linux-kernel - Re: [PATCH] dmatest: flush and invalidate destination buffer before DMA

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Date:	Mon, 5 Jan 2009 11:31:57 -0700
From:	"Dan Williams" <dan.j.williams@...el.com>
To:	"Atsushi Nemoto" <anemo@....ocn.ne.jp>
Cc:	haavard.skinnemoen@...el.com, linux-kernel@...r.kernel.org,
	hskinnemoen@...el.com, maciej.sosnowski@...el.com,
	ralf@...ux-mips.org
Subject: Re: [PATCH] dmatest: flush and invalidate destination buffer before DMA

On Sun, Dec 28, 2008 at 10:53 AM, Atsushi Nemoto <anemo@....ocn.ne.jp> wrote:
> On Sat, 27 Dec 2008 11:10:37 +0100, Haavard Skinnemoen <haavard.skinnemoen@...el.com> wrote:
>> Atsushi Nemoto wrote:
>> > @@ -226,6 +227,12 @@ static int dmatest_func(void *data)
>> >
>> >             dmatest_init_srcbuf(thread->srcbuf, src_off, len);
>> >             dmatest_init_dstbuf(thread->dstbuf, dst_off, len);
>> > +           /* flush and invalidate caches for whole dstbuf */
>> > +           dma_dest = dma_map_single(chan->device->dev,
>> > +                           thread->dstbuf,
>> > +                           test_buf_size, DMA_BIDIRECTIONAL);
>> > +           dma_unmap_single(chan->device->dev, dma_dest,
>> > +                           test_buf_size, DMA_BIDIRECTIONAL);
>>
>> You're supposed to unmap after the DMA operation is done, not before
>> it's submitted.
>>
>> In this case, the DMA engine framework will do the unmapping for you
>> (probably using the wrong primitive, but they're really all the same in
>> practice, right?) so you can just drop the unmap call.
>
> Well, let me explain more.
>
> On nono-coherent MIPS platforms, dma_map_single() for DMA_TO_DEVICE
> writeback the cache, dma_map_single() for DMA_FROM_DEVICE invalidated
> (without writeback) the cache.  dma_unmap_simgle() is a nop.
>
> If dst_off was not cacheline aligned, dma_map_single(...,
> DMA_FROM_DEVICE) in dma_async_memcpy_buf_to_buf() invalidate whole
> cachelines including dst_off.  So, for example, the initialized data
> at dst_off - 1 will be just discarded.  This result mismatch error of
> course.  Same error can be happen at end of the real DMA area.
>
> I added dma_map_single/dma_unmap_single to just flush all dstbuf to
> main memory.
>

I am tempted to say this is a bug in the MIPS dma_map_single
implementation.  It is at least a difference in behavior from ARM
where partial cachelines are cleaned+invalidated in the
DMA_FROM_DEVICE case [1].

--
Dan

[1] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=arch/arm/mm/cache-xsc3l2.c#l82
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