Re: [PATCH] drm/i915/dp: Link train Fallback on eDP only if fallback link BW can fit panel's native mode

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

Thanks Lyude.

So based on the imitial comments from Jani N, the recommendation was to disconnect
downclock_mode from drrs_init so that user can set downclock mode
independently from drrs mode.

Jani,
So we would need following changes:
* Set the panel->downclock_mode in edp_init_connector() using intel_find_panel_downclock()
* Add downclock_mode->clock when we check against available BW in mode_valid
* Also use that in compute_config
* Then check against downclock_mode on link training fallback

Any more changes recommended here?

Manasi

On Fri, Aug 17, 2018 at 04:43:22PM -0400, Lyude Paul wrote:
> On Fri, 2018-08-17 at 13:40 -0700, Manasi Navare wrote:
> > On Fri, Aug 17, 2018 at 04:32:09PM -0400, Lyude Paul wrote:
> > > After reading the discussion so far on this patch, this sounds correct! One
> > > nit
> > > pick below though:
> > > 
> > > On Wed, 2018-05-16 at 19:21 -0700, Manasi Navare wrote:
> > > > This patch fixes the original commit c0cfb10d9e1de49 ("drm/i915/edp:
> > > > Do not do link training fallback or prune modes on EDP") that causes
> > > > a blank screen in case of certain eDP panels (Eg: seen on Dell XPS13 9350)
> > > > where first link training fails and a retraining is required by falling
> > > > back to lower link rate/lane count.
> > > > In case of some panels they advertise higher link rate/lane count
> > > > than whats required for supporting the panel's native mode.
> > > > But we always link train at highest link rate/lane count for eDP
> > > > and if that fails we can still fallback to lower link rate/lane count
> > > > as long as the fallback link BW still fits the native mode to avoid
> > > > pruning the panel's native mode yet retraining at fallback values
> > > > to recover from a blank screen.
> > > > 
> > > > Cc: Clinton Taylor <clinton.a.taylor@intel.com>
> > > > Cc: Jani Nikula <jani.nikula@linux.intel.com>
> > > > Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
> > > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > > > Cc: Lucas De Marchi <lucas.demarchi@intel.com>
> > > > Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
> > > > ---
> > > >  drivers/gpu/drm/i915/intel_dp.c               | 25
> > > > +++++++++++++++++++++++++
> > > >  drivers/gpu/drm/i915/intel_dp_link_training.c | 26 +++++++++-------------
> > > > ----
> > > >  2 files changed, 34 insertions(+), 17 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/i915/intel_dp.c
> > > > b/drivers/gpu/drm/i915/intel_dp.c
> > > > index 2cc58596..7f7202a 100644
> > > > --- a/drivers/gpu/drm/i915/intel_dp.c
> > > > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > > > @@ -387,6 +387,21 @@ static bool intel_dp_link_params_valid(struct
> > > > intel_dp
> > > > *intel_dp, int link_rate,
> > > >  	return true;
> > > >  }
> > > >  
> > > > +static bool intel_dp_can_link_train_fallback_for_edp(struct intel_dp
> > > > *intel_dp,
> > > > +						     int link_rate,
> > > > +						     uint8_t lane_count)
> > > > +{
> > > > +	struct drm_display_mode *fixed_mode = intel_dp->attached_connector-
> > > > > panel.fixed_mode;
> > > > 
> > > > +	int mode_rate, max_rate;
> > > > +
> > > > +	mode_rate = intel_dp_link_required(fixed_mode->clock, 18);
> > > > +	max_rate = intel_dp_max_data_rate(link_rate, lane_count);
> > > > +	if (mode_rate > max_rate)
> > > > +		return false;
> > > > +
> > > > +	return true;
> > > > +}
> > > > +
> > > >  int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
> > > >  					    int link_rate, uint8_t lane_count)
> > > >  {
> > > > @@ -396,9 +411,19 @@ int intel_dp_get_link_train_fallback_values(struct
> > > > intel_dp *intel_dp,
> > > >  				    intel_dp->num_common_rates,
> > > >  				    link_rate);
> > > >  	if (index > 0) {
> > > > +		if (intel_dp_is_edp(intel_dp) &&
> > > > +		    !intel_dp_can_link_train_fallback_for_edp(intel_dp,
> > > > +							     intel_dp-
> > > > > common_rates[index-1],
> > > > 
> > > > +							      lane_count))
> > > > +			return -1;
> > > >  		intel_dp->max_link_rate = intel_dp->common_rates[index - 1];
> > > >  		intel_dp->max_link_lane_count = lane_count;
> > > >  	} else if (lane_count > 1) {
> > > > +		if (intel_dp_is_edp(intel_dp) &&
> > > > +		    !intel_dp_can_link_train_fallback_for_edp(intel_dp,
> > > > +							      intel_dp_max_commo
> > > > n_rate(intel_dp),
> > > > +							      lane_count >> 1))
> > > > +			return -1;
> > > 
> > > The arguments you pass to intel_dp_can_link_train_fallback_for_edp() are the
> > > same ones that you assign to intel_dp->max_link_rate and intel_dp-
> > > > max_link_lane_count, why not just set those latter two first then pass
> > > > them to
> > > 
> > > intel_dp_can_link_train_fallback_for_edp() afterwards?
> > 
> > Actually I had thought of that. However if the
> > intel_dp_can_link_train_fallback_for_edp()
> > returns false then we dont wanna update the intel_dp->max_link_rate and
> > lane_count to
> > reduced fallback values.
> Ahhh, that makes sense! Ignore that nitpick then :)
> 
> > 
> > The other concerns mentioned on this patch were about checking them against
> > downclock mode
> > which will be with lower refresh rate hence might fit the reduced BW. But
> > currently the
> > downclock mode is assigned only as part of intel_dp_drrs_support() so first we
> > would
> > need to may be find that in edp_init and save it as part of intel_dp or
> > something.
> > As well as testing against alternate mode.
> > Any thoughts on that?
> 
> That sounds fine to me! It is probably a bit safer then just blindly downgrading
> the link
> 
> > 
> > Manasi
> > 
> > > >  		intel_dp->max_link_rate = intel_dp_max_common_rate(intel_dp);
> > > >  		intel_dp->max_link_lane_count = lane_count >> 1;
> > > >  	} else {
> > > > diff --git a/drivers/gpu/drm/i915/intel_dp_link_training.c
> > > > b/drivers/gpu/drm/i915/intel_dp_link_training.c
> > > > index 3fcaa98..6673975 100644
> > > > --- a/drivers/gpu/drm/i915/intel_dp_link_training.c
> > > > +++ b/drivers/gpu/drm/i915/intel_dp_link_training.c
> > > > @@ -335,22 +335,14 @@ intel_dp_start_link_train(struct intel_dp *intel_dp)
> > > >  	return;
> > > >  
> > > >   failure_handling:
> > > > -	/* Dont fallback and prune modes if its eDP */
> > > > -	if (!intel_dp_is_edp(intel_dp)) {
> > > > -		DRM_DEBUG_KMS("[CONNECTOR:%d:%s] Link Training failed at link
> > > > rate = %d, lane count = %d",
> > > > -			      intel_connector->base.base.id,
> > > > -			      intel_connector->base.name,
> > > > -			      intel_dp->link_rate, intel_dp->lane_count);
> > > > -		if (!intel_dp_get_link_train_fallback_values(intel_dp,
> > > > -							     intel_dp-
> > > > > link_rate,
> > > > 
> > > > -							     intel_dp-
> > > > > lane_count))
> > > > 
> > > > -			/* Schedule a Hotplug Uevent to userspace to start
> > > > modeset */
> > > > -			schedule_work(&intel_connector->modeset_retry_work);
> > > > -	} else {
> > > > -		DRM_ERROR("[CONNECTOR:%d:%s] Link Training failed at link rate =
> > > > %d, lane count = %d",
> > > > -			  intel_connector->base.base.id,
> > > > -			  intel_connector->base.name,
> > > > -			  intel_dp->link_rate, intel_dp->lane_count);
> > > > -	}
> > > > +	DRM_DEBUG_KMS("[CONNECTOR:%d:%s] Link Training failed at link rate = %d,
> > > > lane count = %d",
> > > > +		      intel_connector->base.base.id,
> > > > +		      intel_connector->base.name,
> > > > +		      intel_dp->link_rate, intel_dp->lane_count);
> > > > +	if (!intel_dp_get_link_train_fallback_values(intel_dp,
> > > > +						     intel_dp->link_rate,
> > > > +						     intel_dp->lane_count))
> > > > +		/* Schedule a Hotplug Uevent to userspace to start modeset */
> > > > +		schedule_work(&intel_connector->modeset_retry_work);
> > > >  	return;
> > > >  }
> 
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