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

[Jesse Barnes]

On Thu, 21 Apr 2011 21:29:16 +0200
Arnd Bergmann <arnd@xxxxxxxx> wrote:

> I think the recent discussions on linaro-mm-sig and the BoF last week
> at ELC have been quite productive, and at least my understanding
> of the missing pieces has improved quite a bit. This is a list of
> things that I think need to be done in the kernel. Please complain
> if any of these still seem controversial:
> 
> 1. Fix the arm version of dma_alloc_coherent. It's in use today and
>    is broken on modern CPUs because it results in both cached and
>    uncached mappings. Rebecca suggested different approaches how to
>    get there.
> 
> 2. Implement dma_alloc_noncoherent on ARM. Marek pointed out
>    that this is needed, and it currently is not implemented, with
>    an outdated comment explaining why it used to not be possible
>    to do it.
> 
> 3. Convert ARM to use asm-generic/dma-mapping-common.h. We need
>    both IOMMU and direct mapped DMA on some machines.

I don't think the DMA mapping and allocation APIs are sufficient for
high performance graphics at least.  It's fairly common to allocate a
bunch of buffers necessary to render a scene, build up a command buffer
that references them, then hand the whole thing off to the kernel to
execute at once on the GPU.  That allows for a lot of extra efficiency,
since it allows you to batch the MMU binding until execution occurs (or
even put it off entirely until the page is referenced by the GPU in the
case of faulting support).  It's also necessary to avoid livelocks
between two clients trying to render; if mapping is incremental on both
sides, it's possible that neither will be able to make forward
progress due to IOMMU space exhaustion.

So that argues for separating allocation from mapping both on the user
side (which I think everyone agrees on) as well as on the kernel side,
both for CPU access (which some drivers won't need) and for GPU access.

-- 
Jesse Barnes, Intel Open Source Technology Center
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