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SubjectRe: OMAPFB: CMA allocation failures

Hi,


>-------- Оригинално писмо --------
>От: Minchan Kim
>Относно: Re: OMAPFB: CMA allocation failures
>До: Ивайло Димитров
>Изпратено на: Понеделник, 2013, Октомври 28 09:37:48 EET
>
>
>Hello,
>
>On Tue, Oct 15, 2013 at 09:49:51AM +0300, Ивайло Димитров wrote:
>> Hi
>>
>> >-------- Оригинално писмо --------
>> >От: Tomi Valkeinen
>> >Относно: Re: OMAPFB: CMA allocation failures
>> >До: Ивайло Димитров
>>
>> >Изпратено на: Понеделник, 2013, Октомври 14 09:04:35 EEST
>> >
>> >
>> >Hi,
>> >
>> >On 12/10/13 17:43, Ивайло Димитров wrote:
>> >> Hi Tomi,
>> >>
>> >> patch http://lists.infradead.org/pipermail/linux-arm-kernel/2012-November/131269.html modifies
>> >> omapfb driver to use DMA API to allocate framebuffer memory instead of preallocating VRAM.
>> >>
>> >> With this patch I see a lot of:
>> >>
>> >> Jan 1 06:33:27 Nokia-N900 kernel: [ 2054.879577] cma: dma_alloc_from_contiguous(cma c05f5844, count 192, align 8)
>> >> Jan 1 06:33:27 Nokia-N900 kernel: [ 2054.914215] cma: dma_alloc_from_contiguous(): memory range at c07df000 is busy, retrying
>> >> Jan 1 06:33:27 Nokia-N900 kernel: [ 2054.933502] cma: dma_alloc_from_contiguous(): memory range at c07e1000 is busy, retrying
>> >> Jan 1 06:33:27 Nokia-N900 kernel: [ 2054.940032] cma: dma_alloc_from_contiguous(): memory range at c07e3000 is busy, retrying
>> >> Jan 1 06:33:27 Nokia-N900 kernel: [ 2054.966644] cma: dma_alloc_from_contiguous(): memory range at c07e5000 is busy, retrying
>> >> Jan 1 06:33:27 Nokia-N900 kernel: [ 2054.976867] cma: dma_alloc_from_contiguous(): memory range at c07e7000 is busy, retrying
>> >> Jan 1 06:33:27 Nokia-N900 kernel: [ 2055.038055] cma: dma_alloc_from_contiguous(): memory range at c07e9000 is busy, retrying
>> >> Jan 1 06:33:27 Nokia-N900 kernel: [ 2055.038116] cma: dma_alloc_from_contiguous(): returned (null)
>> >> Jan 1 06:33:27 Nokia-N900 kernel: [ 2055.038146] omapfb omapfb: failed to allocate framebuffer
>> >>
>> >> errors while trying to play a video on N900 with Maemo 5 (Fremantle) on top of linux-3.12rc1.
>> >> It is deffinitely the CMA that fails to allocate the memory most of the times, but I wonder
>> >> how reliable CMA is to be used in omapfb. I even reserved 64MB for CMA, but that made no
>> >> difference. If CMA is disabled, the memory allocation still fails as obviously it is highly
>> >> unlikely there will be such a big chunk of continuous free memory on RAM limited device like
>> >> N900.
>> >>
>> >> One obvious solution is to just revert the removal of VRAM memory allocator, but that would
>> >> mean I'll have to maintain a separate tree with all the implications that brings.
>> >>
>> >> What would you advise on how to deal with the issue?
>> >
>> >I've not seen such errors, and I'm no expert on CMA. But I guess the
>> >contiguous memory area can get fragmented enough no matter how hard one
>> >tries to avoid it. The old VRAM system had the same issue, although it
>> >was quite difficult to hit it.
>>
>> I am using my n900 as a daily/only device since the beginning of 2010, never seen such an
>> issue with video playback. And as a maintainer of one of the community supported kernels for
>> n900 (kernel-power) I've never had such an issue reported. On stock kernel and derivatives of
>> course. It seems VRAM allocator is virtually impossible to fail, while with CMA OMAPFB fails on
>> the first video after boot-up.
>>
>> When saying you've not seen such an issue - did you actually test video playback, on what
>> device and using which distro? Did you use DSP accelerated decoding?
>>
>> >64MB does sound quite a lot, though. I wonder what other drivers are
>> >using CMA, and how do they manage to allocate so much memory and
>> >fragment it so badly... With double buffering, N900 should only need
>> >something like 3MB for the frame buffer.
>>
>> Sure, 64 MB is a lot, but I just wanted to see if that would make any difference. And for 720p
>> 3MB is not enough, something like 8MB is needed.
>>
>> >With a quick glance I didn't find any debugfs or such files to show
>> >information about the CMA area. It'd be helpful to find out what's going
>> >on there. Or maybe normal allocations are fragmenting the CMA area, but
>> >for some reason they cannot be moved? Just guessing.
>>
>> I was able to track down the failures to:
>> http://lxr.free-electrons.com/source/mm/migrate.c#L320
>
>That path is for anonymous page migration so the culprit I can think of
>is that you did get_user_pages on those anonymous pages for pin them.
>Right?
>

I grepped through the code and there are lots of places where get_user_pages is called, though
I suspect either SGX or DSP (or both) drivers to be the ones to blame. Both of them are active
and needed for HW accelerated video decoding.

>If so, it's no surpse that fails the migration and CMA doesn't work.
>
>--
>Kind regards,
>Minchan Kim
>

Well, if CMA is to be reliable, I would expect some logic to take care about get_user_pages
causing MIGRATE_CMA pages to be effectively made non-migratable, either by migrating them out of
CMA area before they got pinned or by providing a mechanism to migrate them when needed. I am far
from knowing the nuts and bolts of MM and CMA, but so far I failed to see any such logic. Without
it, CMA could be fine for allocating small buffers, but when we talk about framebuffer memory
needed for 720p playback(for example) on a RAM limited embedded device, it is too fragile, IMO.

BTW quick googling shows I am not the first one to encounter similar problems [0], [1], I don't
see solution for.

However, back to omapfb - my understanding is that the way it uses CMA (in its current form) is
prone to allocation failures way beyond acceptable.

Tomi, what do you think about adding module parameters to allow pre-allocating framebuffer memory
from CMA during boot? Or re-implement VRAM allocator to use CMA? As a good side-effect
OMAPFB_GET_VRAM_INFO will no longer return fake values.

Regards,
Ivo

[0] http://lwn.net/Articles/541423/
[1] https://lkml.org/lkml/2012/11/29/69
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