[Intel-gfx] [PATCH v1] drm/i915: Minor link training logic fixes for dp_mst

[Manasi Navare]

On Wed, May 27, 2020 at 11:00:22PM +0300, Stanislav Lisovskiy wrote:
> First of all *_needs_link_retraining function should return
> false is link_train is set to true but not false.
> 
> Also if we detect channel eq problem when checking mst status
> we simply bail out, without setting link_train to false again,
> which might end up in a situation that we don't do link retraining
> when needed.
> 
> There were some issues, when we had several problems with dp mst
> and at the same time the log was floode by messages about
> "channel eq not ok, need retraining" however the actual training
> seems to be never done.
> 
> Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 1768731678a1..9288dc1f8914 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -5627,6 +5627,7 @@ intel_dp_check_mst_status(struct intel_dp *intel_dp)
>  			drm_dbg_kms(&i915->drm,
>  				    "channel EQ not ok, retraining\n");
>  			need_retrain = true;
> +			intel_dp->link_trained = false;
>  		}
>  
>  		drm_dbg_kms(&i915->drm, "got esi %3ph\n", esi);
> @@ -5654,7 +5655,7 @@ intel_dp_needs_link_retrain(struct intel_dp *intel_dp)
>  {
>  	u8 link_status[DP_LINK_STATUS_SIZE];
>  
> -	if (!intel_dp->link_trained)
> +	if (intel_dp->link_trained)

This is not correct. Since link_trained is set when link training is completed as part of a
complete modeset. If link training is not done, like at hotplug, then in that case we should
not retrain since the pipe has not been configured for this new hotplug and link training
has not been done.

Retraining is expected to happen only in cases where there is a short pulse or a spurious long pulse
when link training through modeset is already complete and hence the old logic of returnin a false
when !intel_dp->link_trained is correct.

Regards
Manasi

>  		return false;
>  
>  	/*
> -- 
> 2.24.1.485.gad05a3d8e5
>