drm/i915/edp: read edp display control registers unconditionally

Per my reading of the eDP spec, DP_DPCD_DISPLAY_CONTROL_CAPABLE bit in
DP_EDP_CONFIGURATION_CAP should be set if the eDP display control
registers starting at offset DP_EDP_DPCD_REV are "enabled". Currently we
check the bit before reading the registers, and DP_EDP_DPCD_REV is the
only way to detect eDP revision.

Turns out there are (likely buggy) displays that require eDP 1.4+
features, such as supported link rates and link rate select, but do not
have the bit set. Read the display control registers
unconditionally. They are supposed to read zero anyway if they are not
supported, so there should be no harm in this.

This fixes the referenced bug by enabling the eDP version check, and
thus reading of the supported link rates. The panel in question has 0 in
DP_MAX_LINK_RATE which is only supported in eDP 1.4+. Without the
supported link rates method we default to RBR which is insufficient for
the panel native mode. As a curiosity, the panel also has a bogus value
of 0x12 in DP_EDP_DPCD_REV, but that passes our check for >= DP_EDP_14
(which is 0x03).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103400Reported-and-tested-by: Nicolas P. <issun.artiste@gmail.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171026142932.17737-1-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -3735,9 +3735,16 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp)
 
 	}
 
-	/* Read the eDP Display control capabilities registers */
-	if ((intel_dp->dpcd[DP_EDP_CONFIGURATION_CAP] & DP_DPCD_DISPLAY_CONTROL_CAPABLE) &&
-	    drm_dp_dpcd_read(&intel_dp->aux, DP_EDP_DPCD_REV,
+	/*
+	 * Read the eDP display control registers.
+	 *
+	 * Do this independent of DP_DPCD_DISPLAY_CONTROL_CAPABLE bit in
+	 * DP_EDP_CONFIGURATION_CAP, because some buggy displays do not have it
+	 * set, but require eDP 1.4+ detection (e.g. for supported link rates
+	 * method). The display control registers should read zero if they're
+	 * not supported anyway.
+	 */
+	if (drm_dp_dpcd_read(&intel_dp->aux, DP_EDP_DPCD_REV,
 			     intel_dp->edp_dpcd, sizeof(intel_dp->edp_dpcd)) ==
 			     sizeof(intel_dp->edp_dpcd))
 		DRM_DEBUG_KMS("EDP DPCD : %*ph\n", (int) sizeof(intel_dp->edp_dpcd),