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

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Date:	Thu, 28 Apr 2011 14:59:24 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC] ARM DMA mapping TODO, v1

On Thu, Apr 28, 2011 at 01:42:42PM +0100, Russell King - ARM Linux wrote:

> Sigh.  You're not seeing the point.
> 
> There is _no_ point doing the cache management _if_ we're using something
> like dmabounce or swiotlb, as we'll be using memcpy() at some point with
> the buffer.  Moreover, dmabounce or swiotlb may have to do its own cache
> management _after_ that memcpy() to ensure that the page cache requirements
> are met.

Well, I was talking about a generic dma_ops implementation based on the
iommu-api so that every system that has iommu hardware can use a common
code-set.
If you have to dma-bounce you don't have iommu hardware and thus you
don't use this common implementation of dma_ops (but probably the
swiotlb implementation which is already mostly generic).

> Doing DMA cache management for dmabounce or swiotlb will result in
> unnecessary overhead - and as we can see from the MMC discussions,
> it has a _significant_ performance impact.

Yeah, I see that from your explanation below. But as I said, swiotlb
backend is not a target use-case for a common iommu-api-bound dma_ops
implementation.

> Think about it.  If you're using dmabounce, but still do the cache
> management:
> 
> 1. you flush the data out of the CPU cache back to memory.
> 2. you allocate new memory using dma_alloc_coherent() for the DMA buffer
>    which is accessible to the device.
> 3. you memcpy() the data out of the buffer you just flushed into the
>    DMA buffer - this re-fills the cache, evicting entries which may
>    otherwise be hot due to the cache fill policy.
> 
> Step 1 is entirely unnecessary and is just a complete and utter waste of
> CPU resources.

Thanks for the explanation.

Regards,

	Joerg

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