[PATCH v8] 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:

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

which were eventually hand-reverted by:

commit 34511dce4b35 ("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 4e96c97742f4 ("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
v5: * Checkpatch cleanup
    * Rebase
v6: * is_edp() => intel_dp_is_edp()
    * rebase
v7: * Remove extraneous is_edp() prototype (Rodrigo)
v8: 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")
Reviewed-by: Manasi Navare <manasi.d.navare at intel.com>
Signed-off-by: Jim Bride <jim.bride at linux.intel.com>
---
 drivers/gpu/drm/i915/intel_dp.c               |  2 ++
 drivers/gpu/drm/i915/intel_dp_link_training.c | 15 ++++++++++++++-
 drivers/gpu/drm/i915/intel_drv.h              |  1 +
 3 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 8db6b11..aee7e9b 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -4748,6 +4748,7 @@ 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);
@@ -6017,6 +6018,7 @@ 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 05907fa..79fe369 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 = intel_dp_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 3078076..40e98ed 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -987,6 +987,7 @@ 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;
-- 
2.7.4