Re: [PATCH v5 4/4] drm/fence: add out-fences support

From: Ville Syrjälä
Date: Thu Oct 20 2016 - 12:35:52 EST

On Thu, Oct 20, 2016 at 01:55:38PM -0200, Gustavo Padovan wrote:
> 2016-10-20 Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>:
> > On Thu, Oct 20, 2016 at 12:50:05PM -0200, Gustavo Padovan wrote:
> > > From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx>
> > >
> > > Support DRM out-fences by creating a sync_file with a fence for each CRTC
> > > that sets the OUT_FENCE_PTR property.
> >
> > I still maintain the out fence should also be per fb (well, per plane
> > since we can't add it to fbs). Otherwise it's not going to be at all
> > useful if you do fps>vrefresh and don't include the same set of planes
> > in every atomic ioctl, eg. if you only ever include ones that are
> > somehow dirty.
> How would the kernel signal these dirty planes then? Right now we signal
> at the vblank.

So if I think about it in terms of the previous fbs something like this
comes to mind:

starting point:
plane a, fb 0
plane b, fb 1

plane a, fb 2, fence A
plane b, fb 3, fence B

plane a, fb 4, fence C
-> fb 2 can be reused, so fence C should signal immediately?

-> fb 0,1 can be reused, so fence A,B signal?

It feels a bit wonky since the fence is effectively associated with the
previous fb after the previous fb was already submitted to the kernel.
One might assume fence A to be the one signalled early, but that would
give the impression that fb 0 would be free for reuse when it's not.

Ville Syrjälä
Intel OTC