drm/i915/bxt: Force port A DDI to use 4 lanes

The bspec indicates that DDI A using four lanes is the only valid
configuration for Broxton (Broxton doesn't have a DDI E to split these
lanes with); the DDI_A_4_LANES bit of port A's DDI_BUF_CTL should always
be set by the BIOS.  However some BIOS versions seem to only be setting
this bit if eDP is actually lit up at boot time; if the BIOS doesn't
turn on the eDP panel because an external display is plugged in, then
this bit is never properly initialized.  The end result of this is that
we wind up calculating a lower max data rate than we should and may wind
up rejecting the native mode for panels that we should be able to drive.

Let's workaround this BIOS bug by just turning the DDI_A_4_LANES bit on
in our driver's internal state if we recognize that we're running on BXT
where it should have been on anyway.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Tested-by: Bob Paauwe <bob.j.paauwe@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446764012-27251-1-git-send-email-matthew.d.roper@intel.com

drivers/gpu/drm/i915/intel_ddi.c

@@ -3234,6 +3234,20 @@ void intel_ddi_init(struct drm_device *dev, enum port port)
 					  (DDI_BUF_PORT_REVERSAL |
 					   DDI_A_4_LANES);
 
+	/*
+	 * 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
+	 * configuration so that we use the proper lane count for our
+	 * calculations.
+	 */
+	if (IS_BROXTON(dev) && 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");
+			intel_dig_port->saved_port_bits |= DDI_A_4_LANES;
+		}
+	}
+
 	intel_encoder->type = INTEL_OUTPUT_UNKNOWN;
 	intel_encoder->crtc_mask = (1 << 0) | (1 << 1) | (1 << 2);
 	intel_encoder->cloneable = 0;