[PATCH] drm/i915/cnl: Force DDI_A_4_LANES when needed.

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

As we faced in BXT, on CNL DDI_A_4_LANES is not
set as expected when system is boot with multiple
monitors connected. This result in wrong lane
setup impacting the max data rate available and
consequently blocking modeset on eDP, resulting
in a blank screen.

Most of CNL SKUs don't support DDI-E.
The only SKU that supports DDI-E is the same
that supports the full A/E split called DDI-F.

Also when DDI-F is used DDI-E cannot be used because
they share Interrupts. So DDI-E is almost useless.
Anyways let's consider this is possible and rely on
VBT for that.

Since this become a trend this commit also adds
an extra commit message so in the future we have
an extra insight when we are dealing with this
same case.

This patch was initialy start by Clint, but required
many changes including full commit message. So
Credits entirely to Clint for finding this.

Suggested-by: Clint Taylor <clinton.a.taylor@intel.com>
Cc: Clint Taylor <clinton.a.taylor@intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
---
 drivers/gpu/drm/i915/intel_ddi.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_ddi.c b/drivers/gpu/drm/i915/intel_ddi.c
index adf51b328844..8acacf800a24 100644
--- a/drivers/gpu/drm/i915/intel_ddi.c
+++ b/drivers/gpu/drm/i915/intel_ddi.c
@@ -2803,17 +2803,29 @@ void intel_ddi_init(struct drm_i915_private *dev_priv, enum port port)
 	}
 
 	/*
-	 * Bspec says that DDI_A_4_LANES is the only supported configuration
-	 * for Broxton.  Yet some BIOS fail to set this bit on port A if eDP
-	 * wasn't lit up at boot.  Force this bit on in our internal
+	 * Some BIOS might fail to set this bit on port A if eDP
+	 * wasn't lit up at boot.  Force this bit when needed on in our internal
 	 * configuration so that we use the proper lane count for our
 	 * calculations.
 	 */
-	if (IS_GEN9_LP(dev_priv) && port == PORT_A) {
-		if (!(intel_dig_port->saved_port_bits & DDI_A_4_LANES)) {
-			DRM_DEBUG_KMS("BXT BIOS forgot to set DDI_A_4_LANES for port A; fixing\n");
+	if (port == PORT_A &&
+	    !(intel_dig_port->saved_port_bits & DDI_A_4_LANES)) {
+
+		/* Broxton: Bspec says that DDI_A_4_LANES is the only supported
+		 *          configuration
+		 * Cannonlake: Most of SKUs don't support DDI_E, and the only
+		 *             one who does also have a full A/E split called
+		 *             DDI_F what makes DDI_E useless. However for this
+		 *             case let's trust VBT info.
+		 */
+		if (IS_GEN9_LP(dev_priv) ||
+		    ((IS_CANNONLAKE(dev_priv)) &&
+		     !intel_bios_is_port_present(dev_priv, PORT_E))) {
+			DRM_DEBUG_KMS("Forcing DDI_A_4_LANES for port A\n");
 			intel_dig_port->saved_port_bits |= DDI_A_4_LANES;
 			max_lanes = 4;
+		} else {
+			DRM_DEBUG_KMS("DDI A configured for 2 lanes\n");
 		}
 	}
 
-- 
2.13.5

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