[Intel-gfx] [PATCH] drm/i915: DP link training optimization

[Todd Previte]

Hi Mika,

On 2/26/2015 2:26 AM, Mika Kahola wrote:
> In a case when DP link has been once trained we can reuse
> the existing link training parameters i.e. voltage swing
> and pre-emphasis levels from cache when there is a need to
> restart link training. In a case of eDP we initially try
> to train the link by using the parameter set that we can find
> from VBT. The fallback on both cases is to reset the link
> training parameters and restart.
I think you have a good idea here for the eDP case. But the way it's 
coded, there appear to be cases where the code will try to reuse the 
same training settings for regular DP as well. That will likely have 
some unfortunate results, as there's no guarantee for a normal external 
Displayport connection that training settings which worked once will 
work the next time. This would be of particular concern when 
disconnecting and reconnecting Displayport sink devices, as it appears 
as though it would try to retrain the newly connected device using the 
previously valid settings for the old device.

IMHO, this really needs to be written as eDP only and such that it can 
never occur on an external Displayport device. Otherwise, it seems like 
there's a good chance of ending up with a black screen when you plug in 
a Displayport monitor.

> Signed-off-by: Mika Kahola <mika.kahola at intel.com>
> ---
>   drivers/gpu/drm/i915/intel_dp.c  | 93 +++++++++++++++++++++++++++++++++++-----
>   drivers/gpu/drm/i915/intel_drv.h |  1 +
>   2 files changed, 84 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index d1141d3..8482f58 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -3288,8 +3288,39 @@ static bool
>   intel_dp_reset_link_train(struct intel_dp *intel_dp, uint32_t *DP,
>   			uint8_t dp_train_pat)
>   {
> -	memset(intel_dp->train_set, 0, sizeof(intel_dp->train_set));
> +	struct intel_digital_port *intel_dig_port = dp_to_dig_port(intel_dp);
> +	struct drm_device *dev = intel_dig_port->base.base.dev;
> +	struct drm_i915_private *dev_priv = dev->dev_private;
> +	int i;
> +
> +	/* in case of eDP, get link training parameters from VBT */
> +	if (is_edp(intel_dp)) {
> +		for (i=0; i<intel_dp->lane_count; i++)
> +			intel_dp->train_set[i] =  dev_priv->vbt.edp_vswing | dev_priv->vbt.edp_preemphasis;
> +	}
> +	else
> +		memset(intel_dp->train_set, 0, sizeof(intel_dp->train_set));
> +
> +	intel_dp_set_signal_levels(intel_dp, DP);
> +	return intel_dp_set_link_train(intel_dp, DP, dp_train_pat);
> +}
> +
> +static bool
> +intel_dp_reuse_link_train(struct intel_dp *intel_dp, uint32_t *DP,
> +			  uint8_t dp_train_pat)
> +{
> +	struct intel_digital_port *intel_dig_port = dp_to_dig_port(intel_dp);
> +	struct drm_device *dev = intel_dig_port->base.base.dev;
> +	struct drm_i915_private *dev_priv = dev->dev_private;
> +
>   	intel_dp_set_signal_levels(intel_dp, DP);
> +
> +	I915_WRITE(intel_dp->output_reg, *DP);
> +	POSTING_READ(intel_dp->output_reg);
> +
> +	drm_dp_dpcd_write(&intel_dp->aux, DP_TRAINING_LANE0_SET,
> +			  intel_dp->train_set, intel_dp->lane_count);
> +
>   	return intel_dp_set_link_train(intel_dp, DP, dp_train_pat);
>   }
>   
I'm not sure you want to be updating the sink device at this point. This 
should be checked against the spec to be sure it's not out of sequence.

