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

[Mika Kahola]

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.

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);
 }
 
@@ -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;
+	}
 
 }
 
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 {
-- 
1.9.1