefore I added some printk debugging:
>
> b) Adding some printk statements [1] in the of_iomap/ioremap call, it 
> looks to me that the system hangs in
>
> of_iomap() -> ... -> set_pte_at() -> set_pte_ext() / 
> cpu_v7_set_pte_ext() <= hang
>
I noticed this problem several weeks ago, and did some debug, what i 
found is following:

- at the last stage of reset, all non-boot cpus will call 
ipi_cpu_stop()->cpu_relax(), the cpu_relax() is defined to smp_mb() for 
V6, and smp_mb() is defined to dmb ("mcr p15, 0, %0, c7, c10, 5")

- the boot cpu will call cpu_v7_set_pte_ext() in the proc-v7-2level.S, 
at the end of this function, boot cpu will call flush_pte (mcr p15, 0, 
r0, c7, c10, 1), after executing this function, the system will hang. 
That is to say, all non-boot cpus repeatedly call ("mcr p15, 0, %0, c7, 
c10, 5"), if boot cpu call flush_pte (mcr p15, 0, r0, c7, c10, 1), the 
system will hang, this occurs on the cortexa9 r2p10 cpus (freescale 
i.MX6Q), i don't know the root cause for this problem, but if we split 
imx_v6_v7_defconfig into v7_only and use v7_only to build kernel image 
for imx6q, this problem will disappear, since the cpu_relax() will be 
defined to barrier().

- i think we should split imx_v6_v7_defconfig into v6_only and v7_only, 
since when we use imx_v6_v7_defconfig, we will introduce 
"-D__LINUX_ARM_ARCH__=6" unconditionally, this macro is OK for V6 
platforms (imx3), but it is not good for V7 platforms (imx5, imx6). lets 
grep "__LINUX_ARM_ARCH__" occurrence under the arch/arm, all (#if 
__LINUX_ARM_ARCH__ == 6) and (#if __LINUX_ARM_ARCH__ >= 7) occurrence 
will affect V7 platforms.

Regards,
Hui.