drm/i915/dp: Check error return during DPCD capability queries

The assumptions of these users of drm_dp_dpcd_readb() is that the passed
in output buffer won't change in case of error, but this isn't
guaranteed. Fix this by treating any error as the lack of the given
capability.

In case of DP_SINK_DEVICE_AUX_FRAME_SYNC_CAP an error would leave the
buffer uninitialized even with the above assumption.
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1494408113-379-2-git-send-email-imre.deak@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -3088,7 +3088,8 @@ static bool intel_dp_get_y_cord_status(struct intel_dp *intel_dp)
 {
 	uint8_t psr_caps = 0;
 
-	drm_dp_dpcd_readb(&intel_dp->aux, DP_PSR_CAPS, &psr_caps);
+	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_PSR_CAPS, &psr_caps) != 1)
+		return false;
 	return psr_caps & DP_PSR2_SU_Y_COORDINATE_REQUIRED;
 }
 
@@ -3096,9 +3097,9 @@ static bool intel_dp_get_colorimetry_status(struct intel_dp *intel_dp)
 {
 	uint8_t dprx = 0;
 
-	drm_dp_dpcd_readb(&intel_dp->aux,
-			DP_DPRX_FEATURE_ENUMERATION_LIST,
-			&dprx);
+	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_DPRX_FEATURE_ENUMERATION_LIST,
+			      &dprx) != 1)
+		return false;
 	return dprx & DP_VSC_SDP_EXT_FOR_COLORIMETRY_SUPPORTED;
 }
 
@@ -3106,7 +3107,9 @@ static bool intel_dp_get_alpm_status(struct intel_dp *intel_dp)
 {
 	uint8_t alpm_caps = 0;
 
-	drm_dp_dpcd_readb(&intel_dp->aux, DP_RECEIVER_ALPM_CAP, &alpm_caps);
+	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_RECEIVER_ALPM_CAP,
+			      &alpm_caps) != 1)
+		return false;
 	return alpm_caps & DP_ALPM_CAP;
 }
 
@@ -3679,9 +3682,10 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp)
 		uint8_t frame_sync_cap;
 
 		dev_priv->psr.sink_support = true;
-		drm_dp_dpcd_readb(&intel_dp->aux,
+		if (drm_dp_dpcd_readb(&intel_dp->aux,
 				      DP_SINK_DEVICE_AUX_FRAME_SYNC_CAP,
-				  &frame_sync_cap);
+				      &frame_sync_cap) != 1)
+			frame_sync_cap = 0;
 		dev_priv->psr.aux_frame_sync = frame_sync_cap ? true : false;
 		/* PSR2 needs frame sync as well */
 		dev_priv->psr.psr2_support = dev_priv->psr.aux_frame_sync;