linux-kernel - Re: [Linaro-mm-sig] [PATCH 2/3] dma-buf: add support for kernel cpu access

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Date:	Fri, 02 Mar 2012 22:38:58 +0000
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Daniel Vetter <daniel.vetter@...ll.ch>,
	linaro-mm-sig@...ts.linaro.org,
	LKML <linux-kernel@...r.kernel.org>,
	DRI Development <dri-devel@...ts.freedesktop.org>,
	linux-media@...r.kernel.org
Subject: Re: [Linaro-mm-sig] [PATCH 2/3] dma-buf: add support for kernel cpu access

On Thu,  1 Mar 2012 16:36:00 +0100, Daniel Vetter <daniel.vetter@...ll.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
> patch.
> 
> 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.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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