Re: [PATCH 10/24] drm/vc4: kms: Sort the CRTCs by output before assigning them

Javier Martinez Canillas
2022-11-25T11:01:50Z

> On the vc4 devices (and later), the blending is done by a single device
> called the HVS. The HVS has three FIFO that can operate in parallel, and
> route their output to 6 CRTCs and 7 encoders on the BCM2711.
>
> Each of these CRTCs and encoders have some contraints on which FIFO they

constraints.

> can feed from, so we need some code to take all those constraints into
> account and assign FIFOs to CRTCs.
>
> The problem can be simplified by assigning those FIFOs to CRTCs by
> ascending output index number. We had a comment mentioning it already,
> but we were never actually enforcing it.
>
> It was working still in most situations because the probe order is
> roughly equivalent, except for the (optional, and fairly rarely used on
> the Pi4) VEC which was last in the probe order sequence, but one of the
> earliest device to assign.
>
> This resulted in configurations that were rejected by our code but were
> still valid with a different assignment.
>
> We can fix this by making sure we assign CRTCs to FIFOs by ordering
> them by ascending HVS output index.
>
> Fixes: 87ebcd42fb7b ("drm/vc4: crtc: Assign output to channel automatically")
> Signed-off-by: Maxime Ripard <maxime@cerno.tech>
> ---

[...]

>
> - for_each_oldnew_crtc_in_state(state, crtc, old_crtc_state, new_crtc_state, i) {
> - struct vc4_crtc_state *old_vc4_crtc_state =
> - to_vc4_crtc_state(old_crtc_state);
> - struct vc4_crtc_state *new_vc4_crtc_state =
> - to_vc4_crtc_state(new_crtc_state);
> - struct vc4_crtc *vc4_crtc = to_vc4_crtc(crtc);
> + /*
> + * The problem we have to solve here is that we have up to 7
> + * encoders, connected to up to 6 CRTCs.
> + *
> + * Those CRTCs, depending on the instance, can be routed to 1, 2
> + * or 3 HVS FIFOs, and we need to set the change the muxing

This sentence sounds a little bit off to me. Did you mean:

"we need to set the muxing between" or "we need to change the muxing" ?

I'm not familiar with VC4 but the patch seems to do what the commit message
says, so the changes look good to me.