[PATCH] drm/tidss: dispc: Rewrite naive plane positioning code

[Daniel Vetter]

On Tue, Feb 11, 2020 at 03:00:30PM +0200, Ville Syrjälä wrote:
> On Tue, Feb 11, 2020 at 11:11:34AM +0200, Tomi Valkeinen wrote:
> > Hi Ville,
> > 
> > On 10/02/2020 18:03, Ville Syrjälä wrote:
> > 
> > > The usual approach we follow in i915 for things that affect more
> > > than one plane is is to collect that state into the crtc state.
> > > That way we get to remember it for the planes that are not part
> > > of the current commit.
> > > 
> > > And when we have state that affects more than one crtc that again
> > > get collected up one level up in what we call global state
> > > (basically drm_private_obj with less heavy handed locking scheme).
> > 
> > I'm confused. Don't we always have the full state available? Why do you need to store state into 
> > custom crtc-state?
> > 
> > Here we are interested in the x, y and z positions of all the planes on a crtc. Creating a custom 
> > state object and duplicating that information there seems a bit silly, as surely that information is 
> > tracked by DRM?
> 
> You can have it if you add all the planes to the state, which can be
> a bit expensive. Another option would to peek into the planes' states
> that aren't in the commit, but that's quite gross due to bypassing
> the normal locking rules and instead relying on the crtc mutex to
> sufficiently protect the plane states as well. And I suspect trying
> to do said peeking during the commit phase when the locks have
> already been dropped will end badly.

Yup, don't peek outside of atomic_check.

Also the peeking only works for planes associated to the crtc. Either
because that's how the hw works (i915 has fixed plane routing).

Now if this is only about all the planes currently active on a crtc, then
you the helpers will already add all those plane states for you, and you
can just walk them in your commit function. Not exactly sure what you need
here.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch