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

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Date:	Sat, 27 Dec 2008 11:10:37 +0100
From:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
To:	Atsushi Nemoto <anemo@....ocn.ne.jp>
Cc:	linux-kernel@...r.kernel.org, hskinnemoen@...el.com,
	maciej.sosnowski@...el.com, dan.j.williams@...el.com,
	ralf@...ux-mips.org
Subject: Re: [PATCH] dmatest: flush and invalidate destination buffer before
 DMA

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.

Now, I suspect the dw_dmac driver maps the buffer when it's not
supposed to, masking this kind of error...should probably get that
fixed too.

Haavard
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