linux-kernel - Re: [PATCH v2 8/9] drm/atomic-helper: Re-order bridge chain pre-enable and post-disable

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Date: Fri, 31 May 2024 02:28:42 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Aradhya Bhatia <a-bhatia1@...com>
Cc: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>, 
	Andrzej Hajda <andrzej.hajda@...el.com>, Neil Armstrong <neil.armstrong@...aro.org>, 
	Robert Foss <rfoss@...nel.org>, Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
	Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Jyri Sarha <jyri.sarha@....fi>, Thomas Zimmermann <tzimmermann@...e.de>, 
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	DRI Development List <dri-devel@...ts.freedesktop.org>, Linux Kernel List <linux-kernel@...r.kernel.org>, 
	Sam Ravnborg <sam@...nborg.org>, Thierry Reding <treding@...dia.com>, 
	Kieran Bingham <kieran.bingham+renesas@...asonboard.com>, Nishanth Menon <nm@...com>, 
	Vignesh Raghavendra <vigneshr@...com>, Praneeth Bajjuri <praneeth@...com>, Udit Kumar <u-kumar1@...com>, 
	Devarsh Thakkar <devarsht@...com>, Jayesh Choudhary <j-choudhary@...com>, 
	Jai Luthra <j-luthra@...com>
Subject: Re: [PATCH v2 8/9] drm/atomic-helper: Re-order bridge chain
 pre-enable and post-disable

On Thu, May 30, 2024 at 03:06:20PM +0530, Aradhya Bhatia wrote:
> Move the bridge pre_enable call before crtc enable, and the bridge
> post_disable call after the crtc disable.
> 
> The sequence of enable after this patch will look like:
> 
> 	bridge[n]_pre_enable
> 	...
> 	bridge[1]_pre_enable
> 
> 	crtc_enable
> 	encoder_enable
> 
> 	bridge[1]_enable
> 	...
> 	bridge[n]__enable
> 
> and vice-versa for the bridge chain disable sequence.
> 
> The definition of bridge pre_enable hook says that,
> "The display pipe (i.e. clocks and timing signals) feeding this bridge
> will not yet be running when this callback is called".
> 
> Since CRTC is also a source feeding the bridge, it should not be enabled
> before the bridges in the pipeline are pre_enabled. Fix that by
> re-ordering the sequence of bridge pre_enable and bridge post_disable.
> 
> Signed-off-by: Aradhya Bhatia <a-bhatia1@...com>
> ---
>  drivers/gpu/drm/drm_atomic_helper.c | 70 +++++++++++++++++++++++++++--
>  1 file changed, 67 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index fb97b51b38f1..02e093950218 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -1186,8 +1186,6 @@ disable_outputs(struct drm_device *dev, struct drm_atomic_state *old_state)
>  			else if (funcs->dpms)
>  				funcs->dpms(encoder, DRM_MODE_DPMS_OFF);
>  		}
> -
> -		drm_atomic_bridge_chain_post_disable(bridge, old_state);
>  	}
>  
>  	for_each_oldnew_crtc_in_state(old_state, crtc, old_crtc_state, new_crtc_state, i) {
> @@ -1234,6 +1232,49 @@ disable_outputs(struct drm_device *dev, struct drm_atomic_state *old_state)
>  		if (ret == 0)
>  			drm_crtc_vblank_put(crtc);
>  	}
> +
> +	for_each_oldnew_connector_in_state(old_state, connector, old_conn_state,
> +					   new_conn_state, i) {
> +		struct drm_encoder *encoder;
> +		struct drm_bridge *bridge;
> +
> +		/*
> +		 * Shut down everything that's in the changeset and currently
> +		 * still on. So need to check the old, saved state.
> +		 */
> +		if (!old_conn_state->crtc)
> +			continue;
> +
> +		old_crtc_state = drm_atomic_get_old_crtc_state(old_state, old_conn_state->crtc);
> +
> +		if (new_conn_state->crtc)
> +			new_crtc_state = drm_atomic_get_new_crtc_state(old_state,
> +								       new_conn_state->crtc);
> +		else
> +			new_crtc_state = NULL;
> +
> +		if (!crtc_needs_disable(old_crtc_state, new_crtc_state) ||
> +		    !drm_atomic_crtc_needs_modeset(old_conn_state->crtc->state))
> +			continue;
> +
> +		encoder = old_conn_state->best_encoder;
> +
> +		/* We shouldn't get this far if we didn't previously have
> +		 * an encoder.. but WARN_ON() rather than explode.
> +		 */
> +		if (WARN_ON(!encoder))
> +			continue;
> +
> +		drm_dbg_atomic(dev, "disabling [ENCODER:%d:%s]\n",
> +			       encoder->base.id, encoder->name);

This duplicates the code in the loop around
drm_atomic_bridge_chain_disable(). Can it be extracted to a separate
function? Code duplication is nearly always a bad idea.

The same comment applies to the next chunk too.

> +
> +		/*
> +		 * Each encoder has at most one connector (since we always steal
> +		 * it away), so we won't call disable hooks twice.
> +		 */
> +		bridge = drm_bridge_chain_get_first_bridge(encoder);
> +		drm_atomic_bridge_chain_post_disable(bridge, old_state);
> +	}
>  }
>  
>  /**

-- 
With best wishes
Dmitry