drm/i915/display: Configure HDMI2.1 Pcon for FRL only if Src-Ctl mode is available

Add the check if source control mode is supported by the
PCON, before starting configuring PCON for FRL training,
as per spec VESA DP2.0-HDMI2.1 PCON Draft-1 Sec-7.

v2: Added spec details for the change. (Uma)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210323112422.1211-3-ankit.k.nautiyal@intel.com

drivers/gpu/drm/i915/display/intel_dp.c

@@ -2155,8 +2155,13 @@ void intel_dp_check_frl_training(struct intel_dp *intel_dp)
 {
 	struct drm_i915_private *dev_priv = dp_to_i915(intel_dp);
 
-	/* Always go for FRL training if supported */
-	if (!intel_dp_is_hdmi_2_1_sink(intel_dp) ||
+	/*
+	 * Always go for FRL training if:
+	 * -PCON supports SRC_CTL_MODE (VESA DP2.0-HDMI2.1 PCON Spec Draft-1 Sec-7)
+	 * -sink is HDMI2.1
+	 */
+	if (!(intel_dp->dpcd[2] & DP_PCON_SOURCE_CTL_MODE) ||
+	    !intel_dp_is_hdmi_2_1_sink(intel_dp) ||
 	    intel_dp->frl.is_trained)
 		return;