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

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Message-ID: <20160426200505.GD2558@phenom.ffwll.local>
Date:	Tue, 26 Apr 2016 22:05:05 +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 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:
> > > On Tue, Apr 26, 2016 at 07:20:49PM +0200, Daniel Vetter wrote:
> > > > 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.
> > > 
> > > What else would it mean? It's attached to a specific plane, so why would
> > > it affect other planes?
> > > 
> > > > 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.
> > > 
> > > Well it doesn't belong on the crtc either. I would just stick in the
> > > ioctl as a separate thing, then it's clear it's related to the whole
> > > operation rather than any kms object.
> > 
> > We want it per-crtc I'd say, so that you could flip each crtc
> > individually.
> 
> Then you could just issue multiple ioctls. For eg. those nasty 4k MST
> display (or just otherwise neatly synced displayes) you want to wait for
> all the fences upfront and the flip everything at once, otherwise you'll
> get nasty tears at the seams.
> 
> > 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
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch