[PATCH v2] drm/amd/display: Reduce HDMI pixel encoding if max clock is exceeded

[Thomas Anderson]

For high-res (8K) or HFR (4K120) displays, using uncompressed pixel
formats like YCbCr444 would exceed the bandwidth of HDMI 2.0, so the
"interesting" modes would be disabled, leaving only low-res or low
framerate modes.

This change lowers the pixel encoding to 4:2:2 or 4:2:0 if the max TMDS
clock is exceeded. Verified that 8K30 and 4K120 are now available and
working with a Samsung Q900R over an HDMI 2.0b link from a Radeon 5700.

Signed-off-by: Thomas Anderson <thomasanderson at google.com>
---
 .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 45 ++++++++++---------
 1 file changed, 23 insertions(+), 22 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index 7aac9568d3be..803e59d97411 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -3356,27 +3356,21 @@ get_output_color_space(const struct dc_crtc_timing *dc_crtc_timing)
 	return color_space;
 }
 
-static void reduce_mode_colour_depth(struct dc_crtc_timing *timing_out)
-{
-	if (timing_out->display_color_depth <= COLOR_DEPTH_888)
-		return;
-
-	timing_out->display_color_depth--;
-}
-
-static void adjust_colour_depth_from_display_info(struct dc_crtc_timing *timing_out,
-						const struct drm_display_info *info)
+static bool adjust_colour_depth_from_display_info(
+	struct dc_crtc_timing *timing_out,
+	const struct drm_display_info *info)
 {
+	enum dc_color_depth depth = timing_out->display_color_depth;
 	int normalized_clk;
-	if (timing_out->display_color_depth <= COLOR_DEPTH_888)
-		return;
 	do {
 		normalized_clk = timing_out->pix_clk_100hz / 10;
 		/* YCbCr 4:2:0 requires additional adjustment of 1/2 */
 		if (timing_out->pixel_encoding == PIXEL_ENCODING_YCBCR420)
 			normalized_clk /= 2;
 		/* Adjusting pix clock following on HDMI spec based on colour depth */
-		switch (timing_out->display_color_depth) {
+		switch (depth) {
+		case COLOR_DEPTH_888:
+			break;
 		case COLOR_DEPTH_101010:
 			normalized_clk = (normalized_clk * 30) / 24;
 			break;
@@ -3387,14 +3381,15 @@ static void adjust_colour_depth_from_display_info(struct dc_crtc_timing *timing_
 			normalized_clk = (normalized_clk * 48) / 24;
 			break;
 		default:
-			return;
+			/* The above depths are the only ones valid for HDMI. */
+			return false;
 		}
-		if (normalized_clk <= info->max_tmds_clock)
-			return;
-		reduce_mode_colour_depth(timing_out);
-
-	} while (timing_out->display_color_depth > COLOR_DEPTH_888);
-
+		if (normalized_clk <= info->max_tmds_clock) {
+			timing_out->display_color_depth = depth;
+			return true;
+		}
+	} while (--depth > COLOR_DEPTH_666);
+	return false;
 }
 
 static void fill_stream_properties_from_drm_display_mode(
@@ -3474,8 +3469,14 @@ static void fill_stream_properties_from_drm_display_mode(
 
 	stream->out_transfer_func->type = TF_TYPE_PREDEFINED;
 	stream->out_transfer_func->tf = TRANSFER_FUNCTION_SRGB;
-	if (stream->signal == SIGNAL_TYPE_HDMI_TYPE_A)
-		adjust_colour_depth_from_display_info(timing_out, info);
+	if (stream->signal == SIGNAL_TYPE_HDMI_TYPE_A) {
+		if (!adjust_colour_depth_from_display_info(timing_out, info) &&
+		    drm_mode_is_420_also(info, mode_in) &&
+		    timing_out->pixel_encoding != PIXEL_ENCODING_YCBCR420) {
+			timing_out->pixel_encoding = PIXEL_ENCODING_YCBCR420;
+			adjust_colour_depth_from_display_info(timing_out, info);
+		}
+	}
 }
 
 static void fill_audio_info(struct audio_info *audio_info,
-- 
2.24.0.393.g34dc348eaf-goog