drm/i915: hsw: fix link training for eDP on port-A

According to BSpec the link training sequence for eDP on HSW port-A
should be as follows:

1. link training: clock recovery
2. link training: equalization
3. link training: set idle transmission mode
4. display pipe enable
5. link training: disable (set normal mode)

Contrary to this at the moment we don't do step 3. and we do step 5.
before step 4. Fix this by setting idle transmission mode for eDP at
the end of intel_dp_complete_link_train and adding a new
intel_dp_stop_link_training function to disable link training. With
these changes we'll end up with the following functions corresponding
to the above steps:

intel_dp_start_link_train    -> step 1.
intel_dp_complete_link_train -> step 2., step 3.
intel_dp_stop_link_train     -> step 5.

For port-A we'll call intel_dp_stop_link_train only after enabling the
pipe, for everything else we'll call it right after
intel_dp_complete_link_train to preserve the current behavior.

Tested on HSW/HSW-ULT.

In v2:
- Due to a HW issue we must set idle transmission mode for port-A too
  before enabling the pipe. Thanks for Arthur Runyan for explaining
  this.
- Update the patch subject to make it clear that it's an eDP fix, DP is
  not affected.

v3:
- rename intel_dp_link_train() to intel_dp_set_link_train(), use 'val'
  instead 'l' as var name. (Paulo)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 5 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/intel_ddi.c b/drivers/gpu/drm/i915/intel_ddi.c
index 26a0a570f92..fb961bb8190 100644
--- a/drivers/gpu/drm/i915/intel_ddi.c
+++ b/drivers/gpu/drm/i915/intel_ddi.c
@@ -1265,6 +1265,8 @@ static void intel_ddi_pre_enable(struct intel_encoder *intel_encoder)
 		intel_dp_sink_dpms(intel_dp, DRM_MODE_DPMS_ON);
 		intel_dp_start_link_train(intel_dp);
 		intel_dp_complete_link_train(intel_dp);
+		if (port != PORT_A)
+			intel_dp_stop_link_train(intel_dp);
 	}
 }
 
@@ -1326,6 +1328,9 @@ static void intel_enable_ddi(struct intel_encoder *intel_encoder)
 	} else if (type == INTEL_OUTPUT_EDP) {
 		struct intel_dp *intel_dp = enc_to_intel_dp(encoder);
 
+		if (port == PORT_A)
+			intel_dp_stop_link_train(intel_dp);
+
 		ironlake_edp_backlight_on(intel_dp);
 	}