drm/bridge: dw-hdmi: disable SCDC configuration for invalid setups

This patch is an attempt to limit HDMI 2.0 SCDC setup when :
- the SoC embeds an HDMI 1.4 only controller
- the EDID supports SCDC but not scrambling
- the EDID supports SCDC scrambling but not for low TMDS bit rates,
  while only supporting low TMDS bit rates

This to avoid communicating with the SCDC DDC slave uncessary, and
setting the DW-HDMI TMDS Scrambler setup when not supported by the
underlying hardware.

Reported-by: Rob Herring <robh@kernel.org>
Fixes: 264fce6 ("drm/bridge: dw-hdmi: Add SCDC and TMDS Scrambling support")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190315095414.28520-1-narmstrong@baylibre.com

drivers/gpu/drm/bridge/synopsys/dw-hdmi.c

@@ -1037,6 +1037,31 @@ void dw_hdmi_phy_i2c_write(struct dw_hdmi *hdmi, unsigned short data,
 }
 EXPORT_SYMBOL_GPL(dw_hdmi_phy_i2c_write);
 
+/* Filter out invalid setups to avoid configuring SCDC and scrambling */
+static bool dw_hdmi_support_scdc(struct dw_hdmi *hdmi)
+{
+	struct drm_display_info *display = &hdmi->connector.display_info;
+
+	/* Completely disable SCDC support for older controllers */
+	if (hdmi->version < 0x200a)
+		return false;
+
+	/* Disable if SCDC is not supported, or if an HF-VSDB block is absent */
+	if (!display->hdmi.scdc.supported ||
+	    !display->hdmi.scdc.scrambling.supported)
+		return false;
+
+	/*
+	 * Disable if display only support low TMDS rates and scrambling
+	 * for low rates is not supported either
+	 */
+	if (!display->hdmi.scdc.scrambling.low_rates &&
+	    display->max_tmds_clock <= 340000)
+		return false;
+
+	return true;
+}
+
 /*
  * HDMI2.0 Specifies the following procedure for High TMDS Bit Rates:
  * - The Source shall suspend transmission of the TMDS clock and data
@@ -1055,7 +1080,7 @@ void dw_hdmi_set_high_tmds_clock_ratio(struct dw_hdmi *hdmi)
 	unsigned long mtmdsclock = hdmi->hdmi_data.video_mode.mtmdsclock;
 
 	/* Control for TMDS Bit Period/TMDS Clock-Period Ratio */
-	if (hdmi->connector.display_info.hdmi.scdc.supported) {
+	if (dw_hdmi_support_scdc(hdmi)) {
 		if (mtmdsclock > HDMI14_MAX_TMDSCLK)
 			drm_scdc_set_high_tmds_clock_ratio(hdmi->ddc, 1);
 		else
@@ -1579,8 +1604,9 @@ static void hdmi_av_composer(struct dw_hdmi *hdmi,
 
 	/* Set up HDMI_FC_INVIDCONF */
 	inv_val = (hdmi->hdmi_data.hdcp_enable ||
-		   vmode->mtmdsclock > HDMI14_MAX_TMDSCLK ||
-		   hdmi_info->scdc.scrambling.low_rates ?
+		   (dw_hdmi_support_scdc(hdmi) &&
+		    (vmode->mtmdsclock > HDMI14_MAX_TMDSCLK ||
+		     hdmi_info->scdc.scrambling.low_rates)) ?
 		HDMI_FC_INVIDCONF_HDCP_KEEPOUT_ACTIVE :
 		HDMI_FC_INVIDCONF_HDCP_KEEPOUT_INACTIVE);
 
@@ -1646,7 +1672,7 @@ static void hdmi_av_composer(struct dw_hdmi *hdmi,
 	}
 
 	/* Scrambling Control */
-	if (hdmi_info->scdc.supported) {
+	if (dw_hdmi_support_scdc(hdmi)) {
 		if (vmode->mtmdsclock > HDMI14_MAX_TMDSCLK ||
 		    hdmi_info->scdc.scrambling.low_rates) {
 			/*