Re: [Intel-gfx] [PATCH] drm/i915: Perform link quality check unconditionally during long pulse

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Feb 16, 2017 at 09:07:53AM -0800, Manasi Navare wrote:
> On Thu, Feb 16, 2017 at 05:30:07PM +0200, ville.syrjala@linux.intel.com wrote:
> > From: Ville Syrj�l� <ville.syrjala@linux.intel.com>
> > 
> > Apparently some DP sinks are a little nuts and cause HPD to drop
> > intermittently during modesets. This happens eg. on an ASUS PB287Q.
> > In oder to recover from this we can't really use the previous
> > connector status to determine if the link needs retraining, so let's
> > just ignore that piece of information and do the retrain
> > unconditionally. We do of course still check whether the link is
> > supposed to be running or not.
> > 
> > Cc: stable@vger.kernel.org
> > Cc: Palmer Dabbelt <palmer@dabbelt.com>
> > Reported-by: Palmer Dabbelt <palmer@dabbelt.com>
> > References: https://lists.freedesktop.org/archives/intel-gfx/2017-February/119779.html
> > Signed-off-by: Ville Syrj�l� <ville.syrjala@linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_dp.c | 15 +++++++++++----
> >  1 file changed, 11 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> > index 024798a9c016..37a746f7fbc3 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -4648,11 +4648,18 @@ intel_dp_long_pulse(struct intel_connector *intel_connector)
> >  		 */
> >  		status = connector_status_disconnected;
> >  		goto out;
> > -	} else if (connector->status == connector_status_connected) {
> > +	} else {
> >  		/*
> > -		 * If display was connected already and is still connected
> > -		 * check links status, there has been known issues of
> > -		 * link loss triggerring long pulse!!!!
> > +		 * If display is now connected check links status,
> > +		 * there has been known issues of link loss triggerring
> > +		 * long pulse.
> > +		 *
> > +		 * Some sinks (eg. ASUS PB287Q) seem to perform some
> > +		 * weird HPD ping pong during modesets. So we can apparely
> > +		 * end up with HPD going low during a modeset, and then
> > +		 * going back up soon after. And once that happens we must
> > +		 * retrain the link to get a picture. That's in case no
> > +		 * userspace component reacted to intermittent HPD dip.
> >  		 */
> >  		drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
> >  		intel_dp_check_link_status(intel_dp);
> > --
> 
> So here we basically just ignore the connector status and retrain irrespectively.

We ignore the _previous_ connector status.

> But that means even if we have newer values now for max link rate/lane count from
> DPCD, during this retrain we are just using the stale value of intel_dp->link_rate
> and intel_dp->lane_count. I think intel_dp->link_rate and lane count values
> should be set to 0 on HPD pulse, they would be set only during a modeset.

The DPCD has already been parsed by this time.

-- 
Ville Syrj�l�
Intel OTC