linux-kernel - Re: [PATCH] drm: do not call detect for connectors which are forced on

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Message-ID: <YzYrNJbfGcch1UtX@pendragon.ideasonboard.com>
Date:   Fri, 30 Sep 2022 02:33:08 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Michael Rodin <mrodin@...adit-jv.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Alex Deucher <alexander.deucher@....com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Sean Paul <seanpaul@...omium.org>,
        Vincent Abriou <vincent.abriou@...com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        michael@...in.online, erosca@...adit-jv.com
Subject: Re: [PATCH] drm: do not call detect for connectors which are forced
 on

Hello Michael,

Thank you for the patch. Sorry for the late reply, I wasn't on the CC
list so I didn't notice it.

On Fri, Aug 26, 2022 at 11:11:21AM +0200, Michael Rodin wrote:
> "detect" should not be called and its return value shall not be used when a
> connector is forced as hinted in the commit d50ba256b5f1 ("drm/kms: start
> adding command line interface using fb.") and the commit 6fe14acd496e
> ("drm: Document drm_connector_funcs"). One negative side effect of doing
> this is observed on the RCar3 SoCs which use the dw-hdmi driver. It
> continues executing drm_helper_hpd_irq_event even if its connector is
> forced to ON. As consequence drm_helper_hpd_irq_event calls "detect" so the
> connector status is updated to "disconnected":
> 
> [  420.201527] [drm:drm_helper_hpd_irq_event] [CONNECTOR:76:HDMI-A-1] status updated from connected to disconnected
> 
> This status is corrected by drm_helper_probe_single_connector_modes shortly
> after this because this function checks if a connector is forced:
> 
> [  420.218703] [drm:drm_helper_probe_single_connector_modes] [CONNECTOR:76:HDMI-A-1] status updated from disconnected to connected
> 
> To avoid similar issues this commit adapts functions which call "detect"
> so they check if a connector is forced and return the correct status.
> 
> Fixes: 949f08862d66 ("drm: Make the connector .detect() callback optional")

Is this really the right fixes tag ? The call to .detect() in
drm_helper_hpd_irq_event() predates that commit.

> Signed-off-by: Michael Rodin <mrodin@...adit-jv.com>
> ---
>  drivers/gpu/drm/drm_probe_helper.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
> index bb427c5a4f1f..1691047d0472 100644
> --- a/drivers/gpu/drm/drm_probe_helper.c
> +++ b/drivers/gpu/drm/drm_probe_helper.c
> @@ -289,7 +289,12 @@ drm_helper_probe_detect_ctx(struct drm_connector *connector, bool force)
>  retry:
>  	ret = drm_modeset_lock(&connector->dev->mode_config.connection_mutex, &ctx);
>  	if (!ret) {
> -		if (funcs->detect_ctx)
> +		if (connector->force == DRM_FORCE_ON ||
> +		    connector->force == DRM_FORCE_ON_DIGITAL)
> +			ret = connector_status_connected;
> +		else if (connector->force == DRM_FORCE_OFF)
> +			ret = connector_status_disconnected;
> +		else if (funcs->detect_ctx)
>  			ret = funcs->detect_ctx(connector, &ctx, force);
>  		else if (connector->funcs->detect)
>  			ret = connector->funcs->detect(connector, force);
> @@ -340,7 +345,14 @@ drm_helper_probe_detect(struct drm_connector *connector,
>  	if (ret)
>  		return ret;
>  
> -	if (funcs->detect_ctx)
> +	if (connector->force == DRM_FORCE_ON ||
> +	    connector->force == DRM_FORCE_ON_DIGITAL)
> +		ret = connector_status_connected;
> +	else if (connector->force == DRM_FORCE_OFF)
> +		ret = connector_status_disconnected;
> +	else if (funcs->detect_ctx)
> +		ret = funcs->detect_ctx(connector, ctx, force);
> +	else if (funcs->detect_ctx)
>  		ret = funcs->detect_ctx(connector, ctx, force);

Those two conditions are identical.

>  	else if (connector->funcs->detect)
>  		ret = connector->funcs->detect(connector, force);

The same logic is used in two places in this patch. Could this be
factored out to a separate function ? It may even be possible to
refactor drm_helper_probe_detect() and drm_helper_probe_detect_ctx() to
share more code between the two functions.

This being said, I'm not sure this is the right fix. Beside the i915 and
nouveau drivers, the only callers of drm_helper_probe_detect() are
drm_helper_probe_single_connector_modes(), output_poll_execute() and
check_connector_changed() in this file. The first two functions already
check connector->force and skip the call to drm_helper_probe_detect() if
the connector is forced. Only check_connector_changed() is missing that
check. Wouldn't it be simpler to return false in that function if
connector->force is set ?

Another question is whether it is valid for the dw-hdmi driver to call
drm_helper_hpd_irq_event() when the connector status is forced.
Shouldn't HPD events be ignored in that case ?

The detection code has grown quite complex over time, I would really
appreciate input from Maxime and Maarten on this.

-- 
Regards,

Laurent Pinchart