> @@ -3356,6 +3387,8 @@ intel_dp_start_link_train(struct intel_dp *intel_dp)
>   	int voltage_tries, loop_tries;
>   	uint32_t DP = intel_dp->DP;
>   	uint8_t link_config[2];
> +	uint8_t link_status[DP_LINK_STATUS_SIZE];
> +	bool reuse_train_set = false;
>   
>   	if (HAS_DDI(dev))
>   		intel_ddi_prepare_link_retrain(encoder);
> @@ -3373,20 +3406,44 @@ intel_dp_start_link_train(struct intel_dp *intel_dp)
>   
>   	DP |= DP_PORT_EN;
>   
> -	/* clock recovery */
> -	if (!intel_dp_reset_link_train(intel_dp, &DP,
> -				       DP_TRAINING_PATTERN_1 |
> -				       DP_LINK_SCRAMBLING_DISABLE)) {
> -		DRM_ERROR("failed to enable link training\n");
> -		return;
> +	/*
> +	 * check if DP has already trained. Reset voltage swing and
> +	 * pre-emphasis levels if that's not the case.
> +	 */
> +	if (intel_dp->link_trained) {
> +		if (!intel_dp_reuse_link_train(intel_dp, &DP,
> +					       DP_TRAINING_PATTERN_1 |
> +					       DP_LINK_SCRAMBLING_DISABLE)) {
> +			DRM_DEBUG_KMS("unable to set known link training values\n");
> +			return;
> +		}
> +		DRM_DEBUG_KMS("reuse current link train set\n");
> +		reuse_train_set = true;
> +	}
> +	else {
> +		/* reset link training values */
> +		if (!intel_dp_reset_link_train(intel_dp, &DP,
> +					       DP_TRAINING_PATTERN_1 |
> +					       DP_LINK_SCRAMBLING_DISABLE)) {
> +			DRM_ERROR("failed to enable link training\n");
> +			return;
> +		}
> +		DRM_DEBUG_KMS("reset link train set\n");
> +
> +		/*
> +		 * in case of eDP, allow fallback to reset link training set
> +		 * if VBT parameters doesn't apply
> +		 */
> +		if (is_edp(intel_dp))
> +			reuse_train_set = true;
> +		else
> +			reuse_train_set = false;
>   	}
>   
>   	voltage = 0xff;
>   	voltage_tries = 0;
>   	loop_tries = 0;
>   	for (;;) {
> -		uint8_t link_status[DP_LINK_STATUS_SIZE];
> -
>   		drm_dp_link_train_clock_recovery_delay(intel_dp->dpcd);
>   		if (!intel_dp_get_link_status(intel_dp, link_status)) {
>   			DRM_ERROR("failed to get link status\n");
> @@ -3397,6 +3454,20 @@ intel_dp_start_link_train(struct intel_dp *intel_dp)
>   			DRM_DEBUG_KMS("clock recovery OK\n");
>   			break;
>   		}
> +		/*
> +		 * if we used previously trained voltage and pre-emphasis values
> +		 * and we don't get clock recovery, reset link training values
> +		 */
> +		if (reuse_train_set) {
> +			DRM_DEBUG_KMS("clock recovery not ok, resetting DP train set\n");
> +			if (!intel_dp_reset_link_train(intel_dp, &DP,
> +						       DP_TRAINING_PATTERN_1 |
> +						       DP_LINK_SCRAMBLING_DISABLE)) {
> +				DRM_ERROR("failed to enable link training\n");
> +				return;
> +			}
> +			reuse_train_set = false;
> +		}
>   
>   		/* Check to see if we've tried the max voltage */
>   		for (i = 0; i < intel_dp->lane_count; i++)
> @@ -3511,8 +3582,10 @@ intel_dp_complete_link_train(struct intel_dp *intel_dp)
>   
>   	intel_dp->DP = DP;
>   
> -	if (channel_eq)
> +	if (channel_eq) {
>   		DRM_DEBUG_KMS("Channel EQ done. DP Training successful\n");
> +		intel_dp->link_trained = true;
The link_trained flag is set here but never initialized or cleared 
anywhere else. So it looks like once it's set, it's there forever. That 
being the case, this code would never hit the else() case where 
intel_dp->link_trained is checked further up. I'm pretty sure that's not 
what you intended... :)

> +	}
>   
>   }
>   
> diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
> index 1fb1529..32ece0d 100644
> --- a/drivers/gpu/drm/i915/intel_drv.h
> +++ b/drivers/gpu/drm/i915/intel_drv.h
> @@ -668,6 +668,7 @@ struct intel_dp {
>   				     bool has_aux_irq,
>   				     int send_bytes,
>   				     uint32_t aux_clock_divider);
> +	bool link_trained; /* flag to indicate if DP link has been trained or not */
>   };
>   
>   struct intel_digital_port {