[Intel-gfx] [PATCH v4 3/4] drm/i915/edp: Be less aggressive about changing link config on eDP

[Jim Bride]

This set of changes has some history to them.  There were several attempts
to add what was called "fast link training" to i915, which actually wasn't
fast link training as per the DP spec.  These changes were

5fa836a9d859 ("drm/i915: DP link training optimization")
4e96c97742f4 ("drm/i915: eDP link training optimization")

which were eventually hand-reverted by

34511dce4 ("drm/i915: Revert DisplayPort fast link training feature")

in kernel 4.7-rc4.  The eDP pieces of the above revert, however, had some
very bad side-effects on PSR functionality on Skylake. The issue at
hand is that when PSR exits i915 briefly emits TP1 followed by TP2/3
(depending on the original link configuration) in order to quickly get
the source and sink back in synchronization across the link before handing
control back to the i915.  There's an assumption that none of the link
configuration information has changed (and thus it's still valid) since the
last full link training operation.  The revert above was identified via a
bisect as the cause of some of Skylake's PSR woes.  This patch, largely
based on

commit 4e96c97742f4201edf1b0f8e1b1b6b2ac6ff33e7
Author: Mika Kahola <mika.kahola at intel.com>
Date:   Wed Apr 29 09:17:39 2015 +0300
drm/i915: eDP link training optimization

puts the eDP portions of this patch back in place.  None of the flickering
issues that spurred the revert have been seen, and I suspect the real
culprits here were addressed by some of the recent link training changes
that Manasi has implemented, and PSR on Skylake is definitely more happy
with these changes in-place.

v2 and v3: Rebase
v4: * Clean up accesses to train_set_valid a bit for easier reading. (Chris)
    * Rebase

Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni at intel.com>
Cc: Manasi D Navare <manasi.d.navare at intel.com>
Cc: Mika Kahola <mika.kahola at intel.com>
Cc: Jani Nikula <jani.nikula at intel.com>
Fixes: 34511dce4 ("drm/i915: Revert DisplayPort fast link training feature")
Signed-off-by: Jim Bride <jim.bride at linux.intel.com>
---
 drivers/gpu/drm/i915/intel_dp.c               |  4 +++-
 drivers/gpu/drm/i915/intel_dp_link_training.c | 15 ++++++++++++++-
 drivers/gpu/drm/i915/intel_drv.h              |  2 ++
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index c90ca1c..7c0e530 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -106,7 +106,7 @@ static const int default_rates[] = { 162000, 270000, 540000 };
  * If a CPU or PCH DP output is attached to an eDP panel, this function
  * will return true, and false otherwise.
  */
-static bool is_edp(struct intel_dp *intel_dp)
+bool is_edp(struct intel_dp *intel_dp)
 {
 	struct intel_digital_port *intel_dig_port = dp_to_dig_port(intel_dp);
 
@@ -4738,6 +4738,7 @@ intel_dp_long_pulse(struct intel_connector *intel_connector)
 		intel_dp->max_link_rate = intel_dp_max_common_rate(intel_dp);
 
 		intel_dp->reset_link_params = false;
+		intel_dp->train_set_valid = false;
 	}
 
 	intel_dp_print_rates(intel_dp);
@@ -6008,6 +6009,7 @@ intel_dp_init_connector(struct intel_digital_port *intel_dig_port,
 	intel_dp_set_source_rates(intel_dp);
 
 	intel_dp->reset_link_params = true;
+	intel_dp->train_set_valid = false;
 	intel_dp->pps_pipe = INVALID_PIPE;
 	intel_dp->active_pipe = INVALID_PIPE;
 
diff --git a/drivers/gpu/drm/i915/intel_dp_link_training.c b/drivers/gpu/drm/i915/intel_dp_link_training.c
index b79c1c0..d12200d 100644
--- a/drivers/gpu/drm/i915/intel_dp_link_training.c
+++ b/drivers/gpu/drm/i915/intel_dp_link_training.c
@@ -94,7 +94,8 @@ static bool
 intel_dp_reset_link_train(struct intel_dp *intel_dp,
 			uint8_t dp_train_pat)
 {
-	memset(intel_dp->train_set, 0, sizeof(intel_dp->train_set));
+	if (!intel_dp->train_set_valid)
+		memset(intel_dp->train_set, 0, sizeof(intel_dp->train_set));
 	intel_dp_set_signal_levels(intel_dp);
 	return intel_dp_set_link_train(intel_dp, dp_train_pat);
 }
@@ -162,9 +163,18 @@ intel_dp_link_training_clock_recovery(struct intel_dp *intel_dp)
 				       DP_TRAINING_PATTERN_1 |
 				       DP_LINK_SCRAMBLING_DISABLE)) {
 		DRM_ERROR("failed to enable link training\n");
+		intel_dp->train_set_valid = false;
 		return false;
 	}
 
+	/*
+	 * The initial set of link parameters are set by this point, so go
+	 * ahead and set intel_dp->train_set_valid to false in case any of
+	 * the succeeding steps fail.  It will be set back to true if we were
+	 * able to achieve clock recovery in the specified configuration.
+	 */
+	intel_dp->train_set_valid = false;
+
 	voltage_tries = 1;
 	max_vswing_tries = 0;
 	for (;;) {
@@ -179,6 +189,7 @@ intel_dp_link_training_clock_recovery(struct intel_dp *intel_dp)
 
 		if (drm_dp_clock_recovery_ok(link_status, intel_dp->lane_count)) {
 			DRM_DEBUG_KMS("clock recovery OK\n");
+			intel_dp->train_set_valid = is_edp(intel_dp);
 			return true;
 		}
 
@@ -256,6 +267,7 @@ intel_dp_link_training_channel_equalization(struct intel_dp *intel_dp)
 				     training_pattern |
 				     DP_LINK_SCRAMBLING_DISABLE)) {
 		DRM_ERROR("failed to start channel equalization\n");
+		intel_dp->train_set_valid = false;
 		return false;
 	}
 
@@ -296,6 +308,7 @@ intel_dp_link_training_channel_equalization(struct intel_dp *intel_dp)
 	/* Try 5 times, else fail and try at lower BW */
 	if (tries == 5) {
 		intel_dp_dump_link_status(link_status);
+		intel_dp->train_set_valid = false;
 		DRM_DEBUG_KMS("Channel equalization failed 5 times\n");
 	}
 
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 0902d7c..e45163a 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -995,6 +995,7 @@ struct intel_dp {
 	struct drm_dp_aux aux;
 	enum intel_display_power_domain aux_power_domain;
 	uint8_t train_set[4];
+	bool train_set_valid;
 	int panel_power_up_delay;
 	int panel_power_down_delay;
 	int panel_power_cycle_delay;
@@ -1570,6 +1571,7 @@ static inline unsigned int intel_dp_unused_lane_mask(int lane_count)
 }
 
 bool intel_dp_read_dpcd(struct intel_dp *intel_dp);
+bool is_edp(struct intel_dp *intel_dp);
 int intel_dp_link_required(int pixel_clock, int bpp);
 int intel_dp_max_data_rate(int max_link_clock, int max_lanes);
 bool intel_digital_port_connected(struct drm_i915_private *dev_priv,
-- 
2.7.4