linux-kernel - Re: [PATCH] drm/probe_helper: Don't bother probing when connectors are forced off

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Message-ID: <20180921085956.GX11082@phenom.ffwll.local>
Date:   Fri, 21 Sep 2018 10:59:56 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Lyude Paul <lyude@...hat.com>
Cc:     Ville Syrjälä 
        <ville.syrjala@...ux.intel.com>, David Airlie <airlied@...ux.ie>,
        Sean Paul <sean@...rly.run>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/probe_helper: Don't bother probing when connectors
 are forced off

On Mon, Sep 17, 2018 at 05:12:04PM -0400, Lyude Paul wrote:
> On Mon, 2018-09-17 at 21:16 +0300, Ville Syrjälä wrote:
> > On Mon, Sep 17, 2018 at 02:10:02PM -0400, Lyude Paul wrote:
> > > On Mon, 2018-09-17 at 20:55 +0300, Ville Syrjälä wrote:
> > > > On Mon, Sep 17, 2018 at 01:43:44PM -0400, Lyude Paul wrote:
> > > > > Userspace asked them to be forced off, so why would we care about what a
> > > > > probe tells us?
> > > > 
> > > > I believe there should be force checks in the callers already.
> > > > Or are we missing some?
> > > 
> > > JFYI, what triggered me to send this patch are these error messages that
> > > come
> > > from nouveau when a hotplug happens on a port that we've forced off:
> > > 
> > > [ 1903.918104] nouveau 0000:01:00.0: DRM: DDC responded, but no EDID for DP-
> > > 2
> > > [ 1903.918123] [drm:drm_helper_hpd_irq_event [drm_kms_helper]]
> > > [CONNECTOR:61:DP-2] status updated from disconnected to disconnected
> > > 
> > > That being said; I'm sure there are probably some checks missing, but I
> > > don't
> > > really see the purpose in calling the driver's probe functions at all if
> > > they're
> > > just supposed to return the status we forced.
> > 
> > Digging through my cobweb ridden local git repository I found this:
> > 
> > commit bbd17813a7c7d0210c619365707044d0fb29e3f0
> > Author: Ville Syrjälä <ville.syrjala@...ux.intel.com>
> > Date:   Mon Jun 10 15:28:55 2013 +0300
> > 
> >     drm: Ignore forced connectors in drm_helper_hpd_irq_event()
> >     
> >     drm_helper_hpd_irq_event() calls the connector's .detect() function
> >     even for forced connectors. If the returned status doesn't match the
> >     forced status, we will send the hotplug event, causing userspace to
> >     re-probe all the connectors. Eventually we should end up back where
> >     we started when drm_helper_probe_single_connector_modes() overwrites
> >     the connector status with the forced status.
> >     
> >     We can avoid all that pointles work if we just skip forced connectors
> >     in drm_helper_hpd_irq_event().
> >     
> >     Signed-off-by: Ville Syrjälä <ville.syrjala@...ux.intel.com>
> > 
> > diff --git a/drivers/gpu/drm/drm_crtc_helper.c
> > b/drivers/gpu/drm/drm_crtc_helper.c
> > index ed1334e27c33..4fc2ad76c107 100644
> > --- a/drivers/gpu/drm/drm_crtc_helper.c
> > +++ b/drivers/gpu/drm/drm_crtc_helper.c
> > @@ -1086,6 +1086,10 @@ void drm_helper_hpd_irq_event(struct drm_device *dev)
> >  	mutex_lock(&dev->mode_config.mutex);
> >  	list_for_each_entry(connector, &dev->mode_config.connector_list, head) {
> >  
> > +		/* Ignore forced connectors. */
> > +		if (connector->force)
> > +			continue;
> > +
> >  		/* Only handle HPD capable connectors. */
> >  		if (!(connector->polled & DRM_CONNECTOR_POLL_HPD))
> >  			continue;
> > 
> > 
> > I guess I never sent it out.
> 
> Ahhh, to be honest though this patch isn't really enough.
> drm_helper_hpd_irq_event() isn't going to be used by all drivers (I may remove
> some usage of it in nouveau in the near future, even) so I still think it would
> be a better idea to just add this into drm_helper_probe_detect() and
> drm_helper_probe_detect_ctx() so everything gets covered

Atm all connector->force handling is outside of drm_helper_probe_detect. I
guess we could try to push (some) of it into this helper, if that's
useful. There seems to be some duplication already. But adding a redundant
check like you do here feels a bit funky.  Maybe makes more sense in
context of the nouveau stuff you're working on?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch