drm/i915/dp: Rework HDMI DFP TMDS clock handling

Rework the HDMI DFP TMDS clock checks to also check at 8bpc.
Previously we only checked the deep color cases. But I suppose
a sink could potentially declare "4:2:0 also" modes that only
actually fit within its own limits when using 4:2:0. Even if
that is too nuts to be real there is no real harm in running
through the full checks for everything.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220322120015.28074-9-ville.syrjala@linux.intel.comReviewed-by: Uma Shankar <uma.shankar@intel.com>

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

@@ -1136,16 +1136,28 @@ static bool intel_dp_is_ycbcr420(struct intel_dp *intel_dp,
 		 intel_dp->dfp.ycbcr_444_to_420);
 }
 
-static bool intel_dp_hdmi_bpc_possible(struct intel_dp *intel_dp,
+static int intel_dp_hdmi_compute_bpc(struct intel_dp *intel_dp,
 				     const struct intel_crtc_state *crtc_state,
 				     int bpc)
 {
 	bool ycbcr420_output = intel_dp_is_ycbcr420(intel_dp, crtc_state);
 	int clock = crtc_state->hw.adjusted_mode.crtc_clock;
 
-	return intel_hdmi_bpc_possible(crtc_state, bpc,
+	/*
+	 * Current bpc could already be below 8bpc due to
+	 * FDI bandwidth constraints or other limits.
+	 * HDMI minimum is 8bpc however.
+	 */
+	bpc = max(bpc, 8);
+
+	for (; bpc >= 8; bpc -= 2) {
+		if (intel_hdmi_bpc_possible(crtc_state, bpc,
 					    intel_dp->has_hdmi_sink, ycbcr420_output) &&
-		intel_dp_tmds_clock_valid(intel_dp, clock, bpc, ycbcr420_output) == MODE_OK;
+		    intel_dp_tmds_clock_valid(intel_dp, clock, bpc, ycbcr420_output) == MODE_OK)
+			return bpc;
+	}
+
+	return -EINVAL;
 }
 
 static int intel_dp_max_bpp(struct intel_dp *intel_dp,
@@ -1161,10 +1173,13 @@ static int intel_dp_max_bpp(struct intel_dp *intel_dp,
 		bpc = min_t(int, bpc, intel_dp->dfp.max_bpc);
 
 	if (intel_dp->dfp.min_tmds_clock) {
-		for (; bpc >= 10; bpc -= 2) {
-			if (intel_dp_hdmi_bpc_possible(intel_dp, crtc_state, bpc))
-				break;
-		}
+		int max_hdmi_bpc;
+
+		max_hdmi_bpc = intel_dp_hdmi_compute_bpc(intel_dp, crtc_state, bpc);
+		if (max_hdmi_bpc < 0)
+			return 0;
+
+		bpc = min(bpc, max_hdmi_bpc);
 	}
 
 	bpp = bpc * 3;