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

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Date:   Fri, 25 Nov 2022 12:00:44 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Maxime Ripard <maxime@...no.tech>,
        Maxime Ripard <mripard@...nel.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...il.com>,
        Thomas Zimmermann <tzimmermann@...e.de>
Cc:     David Gow <davidgow@...gle.com>, linaro-mm-sig@...ts.linaro.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kselftest@...r.kernel.org,
        Maíra Canal <mairacanal@...eup.net>,
        linux-media@...r.kernel.org, kunit-dev@...glegroups.com,
        dri-devel@...ts.freedesktop.org,
        Brendan Higgins <brendan.higgins@...ux.dev>,
        linux-kernel@...r.kernel.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>
Subject: Re: [PATCH 10/24] drm/vc4: kms: Sort the CRTCs by output before
 assigning them

On 11/23/22 16:25, Maxime Ripard wrote:
> 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@...no.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.

Reviewed-by: Javier Martinez Canillas <javierm@...hat.com>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat