drm/i915/edp: don't write to DP_LINK_BW_SET when using rate select

The eDP 1.5 spec adds a clarification for eDP 1.4x:

> For eDP v1.4x, if the Source device chooses the Main-Link rate by way
> of DPCD 00100h, the Sink device shall ignore DPCD 00115h[2:0].

We write 0 to DP_LINK_BW_SET (DPCD 100h) even when using
DP_LINK_RATE_SET (DPCD 114h). Stop doing that, as it can cause the panel
to ignore the rate set method.

Moreover, 0 is a reserved value for DP_LINK_BW_SET, and should not be
used.

v2: Improve the comments (Ville)

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/9081Tested-by: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231205180551.2476228-1-jani.nikula@intel.com

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

@@ -650,19 +650,30 @@ intel_dp_update_link_bw_set(struct intel_dp *intel_dp,
 			    const struct intel_crtc_state *crtc_state,
 			    u8 link_bw, u8 rate_select)
 {
-	u8 link_config[2];
+	u8 lane_count = crtc_state->lane_count;
 
-	/* Write the link configuration data */
-	link_config[0] = link_bw;
-	link_config[1] = crtc_state->lane_count;
 	if (crtc_state->enhanced_framing)
-		link_config[1] |= DP_LANE_COUNT_ENHANCED_FRAME_EN;
-	drm_dp_dpcd_write(&intel_dp->aux, DP_LINK_BW_SET, link_config, 2);
+		lane_count |= DP_LANE_COUNT_ENHANCED_FRAME_EN;
+
+	if (link_bw) {
+		/* DP and eDP v1.3 and earlier link bw set method. */
+		u8 link_config[] = { link_bw, lane_count };
 
-	/* eDP 1.4 rate select method. */
-	if (!link_bw)
-		drm_dp_dpcd_write(&intel_dp->aux, DP_LINK_RATE_SET,
-				  &rate_select, 1);
+		drm_dp_dpcd_write(&intel_dp->aux, DP_LINK_BW_SET, link_config,
+				  ARRAY_SIZE(link_config));
+	} else {
+		/*
+		 * eDP v1.4 and later link rate set method.
+		 *
+		 * eDP v1.4x sinks shall ignore DP_LINK_RATE_SET if
+		 * DP_LINK_BW_SET is set. Avoid writing DP_LINK_BW_SET.
+		 *
+		 * eDP v1.5 sinks allow choosing either, and the last choice
+		 * shall be active.
+		 */
+		drm_dp_dpcd_writeb(&intel_dp->aux, DP_LANE_COUNT_SET, lane_count);
+		drm_dp_dpcd_writeb(&intel_dp->aux, DP_LINK_RATE_SET, rate_select);
+	}
 }
 
 /*