dma_alloc_coherent fails in framebuffer

[Bob Liu]

On Tue, Oct 16, 2012 at 2:28 AM, Tony Prisk <linux at prisktech.co.nz> wrote:
> On Mon, 2012-10-15 at 10:45 +0100, Mel Gorman wrote:
>> On Mon, Oct 15, 2012 at 09:34:55AM +1300, Tony Prisk wrote:
>> > On Sun, 2012-10-14 at 18:28 +1300, Tony Prisk wrote:
>> > > Up until 07 Oct, drivers/video/wm8505-fb.c was working fine, but on the
>> > > 11 Oct when I did another pull from linus all of a sudden
>> > > dma_alloc_coherent is failing to allocate the framebuffer any longer.
>> > >
>> > > I did a quick look back and found this:
>> > >
>> > > ARM: add coherent dma ops
>> > >
>> > > arch_is_coherent is problematic as it is a global symbol. This
>> > > doesn't work for multi-platform kernels or platforms which can support
>> > > per device coherent DMA.
>> > >
>> > > This adds arm_coherent_dma_ops to be used for devices which connected
>> > > coherently (i.e. to the ACP port on Cortex-A9 or A15). The arm_dma_ops
>> > > are modified at boot when arch_is_coherent is true.
>> > >
>> > > Signed-off-by: Rob Herring <rob.herring at calxeda.com>
>> > > Cc: Russell King <linux at arm.linux.org.uk>
>> > > Cc: Marek Szyprowski <m.szyprowski at samsung.com>
>> > > Signed-off-by: Marek Szyprowski <m.szyprowski at samsung.com>
>> > >
>> > >
>> > > This is the only patch lately that I could find (not that I would claim
>> > > to be any good at finding things) that is related to the problem. Could
>> > > it have caused the allocations to fail?
>> > >
>> > > Regards
>> > > Tony P
>> >
>> > Have done a bit more digging and found the cause - not Rob's patch so
>> > apologies.
>> >
>> > The cause of the regression is this patch:
>> >
>> > From f40d1e42bb988d2a26e8e111ea4c4c7bac819b7e Mon Sep 17 00:00:00 2001
>> > From: Mel Gorman <mgorman at suse.de>
>> > Date: Mon, 8 Oct 2012 16:32:36 -0700
>> > Subject: [PATCH 2/3] mm: compaction: acquire the zone->lock as late as
>> >  possible
>> >
>> > Up until then, the framebuffer allocation with dma_alloc_coherent(...)
>> > was fine. From this patch onwards, allocations fail.
>> >
>>
>> Was this found through bisection or some other means?
>>
>> There was a bug in that series that broke CMA but it was commit bb13ffeb
>> (mm: compaction: cache if a pageblock was scanned and no pages were
>> isolated) and it was fixed by 62726059 (mm: compaction: fix bit ranges
>> in {get,clear,set}_pageblock_skip()). So it should have been fixed by
>> 3.7-rc1 and probably was included by the time you pulled in October 11th
>> but bisection would be a pain. There were problems with that series during
>> development but tests were completing for other people.
>>
>> Just in case, is this still broken in 3.7-rc1?
>
> Still broken. Although the printk's might have cleared it up a bit.
>>
>> > I don't know how this patch would effect CMA allocations, but it seems
>> > to be causing the issue (or at least, it's caused an error in
>> > arch-vt8500 to become visible).
>> >
>> > Perhaps someone who understand -mm could explain the best way to
>> > troubleshoot the cause of this problem?
>> >
>>
>> If you are comfortable with ftrace, it can be used to narrow down where
>> the exact failure is occurring but if you're not comfortable with that
>> then the easiest is a bunch of printks starting in alloc_contig_range()
>> to see at what point and why it returns failure.
>>
>> It's not obvious at the moment why that patch would cause an allocation
>> problem. It's the type of patch that if it was wrong it would fail every
>> time for everyone, not just for a single driver.
>>
>
> I added some printk's to see what was happening.
>
> from arch/arm/mm/dma-mapping.c: arm_dma_alloc(..) it calls out to:
> dma_alloc_from_coherent().
>
> This returns 0, because:
> mem = dev->dma_mem
> if (!mem) return 0;
>
> and then arm_dma_alloc() falls back on __dma_alloc(..)
>
>
> I suspect the reason this fault is a bit 'weird' is because its
> effectively not using alloc_from_coherent at all, but falling back on
> __dma_alloc all the time, and sometimes it fails.
>

I think you need to declare that memory using
dma_declare_coherent_memory() before
alloc_from_coherent.

> Why it caused a problem on that particular commit I don't know - but it
> was reproducible by adding/removing it.
>
>
> Regards
> Tony P
>
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-- 
Regards,
--Bob