[Linaro-mm-sig] [PATCH 2/3] dma-buf: add support for kernel cpu access
rob.clark at linaro.org
Fri Mar 2 14:53:11 PST 2012
On Fri, Mar 2, 2012 at 4:38 PM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> On Thu, 1 Mar 2012 16:36:00 +0100, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>> Big differences to other contenders in the field (like ion) is
>> that this also supports highmem, so we have to split up the cpu
>> access from the kernel side into a prepare and a kmap step.
>> Prepare is allowed to fail and should do everything required so that
>> the kmap calls can succeed (like swapin/backing storage allocation,
>> flushing, ...).
>> More in-depth explanations will follow in the follow-up documentation
>> Changes in v2:
>> - Clear up begin_cpu_access confusion noticed by Sumit Semwal.
>> - Don't automatically fallback from the _atomic variants to the
>> non-atomic variants. The _atomic callbacks are not allowed to
>> sleep, so we want exporters to make this decision explicit. The
>> function signatures are explicit, so simpler exporters can still
>> use the same function for both.
>> - Make the unmap functions optional. Simpler exporters with permanent
>> mappings don't need to do anything at unmap time.
> Are we going to have to have a dma_buf->ops->begin_async_access(&me, dir)
> variant for coherency control of rendering with an imported dma_buf?
> There is also no concurrency control here between multiple importers
> doing simultaneous begin_cpu_access(). I presume that is going to be a
> common function across all exporters so the midlayer might offer a
> semaphore as a library function and then the
> dma_buf->ops->begin_cpu_access() becomes mandatory as at a minimum it
> has to point to the default implementation.
Initially the expectation was that userspace would not pass a buffer
to multiple subsystems for writing (or that if it did, it would get
the undefined results that one could expect).. so dealing w/
synchronization was punted.
I expect, though, that one of the next steps is some sort of
sync-object mechanism to supplement dmabuf
> Chris Wilson, Intel Open Source Technology Centre
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