[Intel-gfx] [PATCH] drm/i915: Fall back to zero vswing/preemph if the sink doesn't like the last good values

[Conselvan De Oliveira, Ander]

On Fri, 2015-10-30 at 18:47 +0200, ville.syrjala at linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> My Lenovo STM STDP3100 miniDP->VGA dongle doesn't seem to like it when
> we try to start link training with non-zero vswing/preemphasis. So when
> the initial link training DPCD write fails, retry it with zero values.

Does the device NACKs the request?

Ander

> 
> Fixes a bunch of errors like so:
> [drm:intel_dp_start_link_train [i915]] *ERROR* failed to enable link training
> 
> Cc: Mika Kahola <mika.kahola at intel.com>
> Cc: Sivakumar Thulasimani <sivakumar.thulasimani at intel.com>
> Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira at intel.com>
> Fixes: 5fa836a9d859 ("drm/i915: DP link training optimization")
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_dp.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index ba4cbf5..9529a6e 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -3750,10 +3750,17 @@ intel_dp_link_training_clock_recovery(struct intel_dp
> *intel_dp)
>  
>  	DP |= DP_PORT_EN;
>  
> +again:
>  	/* clock recovery */
>  	if (!intel_dp_reset_link_train(intel_dp, &DP,
>  				       DP_TRAINING_PATTERN_1 |
>  				       DP_LINK_SCRAMBLING_DISABLE)) {
> +		if (intel_dp->train_set_valid) {
> +			DRM_DEBUG_KMS("Sink rejected link training request,
> trying again with zero values\n");
> +			intel_dp->train_set_valid = false;
> +			goto again;
> +		}
> +
>  		DRM_ERROR("failed to enable link training\n");
>  		return;
>  	}
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