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

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Message-ID: <CAF6AEGs4QyGfZyOnewigYDMi6K=62Cmb8Ntd-zQRbasTXnjy-g@mail.gmail.com>
Date:	Fri, 2 Mar 2012 16:53:11 -0600
From:	Rob Clark <rob.clark@...aro.org>
To:	Chris Wilson <chris@...is-wilson.co.uk>
Cc:	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 Fri, Mar 2, 2012 at 4:38 PM, Chris Wilson <chris@...is-wilson.co.uk> wrote:
> 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.

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

BR,
-R

> -Chris
>
> --
> Chris Wilson, Intel Open Source Technology Centre
> --
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