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

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Message-ID: <20160426172049.GB2558@phenom.ffwll.local>
Date:	Tue, 26 Apr 2016 19:20:49 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:	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 07:26:21PM +0300, Ville Syrjälä wrote:
> On Tue, Apr 26, 2016 at 04:36:36PM +0200, Daniel Vetter wrote:
> > On Tue, Apr 26, 2016 at 11:14:22AM -0300, Gustavo Padovan wrote:
> > > 2016-04-26 Ville Syrjälä <ville.syrjala@...ux.intel.com>:
> > > 
> > > > On Mon, Apr 25, 2016 at 07:33:25PM -0300, Gustavo Padovan wrote:
> > > > > From: Gustavo Padovan <gustavo.padovan@...labora.co.uk>
> > > > > 
> > > > > There is now a new property called FENCE_FD attached to every plane
> > > > > state that receives the sync_file fd from userspace via the atomic commit
> > > > > IOCTL.
> > > > 
> > > > I still don't like this property abuse. Also with atomic, all passed
> > > > fences must be waited upon before anything is done, so attaching them
> > > > to planes seems like it might just give people the wrong idea.
> > > 
> > > I'm actually fine with this as property, but another solutions is use
> > > an array of {plane, fence_fd} and extend drm_atomic_ioctl args just like
> > > we have done for out fences. However the FENCE_FD property is easier to
> > > handle in userspace than the array. Any other idea?
> > 
> > Imo FENCE_FD is perfectly fine. But what's the concern around giving
> > people the wrong idea with attaching fences to planes? For nonblocking
> > commits we need to store them somewhere for the worker, drm_plane_state
> > seems like an as good place as any other.
> 
> It gives the impression that each plane might flip as soon as its fence
> signals.

That wouldn't be atomic. Not sure how someone could come up with that
idea. I mean we could move FENCE_FD to the crtc (fence fds can be merged),
but that's just a needless difference to what hwc expects. I think
aligning with the only real-world users in this case here makes sense.

Plus docs in case someone has funny ideas.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch