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

[Lyude Paul]

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 at intel.com>
> Cc: Jani Nikula <jani.nikula at linux.intel.com>
> Cc: Ville Syrjala <ville.syrjala at linux.intel.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Lucas De Marchi <lucas.demarchi at intel.com>
> Signed-off-by: Manasi Navare <manasi.d.navare at 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?
>  		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;
>  }