linux-kernel - Re: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

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Date:	Fri, 29 Apr 2011 10:25:54 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1


> However, dma_alloc_coherent() memory can't be used with the dma_sync_*
> API as its return address (unlike other architectures) is not in the
> kernel direct mapped memory range.

Well, on non-coherent architectures, dma_sync_* are cache flushes, I
don't see the point of doing those on a non-cachable mapping anyways.

> The only thing valid for dma_sync_* are buffers which have been passed
> to the dma_map_* APIs.

Right, at least that's our expectation on powerpc as well.

> Instead, I think what you're referring to is dma_cache_sync(), which is
> the API to be used with dma_alloc_noncoherent(), which we don't
> implement.

Too may confusing APIs....

Ben.

> As we have problems with some SMP implementations, and the noncoherent
> API doesn't have the idea of buffer ownership, it's rather hard to deal
> with the DMA cache implications with the existing API, especially with
> the issues of speculative prefetching.  The current usage (looking at
> drivers/scsi/53c700.c) doesn't cater for speculative prefetch as the
> dma_cache_sync(,,,DMA_FROM_DEVICE) is done well in advance of the DMA
> actually happening.
> 
> So all in all, I think the noncoherent API is broken as currently
> designed - and until we have devices on ARM which use it, I don't see
> much point into trying to fix the current thing especially as we'd be
> unable to test.
> --
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