linux-kernel - Re: [RFC v2 5/8] drm/fence: add in-fences support

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Date:	Tue, 26 Apr 2016 13:48:02 -0700
From:	Greg Hackmann <ghackmann@...gle.com>
To:	Ville Syrjälä <ville.syrjala@...ux.intel.com>,
	Gustavo Padovan <gustavo.padovan@...labora.co.uk>,
	Gustavo Padovan <gustavo@...ovan.org>,
	Daniel Stone <daniels@...labora.com>,
	Riley Andrews <riandrews@...roid.com>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	Arve Hjønnevåg <arve@...roid.com>,
	John Harrison <John.C.Harrison@...el.com>
Subject: Re: [RFC v2 5/8] drm/fence: add in-fences support

On 04/26/2016 01:05 PM, Daniel Vetter wrote:
> On Tue, Apr 26, 2016 at 09:55:06PM +0300, Ville Syrjälä wrote:
>> On Tue, Apr 26, 2016 at 08:23:46PM +0200, Daniel Vetter wrote:
>>> On Tue, Apr 26, 2016 at 08:40:45PM +0300, Ville Syrjälä wrote:
>>> But really the reason for per-plane is hw composer from
>>> Android. I don't see any point in designing an api that's needlessly
>>> different from what the main user expects (even if it may be silly).
>>
>> What are they doing that can't stuff the fences into an array
>> instead of props?
>
> The hw composer interface is one in-fence per plane. That's really the
> major reason why the kernel interface is built to match. And I really
> don't think we should diverge just because we have a slight different
> color preference ;-)
>
> As long as you end up with a pile of fences somehow it'll work.
> -Daniel
>

The relationship between layers and fences is only fuzzy and indirect 
though.  The relationship is really between the buffer you're displaying 
on that layer, and the fence representing the work done to render into 
that buffer.  SurfaceFlinger just happens to bundle them together inside 
the same struct hwc_layer_1 as an API convenience.

Which is kind of splitting hairs as long as you have a 1-to-1 
relationship between layers and DRM planes.  But that's not always the case.

A (per-CRTC?) array of fences would be more flexible.  And even in the 
cases where you could make a 1-to-1 mapping between planes and fences, 
it's not that much more work for userspace to assemble those fences into 
an array anyway.