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SubjectRe: OMAPFB: CMA allocation failures
Hello, Ивайло


On Tue, Oct 29, 2013 at 02:47:35PM +0200, Ивайло Димитров wrote:
>
> 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

First of all, CMA is never reliable.

> 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

If you read below links you attached, you could know why it doesn't accept.

> 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.

True.

>
> 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.

Basically, fragile subsystem shouldn't use CMA, otherwise, your platform should support process
killing to unpin some pages, Yeah I know it's not 100% solution and very horrible but I know
some insane people have done it.

I just post an idea.
http://marc.info/?l=linux-mm&m=138311160522311&w=2
If anybody has a interest, maybe we will move that way.

Thanks.

>
> 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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--
Kind regards,
Minchan Kim
--
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