linux-kernel - Re: [PATCH v2 3/3] drm/vc4: hdmi: Actually check for the connector status in hotplug

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Message-ID: <YUCy4AmYDFD2jtG6@phenom.ffwll.local>
Date:   Tue, 14 Sep 2021 16:34:08 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     dri-devel@...ts.freedesktop.org,
        Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Emma Anholt <emma@...olt.net>,
        linux-rpi-kernel@...ts.infradead.org,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Phil Elwell <phil@...pberrypi.com>,
        Tim Gover <tim.gover@...pberrypi.com>,
        Dom Cobley <dom@...pberrypi.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        bcm-kernel-feedback-list@...adcom.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] drm/vc4: hdmi: Actually check for the connector
 status in hotplug

On Tue, Sep 14, 2021 at 12:17:24PM +0200, Maxime Ripard wrote:
> The drm_helper_hpd_irq_event() documentation states that this function
> is "useful for drivers which can't or don't track hotplug interrupts for
> each connector." and that "Drivers which support hotplug interrupts for
> each connector individually and which have a more fine-grained detect
> logic should bypass this code and directly call
> drm_kms_helper_hotplug_event()". This is thus what we ended-up doing.
> 
> However, what this actually means, and is further explained in the
> drm_kms_helper_hotplug_event() documentation, is that
> drm_kms_helper_hotplug_event() should be called by drivers that can
> track the connection status change, and if it has changed we should call
> that function.
> 
> This underlying expectation we failed to provide is that the caller of
> drm_kms_helper_hotplug_event() should call drm_helper_probe_detect() to
> probe the new status of the connector.
> 
> Since we didn't do it, it meant that even though we were sending the
> notification to user-space and the DRM clients that something changed we
> never probed or updated our internal connector status ourselves.
> 
> This went mostly unnoticed since the detect callback usually doesn't
> have any side-effect. Also, if we were using the DRM fbdev emulation
> (which is a DRM client), or any user-space application that can deal
> with hotplug events, chances are they would react to the hotplug event
> by probing the connector status eventually.
> 
> However, now that we have to enable the scrambler in detect() if it was
> enabled it has a side effect, and an application such as Kodi or
> modetest doesn't deal with hotplug events. This resulted with a black
> screen when Kodi or modetest was running when a screen was disconnected
> and then reconnected, or switched off and on.

Uh, why are you running this scrambler restore in your probe function? I
guess it works, but most drivers that do expensive hotplug restore to
handle the "no black screen for replug" use-case handle that in their own
dedicated code.

But those also tend to have per-output hpd interrupt sources, so maybe
that's why?
-Daniel

> 
> Signed-off-by: Maxime Ripard <maxime@...no.tech>
> ---
>  drivers/gpu/drm/vc4/vc4_hdmi.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> index a3dbd1fdff7d..d9e001b9314f 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> @@ -1578,10 +1578,11 @@ static int vc4_hdmi_audio_init(struct vc4_hdmi *vc4_hdmi)
>  static irqreturn_t vc4_hdmi_hpd_irq_thread(int irq, void *priv)
>  {
>  	struct vc4_hdmi *vc4_hdmi = priv;
> -	struct drm_device *dev = vc4_hdmi->connector.dev;
> +	struct drm_connector *connector = &vc4_hdmi->connector;
> +	struct drm_device *dev = connector->dev;
>  
>  	if (dev && dev->registered)
> -		drm_kms_helper_hotplug_event(dev);
> +		drm_connector_helper_hpd_irq_event(connector);
>  
>  	return IRQ_HANDLED;
>  }
> -- 
> 2.31.1
> 

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