linux-kernel - Re: [PATCH v6 6/7] drm/edid: parse DRM VESA dsc bpp target

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Message-ID: <0e7d2fa46cf76c9f8ed3632c4b00a4eabbbf5590@intel.com>
Date: Wed, 26 Nov 2025 16:25:27 +0200
From: Jani Nikula <jani.nikula@...ux.intel.com>
To: Yaroslav Bolyukin <iam@...h.pw>, Ville Syrjälä
 <ville.syrjala@...ux.intel.com>, 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, Yaroslav
 Bolyukin <iam@...h.pw>
Subject: Re: [PATCH v6 6/7] drm/edid: parse DRM VESA dsc bpp target

On Wed, 26 Nov 2025, Yaroslav Bolyukin <iam@...h.pw> wrote:
> As per DisplayID v2.1a spec "DSC pass-through timing support",
> VESA vendor-specific data block may contain target DSC bits per pixel
> fields, that should be always used for the VII modes that declare they
> only support working with this value (Pass-through Timing Support for
> Target DSC Bits per Pixel).
>
> Signed-off-by: Yaroslav Bolyukin <iam@...h.pw>
> ---
>  drivers/gpu/drm/drm_displayid_internal.h |  4 ++++
>  drivers/gpu/drm/drm_edid.c               | 16 ++++++++++++++++
>  include/drm/drm_connector.h              |  6 ++++++
>  3 files changed, 26 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_displayid_internal.h b/drivers/gpu/drm/drm_displayid_internal.h
> index 55f972d32847..8f1a2f33ca1a 100644
> --- a/drivers/gpu/drm/drm_displayid_internal.h
> +++ b/drivers/gpu/drm/drm_displayid_internal.h
> @@ -148,6 +148,8 @@ struct displayid_formula_timing_block {
>  #define DISPLAYID_VESA_DP_TYPE		GENMASK(2, 0)
>  #define DISPLAYID_VESA_MSO_OVERLAP	GENMASK(3, 0)
>  #define DISPLAYID_VESA_MSO_MODE		GENMASK(6, 5)
> +#define DISPLAYID_VESA_DSC_BPP_INT	GENMASK(5, 0)
> +#define DISPLAYID_VESA_DSC_BPP_FRACT	GENMASK(3, 0)
>  
>  #define DISPLAYID_VESA_DP_TYPE_EDP	0
>  #define DISPLAYID_VESA_DP_TYPE_DP	1
> @@ -157,6 +159,8 @@ struct displayid_vesa_vendor_specific_block {
>  	u8 oui[3];
>  	u8 data_structure_type;
>  	u8 mso;
> +	u8 dsc_bpp_int;
> +	u8 dsc_bpp_fract;
>  } __packed;
>  
>  /*
> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> index 7bdc99d5084a..b2502be2e807 100644
> --- a/drivers/gpu/drm/drm_edid.c
> +++ b/drivers/gpu/drm/drm_edid.c
> @@ -6591,6 +6591,21 @@ static void drm_parse_vesa_specific_block(struct drm_connector *connector,
>  			    connector->base.id, connector->name,
>  			    info->mso_stream_count, info->mso_pixel_overlap);
>  	}
> +
> +	if (block->num_bytes < 7) {
> +		/* DSC bpp is optional */
> +		return;
> +	}
> +
> +	info->dp_dsc_bpp_x16 = FIELD_GET(DISPLAYID_VESA_DSC_BPP_INT, vesa->dsc_bpp_int) << 4 |
> +			       FIELD_GET(DISPLAYID_VESA_DSC_BPP_FRACT, vesa->dsc_bpp_fract);
> +
> +	if (info->dp_dsc_bpp_x16 > 0) {
> +		drm_dbg_kms(connector->dev,
> +			    "[CONNECTOR:%d:%s] DSC bits per pixel x16 %u\n",
> +			    connector->base.id, connector->name,
> +			    info->dp_dsc_bpp_x16);

Use drm_fixed.h, and do something like this:

	drm_dbg_kms(connector->dev,
		    "[CONNECTOR:%d:%s] DSC bits per pixel " FXP_Q4_FMT "\n",
		    connector->base.id, connector->name,
		    FXP_Q4_ARGS(info->dp_dsc_bpp_x16));

and you'll get the actual x.y in the output.


> +	}
>  }
>  
>  static void drm_update_vesa_specific_block(struct drm_connector *connector,
> @@ -6639,6 +6654,7 @@ static void drm_reset_display_info(struct drm_connector *connector)
>  	info->mso_stream_count = 0;
>  	info->mso_pixel_overlap = 0;
>  	info->max_dsc_bpp = 0;
> +	info->dp_dsc_bpp_x16 = 0;
>  
>  	kfree(info->vics);
>  	info->vics = NULL;
> diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> index 8f34f4b8183d..7decfc288aa3 100644
> --- a/include/drm/drm_connector.h
> +++ b/include/drm/drm_connector.h
> @@ -837,6 +837,12 @@ struct drm_display_info {
>  	 */
>  	u32 max_dsc_bpp;
>  
> +	/**
> +	 * @dp_dsc_bpp: DP Display-Stream-Compression (DSC) timing's target
> +	 * DSC bits per pixel in 6.4 fixed point format. 0 means undefined.
> +	 */
> +	u16 dp_dsc_bpp_x16;
> +
>  	/**
>  	 * @vics: Array of vics_len VICs. Internal to EDID parsing.
>  	 */

-- 
Jani Nikula, Intel