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

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Date:   Thu, 20 Oct 2016 19:34:44 +0300
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Gustavo Padovan <gustavo.padovan@...labora.com>
Cc:     Gustavo Padovan <gustavo@...ovan.org>,
        dri-devel@...ts.freedesktop.org, marcheu@...gle.com,
        Daniel Stone <daniels@...labora.com>, seanpaul@...gle.com,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        linux-kernel@...r.kernel.org, laurent.pinchart@...asonboard.com,
        Gustavo Padovan <gustavo.padovan@...labora.co.uk>,
        John Harrison <John.C.Harrison@...el.com>, m.chehab@...sung.com
Subject: Re: [PATCH v5 4/4] drm/fence: add out-fences support

On Thu, Oct 20, 2016 at 01:55:38PM -0200, Gustavo Padovan wrote:
> 2016-10-20 Ville Syrjälä <ville.syrjala@...ux.intel.com>:
> 
> > On Thu, Oct 20, 2016 at 12:50:05PM -0200, Gustavo Padovan wrote:
> > > From: Gustavo Padovan <gustavo.padovan@...labora.co.uk>
> > > 
> > > 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

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

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

 vblank:
 -> 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