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

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Date:	Wed, 27 Apr 2016 08:39:04 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Greg Hackmann <ghackmann@...gle.com>
Cc:	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 Tue, Apr 26, 2016 at 01:48:02PM -0700, Greg Hackmann wrote:
> 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.

I'm ok with an array too if that's what you folks prefer (it's meant to be
used by you after all). I just don't want just 1 fence for the entire op,
forcing userspace to first merge them all together. That seems silly.

One side-effect of that is that we'd also have to rework all the internal
bits and move fences around in atomic. Which means change a pile of
drivers. Not sure that's worth it, but I'd be ok either way really.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch