[ARM] head.S change broke platform device registration?

[Marko Katić]

On Fri, Nov 30, 2012 at 1:17 PM, Dave Martin <dave.martin at linaro.org> wrote:
> On Fri, Nov 30, 2012 at 02:38:14AM +0100, Marko Katić wrote:
>> I recently tried to boot v3.7-rc7 on my akita (Sharp Zaurus C-1000,
>> PXA270 machine).
>> Backlight never turned on but the kernel booted just fine.
>> After some investigation i determined that the gpio expander (max7310,
>> i2c chip) is not even
>> registered and thus not present in /sys/bus/i2c/devices.
>> Backlight gpio controls are hooked to this chip thus, no backlight.
>>
>> I thought that too many things could have caused this so i went
>> straight to bisecting.
>> Bisection pointed me to this commit:
>>
>>  commit 424e5994e63326a42012f003f1174f3c363c7b62
>> Author: Dave Martin <dave.martin at linaro.org>
>> Date:   Fri Feb 10 18:07:07 2012 -0800
>>
>>     ARM: zImage/virt: hyp mode entry support for the zImage loader
>>
>>     The zImage loader needs to turn on the MMU in order to take
>>     advantage of caching while decompressing the zImage.  Running this
>>     in hyp mode would require the LPAE pagetable format to be
>>     supported; to avoid this complexity, this patch switches out of hyp
>>     mode, and returns back to hyp mode just before booting the kernel.
>>
>>     This implementation assumes that the Hyp mode view of memory and the
>>     PL1 view of memory are coherent, providing that the MMU and caches
>>     are off in both, as required by the boot protocol.  The zImage
>>     decompression code must drain the write buffer on completion anyway, and
>>     entry into Hyp mode should flush any prefetch buffer, avoiding hazards
>>     associated with local write buffers and the pipeline.
>>
>>     Signed-off-by: Dave Martin <dave.martin at linaro.org>
>>     Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
>>
>> This commit was merged in v3.7-rc1. And indeed, if i
>> revert this single commit, my gpio expander reappears and
>> so does my backlight.
>> I made no other changes to the vanilla 3.7-rc7
>> except this revert.
>>
>> This seems very strange to me because i, with my limited knowledge,
>> cannot see any correlation between this commit and the bug i am experiencing.
>>
>> Does anyone know what exactly is happening here?
>
> This is indeed strange, since there is not much difference to the kernel
> if CONFIG_ARM_VIRT_EXT is not set.
>
> Unless a bug is causing the kernel to trample on some random memory,
> there is no reason for this to cause I2C devices to disappear.
>
> It could be something unrelated, like additional and removal of code
> changing the position or alignment of other parts of the kernel image.
>
> Have you could try undoing parts of the patch to narrow the bug down
> further?
>
> Cheers
> ---Dave

No, not yet. As i said my knowledge of what goes on in head.S is limited
so i'd rather not mess with it.