On Wed, 16 May 2018, Manasi Navare <manasi.d.nav...@intel.com> 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.

What eDP revision is the faulty panel? Does [1] help for that? Then we'd
not need this.

I think this also ties in with the alternate/downclock mode. It fails
now and should be reverted [2]. However, if we know the panel has a
valid mode with lower refresh rate, we might have a mode that actually
fits a link with reduced bandwidth.

IMO we need [1], [2], and patches to disconnect downclock mode from drrs
support (so we/user can choose the downclock mode independent of drrs),
and finally eDP link fallback selection based on if there's a downclock
mode and if it fits a link with reduced link rate or lane count.

BR,
Jani.


[1] 
http://patchwork.freedesktop.org/patch/msgid/20180509071321.28563-1-jani.nik...@intel.com
[2] 
http://patchwork.freedesktop.org/patch/msgid/20180516080110.22770-1-jani.nik...@intel.com



>
> Cc: Clinton Taylor <clinton.a.tay...@intel.com>
> Cc: Jani Nikula <jani.nik...@linux.intel.com>
> Cc: Ville Syrjala <ville.syrj...@linux.intel.com>
> Cc: Daniel Vetter <daniel.vet...@ffwll.ch>
> Cc: Lucas De Marchi <lucas.demar...@intel.com>
> Signed-off-by: Manasi Navare <manasi.d.nav...@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_common_rate(intel_dp),
> +                                                           lane_count >> 1))
> +                     return -1;
>               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;
>  }

-- 
Jani Nikula, Intel Open Source Technology Center
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