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.