Re: [PATCH v2 1/4] drm/i915/dp: Link train Fallback on eDP only if fallback link BW can fit panel's native mode

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On Wed, Sep 12, 2018 at 03:57:16PM -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.
> 
> v2:
> * Send uevent if link failure on eDP unconditionally
> 
> Cc: Clinton Taylor <clinton.a.taylor@xxxxxxxxx>
> Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> Cc: Ville Syrjala <ville.syrjala@xxxxxxxxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Lucas De Marchi <lucas.demarchi@xxxxxxxxx>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107489
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105338
> Signed-off-by: Manasi Navare <manasi.d.navare@xxxxxxxxx>
> Tested-by: Alexander Wilson <alexander.wilson@xxxxxxx>
> ---
>  drivers/gpu/drm/i915/intel_dp.c               | 29 +++++++++++++++++++
>  drivers/gpu/drm/i915/intel_dp_link_training.c | 26 ++++++-----------
>  2 files changed, 38 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 436c22de33b6..e4de5257cd87 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -557,6 +557,21 @@ 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;

const

> +	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;

I wonder if we should extract this into a helper and share with
mode_valid(). Then again, it's just a few lines so maybe not worth it.

> +
> +	return true;
> +}
> +
>  int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
>  					    int link_rate, uint8_t lane_count)
>  {
> @@ -566,9 +581,23 @@ 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)) {
> +			DRM_DEBUG_KMS("Retrying Link training for eDP with same parameters\n");
> +			return 0;
> +		}
>  		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_common_rate(intel_dp),
> +							      lane_count >> 1)) {
> +			DRM_DEBUG_KMS("Retrying Link training for eDP with same parameters\n");
> +			return 0;
> +		}
>  		intel_dp->max_link_rate = intel_dp_max_common_rate(intel_dp);
>  		intel_dp->max_link_lane_count = lane_count >> 1;

This whole thing is getting a bit messy. I think it would worthwile to
rewrite this as something like:

intel_dp_update_link_train_fallback_values(intel_dp)
{
	int link_rate = intel_dp->link_rate;
	int lane_count = intel_dp->lane_count;

	if (intel_dp_get_link_train_fallback_values(intel_dp, &link_rate, &lane_count))
		return -1;

	if (is_edp() && !edp_can_link_train_thing(link_rate, lane_count)) {
		DRM_DEBUG_KMS("...");
		return 0;
	}

	intel_dp->max_link_rate = link_rate;
	intel_dp->max_link_lane_count = lane_count;

	return 0;
}

But that can be done later. With the missing const added this patch is
Reviewed-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>

>  	} else {
> diff --git a/drivers/gpu/drm/i915/intel_dp_link_training.c b/drivers/gpu/drm/i915/intel_dp_link_training.c
> index a9f40985a621..30be0e39bd5f 100644
> --- a/drivers/gpu/drm/i915/intel_dp_link_training.c
> +++ b/drivers/gpu/drm/i915/intel_dp_link_training.c
> @@ -367,22 +367,14 @@ 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;
>  }
> -- 
> 2.18.0

-- 
Ville Syrjälä
Intel
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