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

[Jyri Sarha]

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().

>>
>> 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.

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