Arnd Bergmann arnd@arndb.de 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:
- 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.
- 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that this is needed, and it currently is not implemented, with an outdated comment explaining why it used to not be possible to do it.
- 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.
I agree with Jesse that the separation of mapping from allocation is central to the current usage models. I realize most people didn't like VCMM, but it provided an abstraction for this - if software can handle the multiple mapper approach in a rational way across ARM than we can solve a lot of problems with all the map and unmap current solutions and we don't have to hack in coherency.
-- Jesse Barnes, Intel Open Source Technology Center
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