linux-kernel - Re: [PATCH 13/21] drm/msm/dpu: Require modeset if clone mode status changes

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Message-ID: <ZtW_S0j5AEr4g0QW@phenom.ffwll.local>
Date: Mon, 2 Sep 2024 15:36:11 +0200
From: Daniel Vetter <daniel.vetter@...ll.ch>
To: Jessica Zhang <quic_jesszhan@...cinc.com>
Cc: Rob Clark <robdclark@...il.com>,
	Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
	quic_abhinavk@...cinc.com, Sean Paul <sean@...rly.run>,
	Marijn Suijten <marijn.suijten@...ainline.org>,
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>, quic_ebharadw@...cinc.com,
	linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	Rob Clark <robdclark@...omium.org>
Subject: Re: [PATCH 13/21] drm/msm/dpu: Require modeset if clone mode status
 changes

On Thu, Aug 29, 2024 at 01:48:34PM -0700, Jessica Zhang wrote:
> If the clone mode enabled status is changing, a modeset needs to happen
> so that the resources can be reassigned
> 
> Signed-off-by: Jessica Zhang <quic_jesszhan@...cinc.com>
> ---
>  drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c
> index 1b0cc899e8c1..99eaaca405a4 100644
> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c
> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c
> @@ -1306,6 +1306,8 @@ static int dpu_crtc_atomic_check(struct drm_crtc *crtc,
>  	int rc = 0;
>  
>  	bool needs_dirtyfb = dpu_crtc_needs_dirtyfb(crtc_state);
> +	bool clone_mode_requested = drm_crtc_in_clone_mode(crtc->state);
> +	bool clone_mode_enabled = drm_crtc_in_clone_mode(crtc_state);
>  
>  	/* there might be cases where encoder needs a modeset too */
>  	drm_for_each_encoder_mask(drm_enc, crtc->dev, crtc_state->encoder_mask) {
> @@ -1313,6 +1315,10 @@ static int dpu_crtc_atomic_check(struct drm_crtc *crtc,
>  			crtc_state->mode_changed = true;
>  	}
>  
> +	if ((clone_mode_requested && !clone_mode_enabled) ||
> +			(!clone_mode_requested && clone_mode_enabled))
> +		crtc_state->mode_changed = true;

So two things, and kinda about the overall patch series:

- msm is confused about crtc_state->mode_changed, e.g. it sets it if
  crtc_state->active_changed, which is wrong. Or msm dpu code doesn't use
  drm_atomic_crtc_needs_modeset() correctly.

- changing crtc_state->mode_changed from your crtc or plane callbacks
  means you cannot use drm_atomic_helper_check directly, but need to roll
  your own that calls drm_atomic_helper_check_modesets again as needed.
  See the kerneldoc comment for drm_atomic_helper_check_modesets()

- the same holds if you set mode_changed from your encoder or bridge
  functions, but I think this doesn't apply here for this patch.

I think it'd be really good to

- review existing kerneldoc and please submit patches where it's not clear
  or detailed enough

- add checks to the atomic helper code to catch this. I think the best way
  would be to check if drm_atomic_crtc_needs_modeset() changes outside of
  areas where the helper code allows it already (essentially connector
  functions setting ->connectors_changed), and set a new
  drm_atomic_state->dirty_needs_modeset. Which
  drm_atomic_helper_check_modeset would clear, and which would result in a
  WARN_ON in drm_atomic_check if it's not yet cleared when the driver
  returns with success.

  Otherwise there's just no way I think to make sure drivers get this
  right.

Can I please sign you up for these patches?

Thanks, Sima

> +
>  	if (drm_atomic_crtc_needs_modeset(crtc_state)) {
>  		rc = dpu_crtc_assign_resources(crtc, crtc_state);
>  		if (rc < 0)
> 
> -- 
> 2.34.1
> 

-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch