Re: [PATCH v2] drm/i915: Perform link quality check unconditionally during long pulse

[Manasi Navare <manasi.d.navare@intel.com>]

On Mon, Mar 13, 2017 at 10:53:23PM +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.
> 
> To actually get read out the EDID and update things properly we
> also need to nuke the goto out added by commit 7d23e3c37bb3
> ("drm/i915: Cleaning up intel_dp_hpd_pulse"). I'm actually not sure
> why that was there. Perhaps to avoid an EDID read if the connector
> status didn't appear to change, but that sort of thing is quite racy
> and would have failed anyway if we failed to keep up with the
> hotplugs (if we missed the HPD down in between two HPD ups). And
> now that we take this codepath unconditionally we definitely need
> to drop the goto as otherwise we would never do the EDID read.
> 
> v2: Drop the goto that made us skip EDID reads entirely. Doh!
> 
> Cc: stable@vger.kernel.org
> Cc: Manasi Navare <manasi.d.navare@intel.com>
> Cc: Palmer Dabbelt <palmer@dabbelt.com>
> Reported-by: Palmer Dabbelt <palmer@dabbelt.com>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99766
> 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 | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index fd96a6cf7326..5c2f1b37b58f 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -4634,16 +4634,22 @@ 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);
>  		drm_modeset_unlock(&dev->mode_config.connection_mutex);
> -		goto out;
>  	}
>

Yes makes sense to me too since we have a check for crtc->active so
it will retrain only if the link is already up and running with an active crtc
and intel_dp->lane_count is not 0.
In case of link failures though when we send the hotplug uevent it will get through
this path and it will try to retrain which we dont want since the retraining
should happen only in the atomic_commit at the fallback rate, not here.
So I guess on link failure, I can set intel_dp->lane_count to 0 to invalidate that
and avoid retraining here.

Right?

Regards
Manasi  
>  	/*
> -- 
> 2.10.2
>