Re: [RFC] ARM DMA mapping TODO, v1

From: Joerg Roedel
Date: Thu Apr 28 2011 - 08:59:35 EST


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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