drm/vc4: hdmi: Always try to have the highest bpc

Currently we take the max_bpc property as the bpc value and do not try
anything else.

However, what the other drivers seem to be doing is that they would try
with the highest bpc allowed by the max_bpc property and the hardware
capabilities, test if it results in an acceptable configuration, and if
not decrease the bpc and try again.

Let's use the same logic.
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220222164042.403112-7-maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_hdmi.c

@@ -351,6 +351,7 @@ vc4_hdmi_connector_duplicate_state(struct drm_connector *connector)
 		return NULL;
 
 	new_state->tmds_char_rate = vc4_state->tmds_char_rate;
+	new_state->output_bpc = vc4_state->output_bpc;
 	__drm_atomic_helper_connector_duplicate_state(connector, &new_state->base);
 
 	return &new_state->base;
@@ -905,6 +906,8 @@ static void vc5_hdmi_set_timings(struct vc4_hdmi *vc4_hdmi,
 				 struct drm_connector_state *state,
 				 struct drm_display_mode *mode)
 {
+	const struct vc4_hdmi_connector_state *vc4_state =
+		conn_state_to_vc4_hdmi_conn_state(state);
 	bool hsync_pos = mode->flags & DRM_MODE_FLAG_PHSYNC;
 	bool vsync_pos = mode->flags & DRM_MODE_FLAG_PVSYNC;
 	bool interlaced = mode->flags & DRM_MODE_FLAG_INTERLACE;
@@ -952,7 +955,7 @@ static void vc5_hdmi_set_timings(struct vc4_hdmi *vc4_hdmi,
 	HDMI_WRITE(HDMI_VERTB0, vertb_even);
 	HDMI_WRITE(HDMI_VERTB1, vertb);
 
-	switch (state->max_bpc) {
+	switch (vc4_state->output_bpc) {
 	case 12:
 		gcp = 6;
 		gcp_en = true;
@@ -1245,11 +1248,13 @@ static void vc4_hdmi_encoder_atomic_mode_set(struct drm_encoder *encoder,
 					     struct drm_connector_state *conn_state)
 {
 	struct vc4_hdmi *vc4_hdmi = encoder_to_vc4_hdmi(encoder);
+	struct vc4_hdmi_connector_state *vc4_state =
+		conn_state_to_vc4_hdmi_conn_state(conn_state);
 
 	mutex_lock(&vc4_hdmi->mutex);
 	drm_mode_copy(&vc4_hdmi->saved_adjusted_mode,
 		      &crtc_state->adjusted_mode);
-	vc4_hdmi->output_bpc = conn_state->max_bpc;
+	vc4_hdmi->output_bpc = vc4_state->output_bpc;
 	mutex_unlock(&vc4_hdmi->mutex);
 }
 
@@ -1304,6 +1309,38 @@ vc4_hdmi_encoder_compute_clock(const struct vc4_hdmi *vc4_hdmi,
 	return 0;
 }
 
+static int
+vc4_hdmi_encoder_compute_config(const struct vc4_hdmi *vc4_hdmi,
+				struct vc4_hdmi_connector_state *vc4_state,
+				const struct drm_display_mode *mode)
+{
+	struct drm_connector_state *conn_state = &vc4_state->base;
+	unsigned int max_bpc = clamp_t(unsigned int, conn_state->max_bpc, 8, 12);
+	unsigned int bpc;
+	int ret;
+
+	for (bpc = max_bpc; bpc >= 8; bpc -= 2) {
+		drm_dbg(dev, "Trying with a %d bpc output\n", bpc);
+
+		ret = vc4_hdmi_encoder_compute_clock(vc4_hdmi, vc4_state,
+						     mode, bpc);
+		if (ret)
+			continue;
+
+		vc4_state->output_bpc = bpc;
+
+		drm_dbg(dev,
+			"Mode %ux%u @ %uHz: Found configuration: bpc: %u, clock: %llu\n",
+			mode->hdisplay, mode->vdisplay, drm_mode_vrefresh(mode),
+			vc4_state->output_bpc,
+			vc4_state->tmds_char_rate);
+
+		break;
+	}
+
+	return ret;
+}
+
 #define WIFI_2_4GHz_CH1_MIN_FREQ	2400000000ULL
 #define WIFI_2_4GHz_CH1_MAX_FREQ	2422000000ULL
 
@@ -1337,8 +1374,7 @@ static int vc4_hdmi_encoder_atomic_check(struct drm_encoder *encoder,
 		tmds_char_rate = mode->clock * 1000;
 	}
 
-	ret = vc4_hdmi_encoder_compute_clock(vc4_hdmi, vc4_state, mode,
-					     conn_state->max_bpc);
+	ret = vc4_hdmi_encoder_compute_config(vc4_hdmi, vc4_state, mode);
 	if (ret)
 		return ret;
 

drivers/gpu/drm/vc4/vc4_hdmi.h

@@ -218,7 +218,8 @@ struct vc4_hdmi {
 	bool scdc_enabled;
 
 	/**
-	 * @output_bpc: BPC currently being used. Protected by @mutex.
+	 * @output_bpc: Copy of @vc4_connector_state.output_bpc for use
+	 * outside of KMS hooks. Protected by @mutex.
 	 */
 	unsigned int output_bpc;
 };
@@ -240,6 +241,7 @@ encoder_to_vc4_hdmi(struct drm_encoder *encoder)
 struct vc4_hdmi_connector_state {
 	struct drm_connector_state	base;
 	unsigned long long		tmds_char_rate;
+	unsigned int 			output_bpc;
 };
 
 static inline struct vc4_hdmi_connector_state *