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

[Ville Syrjälä]

On Sun, Feb 09, 2020 at 02:50:09PM +0200, Jyri Sarha wrote:
> On 07/02/2020 20:45, Ville Syrjälä wrote:
> > On Fri, Feb 07, 2020 at 08:26:17PM +0200, Jyri Sarha wrote:
> >> On 07/02/2020 20:18, Jyri Sarha wrote:
> >>> The old implementation of placing planes on the CRTC while configuring
> >>> the planes was naive and relied on the order in which the planes were
> >>> configured, enabled, and disabled. The situation where a plane's zpos
> >>> was changed on the fly was completely broken. The usual symptoms of
> >>> this problem was scrambled display and a flood of sync lost errors,
> >>> when a plane was active in two layers at the same time, or a missing
> >>> plane, in case when a layer was accidentally disabled.
> >>>
> >>> The rewrite takes a more straight forward approach when when HW is
> >>> concerned. The plane positioning registers are in the CRTC (or
> >>> actually OVR) register space and it is more natural to configure them
> >>> in a one go when configuring the CRTC. This is easy since we have
> >>> access to the whole atomic state when updating the CRTC configuration.
> >>>
> >>
> >> While implementing this fix it caught me by surprise that
> >> crtc->state->state (pointer up to full atomic state) is NULL when
> >> crtc_enable() or -flush() is called. So I take the plane-state directly
> >> from the plane->state and just assume that it is pointing to the same
> >> atomic state with the crtc state I am having. I that alraight?
> > 
> > IMO you should never use plane->state etc. Better pass down the
> > full atomic state everywhere. Otherwise you can never even consider
> > increasing the commit queue depth since you'd end up accessing the
> > wrong state.
> >
> 
> Ok. I did explore this a bit and it starts to look like that I have to
> store the planes' zpos values in the driver after all. Only the changes
> are available in the drm_atomic_state being commited so I have to
> maintain the full state myself. That is if I should not use plane->state
> in crtc_enable() or -flush().

You have the full old and new states around for each
crtc/plane/connector in the state. So not sure what you mean
by having only the changes available?

> 
> >>
> >> Why is the crtc->state->state NULL? Is it a bug or is there some reason
> >> to it?
> > 
> > Currently swap_state() moves that state pointer from the new obj state
> > to the old obj state, and clears the one in the new obj state. Not entirely
> > sure why, but maybe just so there isn't a stale ->state pointer hanging 
> > around in the obj->state after the swap?
> > 
> > I think a better way could be to not clobber the old obj state at
> > all, leave the new_obj_state->state alone, and just clear the ->state
> > pointer .duplicate_state(). But that would require reviewing a bunch
> > of code to find all the places where old_obj_state->state gets used
> > during the commit.
> > 
> 
> I think those places are many, since at least I did not figure out any
> other way to access the full commit behind the atomic helpers.

I haven't examined how many drivers depend on the current behaviour.
But fixing up the core/helpers should be pretty trivial.

-- 
Ville Syrjälä
Intel