linux-kernel - Re: [RFC] dma-fence: dma-buf synchronization (v2)

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Date:	Fri, 13 Jul 2012 23:44:22 +0200
From:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
To:	Rob Clark <rob.clark@...aro.org>
CC:	Tom Cooksey <tom.cooksey@....com>, dri-devel@...ts.freedesktop.org,
	linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
	patches@...aro.org, daniel.vetter@...ll.ch,
	linux-kernel@...r.kernel.org, sumit.semwal@...aro.org
Subject: Re: [RFC] dma-fence: dma-buf synchronization (v2)

Hey,

Op 13-07-12 20:52, Rob Clark schreef:
> On Fri, Jul 13, 2012 at 12:35 PM, Tom Cooksey <tom.cooksey@....com> wrote:
>> My other thought is around atomicity. Could this be extended to
>> (safely) allow for hardware devices which might want to access
>> multiple buffers simultaneously? I think it probably can with
>> some tweaks to the interface? An atomic function which does
>> something like "give me all the fences for all these buffers
>> and add this fence to each instead/as-well-as"?
> fwiw, what I'm leaning towards right now is combining dma-fence w/
> Maarten's idea of dma-buf-mgr (not sure if you saw his patches?).  And
> let dmabufmgr handle the multi-buffer reservation stuff.  And possibly
> the read vs write access, although this I'm not 100% sure on... the
> other option being the concept of read vs write (or
> exclusive/non-exclusive) fences.
Agreed, dmabufmgr is meant for reserving multiple buffers without deadlocks.
The underlying mechanism for synchronization can be dma-fences, it wouldn't
really change dmabufmgr much.
> In the current state, the fence is quite simple, and doesn't care
> *what* it is fencing, which seems advantageous when you get into
> trying to deal with combinations of devices sharing buffers, some of
> whom can do hw sync, and some who can't.  So having a bit of
> partitioning from the code dealing w/ sequencing who can access the
> buffers when and for what purpose seems like it might not be a bad
> idea.  Although I'm still working through the different alternatives.
>
Yeah, I managed to get nouveau hooked up with generating irqs on
completion today using an invalid command. It's also no longer a
performance regression, so software syncing is no longer a problem
for nouveau. i915 already generates irqs and r600 presumably too.

Monday I'll take a better look at your patch, end of day now. :)

~Maarten
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