Re: [PATCH] drm/atomic: pass old crtc state to atomic_begin/flush.

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Jun 15, 2015 at 03:53:43PM +0200, Daniel Vetter wrote:
> On Mon, Jun 15, 2015 at 11:18:46AM +0200, Maarten Lankhorst wrote:
> > Op 15-06-15 om 11:13 schreef Daniel Vetter:
> > > On Mon, Jun 15, 2015 at 09:30:19AM +0200, Maarten Lankhorst wrote:
> > >> Op 15-06-15 om 09:10 schreef Daniel Vetter:
> > >>> On Fri, Jun 12, 2015 at 11:18:22AM +0200, Maarten Lankhorst wrote:
> > >>>> In intel it's useful to keep track of some state changes with old
> > >>>> crtc state vs new state, for example to disable initial planes or
> > >>>> when a modeset's prevented during fastboot.
> > >>>>
> > >>>> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > >>> Hm, thus far the approach has been that the various ->check callbacks diff
> > >>> the state and set appropriate stuff like needs_modeset or planes_changed.
> > >>> And with intel_crtc->atomic we've kinda started to build up similar
> > >>> things for i915. What do you plan to use this for?
> > >>> -Daniel
> > >> On a modeset I want to disable all old planes by calling plane->disable_plane, which is old_crtc_state->plane_mask.
> > >> This is for initial hw readout, where a plane might be active without a fb set. I want to run it during vblank evasion if possible, which
> > >> means in atomic_begin or flush.
> > >>
> > >> commit_plane is not called if the old and new state both have a NULL fb, so the initial plane would stay active in this case.
> > > Hm, so this is for the i915 state readout code. Imo we shouldn't ever leak
> > > this out of the state readout code but instead sanitize the plane state to
> > > make sense. Roughly this would be:
> > > - read out crtc state
> > > - try to reconstruct initial fb for primary plane, if this succeeds then
> > >   fully link up the plane with the crtc in the plane_state.
> > Agreed. Right now get_initial_plane_config takes an initial_plane_state, could we make this atomic too?
> 
> The initial fb takeover code is a bit tricky since we need to temporarily
> store a few things while not everything is set up yet fully. We could try
> to move that information into the plane state, but it would duplicate
> existing information stored in state->fb->i915_gem_object. Not sure
> whether it's worth it to have something fully atomic for plane state
> readout.
> 
> The other option would be to allow enabled planes without a full-blown fb
> object, but experience says this leads to piles of drama in the watermark
> code.

We could create some kind of fake fb without an actual gem object. The
wm code just needs the metadata.

-- 
Ville Syrjälä
Intel OTC
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