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

[Tom Anderson <thomasanderson@google.com>]

I just realized that at 4:2:2, the pixel clock isn't actually decreased to 3/4
of it's value at 4:4:4. I'll send a revised patch on Monday.

On Fri, Nov 22, 2019 at 09:29:00PM -0800, Thomas Anderson wrote:
> 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@google.com>
> ---
>  .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 30 ++++++++++++++-----
>  1 file changed, 23 insertions(+), 7 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 4139f129eafb..a507a6f04c82 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> @@ -3269,13 +3269,15 @@ static void reduce_mode_colour_depth(struct dc_crtc_timing *timing_out)
>  	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 void adjust_timing_from_display_info(
> +	struct dc_crtc_timing *timing_out,
> +	const struct drm_display_info *info,
> +	const struct drm_display_mode *mode_in)
>  {
>  	int normalized_clk;
> -	if (timing_out->display_color_depth <= COLOR_DEPTH_888)
> +	if (timing_out->display_color_depth < COLOR_DEPTH_888)
>  		return;
> -	do {
> +	while (timing_out->display_color_depth > COLOR_DEPTH_888) {
>  		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)
> @@ -3297,9 +3299,23 @@ static void adjust_colour_depth_from_display_info(struct dc_crtc_timing *timing_
>  		if (normalized_clk <= info->max_tmds_clock)
>  			return;
>  		reduce_mode_colour_depth(timing_out);
> +	}
>  
> -	} while (timing_out->display_color_depth > COLOR_DEPTH_888);
> -
> +	/* The color depth is 888 and cannot be reduced any further, but the
> +	 * clock would still exceed the max tmds clock. Try reducing the pixel
> +	 * encoding next.
> +	 */
> +	if (timing_out->pixel_encoding == PIXEL_ENCODING_RGB ||
> +	    timing_out->pixel_encoding == PIXEL_ENCODING_YCBCR444) {
> +		/* YCBCR422 is always supported. */
> +		timing_out->pixel_encoding = PIXEL_ENCODING_YCBCR422;
> +		normalized_clk = (timing_out->pix_clk_100hz * 3) / 40;
> +		if (normalized_clk <= info->max_tmds_clock)
> +			return;
> +	}
> +	/* YCBCR420 may only be supported on specific modes. */
> +	if (drm_mode_is_420_also(info, mode_in))
> +		timing_out->pixel_encoding = PIXEL_ENCODING_YCBCR420;
>  }
>  
>  static void fill_stream_properties_from_drm_display_mode(
> @@ -3366,7 +3382,7 @@ 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);
> +		adjust_timing_from_display_info(timing_out, info, mode_in);
>  }
>  
>  static void fill_audio_info(struct audio_info *audio_info,
> -- 
> 2.24.0.432.g9d3f5f5b63-goog
>