linux-kernel - Re: [PATCH v4 2/2] drm/amd: use fixed dsc bits-per-pixel from edid

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Message-ID: <9c469725-acaf-4450-aff2-4b5d03f8d947@0la.ch>
Date: Thu, 16 Oct 2025 19:22:44 +0200
From: Yaroslav <iam@....ch>
To: Jani Nikula <jani.nikula@...ux.intel.com>, Yaroslav Bolyukin
 <iam@...h.pw>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>
Cc: Harry Wentland <harry.wentland@....com>, Leo Li <sunpeng.li@....com>,
 Rodrigo Siqueira <siqueira@...lia.com>,
 Alex Deucher <alexander.deucher@....com>,
 Christian König <christian.koenig@....com>,
 Wayne Lin <Wayne.Lin@....com>, amd-gfx@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v4 2/2] drm/amd: use fixed dsc bits-per-pixel from edid

On 2025-10-16 18:39, Jani Nikula wrote:
> On Thu, 16 Oct 2025, Yaroslav Bolyukin <iam@...h.pw> wrote:
>> VESA vendor header from DisplayID spec may contain fixed bit per pixel
>> rate, it should be respected by drm driver
>>
>> Signed-off-by: Yaroslav Bolyukin <iam@...h.pw>
>> ---
>>   drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 14 +++++++++++++-
>>   1 file changed, 13 insertions(+), 1 deletion(-)
>>
>> 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 0d03e324d5b9..ebe5bb4eecf8 100644
>> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
>> @@ -6521,6 +6521,11 @@ static void fill_stream_properties_from_drm_display_mode(
>>   
>>   	stream->output_color_space = get_output_color_space(timing_out, connector_state);
>>   	stream->content_type = get_output_content_type(connector_state);
>> +
>> +	/* DisplayID Type VII pass-through timings. */
>> +	if (mode_in->dsc_passthrough_timings_support && info->dp_dsc_bpp != 0) {
>> +		stream->timing.dsc_fixed_bits_per_pixel_x16 = info->dp_dsc_bpp;
>> +	}
> 
> If we had mode->dp_dsc_bpp_x16 directly, or something better named, this
> would be simpler. This will eventually be replicated in a bunch of
> drivers.
> 
> BR,
> Jani.

Unfortunately, that won't solve the problem that there is no good way to 
pass this value from display_modes created from e.g 
DRM_IOCTL_MODE_SETCRTC. I believe it would be better if there was a 
syscall to inherit from a native mode instead of creating a new one, but 
this is too far fetched.

I'll check how this was implemented by nvidia open driver, as they have 
added support for this value in their driver somehow.
  >>   }
>>   
>>   static void fill_audio_info(struct audio_info *audio_info,
>> @@ -7067,6 +7072,13 @@ create_stream_for_sink(struct drm_connector *connector,
>>   					&mode, preferred_mode, scale);
>>   
>>   			preferred_refresh = drm_mode_vrefresh(preferred_mode);
>> +
>> +			/*
>> +			 * HACK: In case of multiple supported modes, we should look at the matching mode to decide this flag.
>> +			 * But what is matching mode, how should it be decided?
>> +			 * Assuming that only preferred mode would have this flag.
>> +			 */
>> +			mode.dsc_passthrough_timings_support = preferred_mode->dsc_passthrough_timings_support;
>>   		}
>>   	}
>>   
>> @@ -7756,7 +7768,7 @@ create_validate_stream_for_sink(struct drm_connector *connector,
>>   			drm_dbg_kms(connector->dev, "%s:%d Validation failed with %d, retrying w/ YUV420\n",
>>   				    __func__, __LINE__, dc_result);
>>   			aconnector->force_yuv420_output = true;
>> -		}
>> +}>>   		stream = create_validate_stream_for_sink(connector, drm_mode,
>>   							 dm_state, old_stream);
>>   		aconnector->force_yuv422_output = false;
> 

Regards,

Lach