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Subject: Re: [PATCH v2] drm/amd/display: Reduce HDMI pixel encoding if max
 clock is exceeded
To:     Thomas Anderson <thomasanderson@google.com>,
        Harry Wentland <harry.wentland@amd.com>,
        Leo Li <sunpeng.li@amd.com>,
        Mikita Lipski <mikita.lipski@amd.com>,
        Alex Deucher <alexander.deucher@amd.com>
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        David Zhou <David1.Zhou@amd.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>,
        Mario Kleiner <mario.kleiner.de@gmail.com>,
        amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org
References: <20191202214713.41001-1-thomasanderson@google.com>
From:   Harry Wentland <hwentlan@amd.com>
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Date:   Thu, 2 Jan 2020 10:14:49 -0500
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On 2019-12-02 4:47 p.m., 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>

Apologies for the late response.

Thanks for getting high-res modes working on HDMI.

This change is
Reviewed-by: Harry Wentland <harry.wentland@amd.com>

Harry

> ---
>  .../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,
>