Re: [PATCH v2 2/3] drm/edid: parse panel type from DisplayID 2.x Display Parameters

[Jani Nikula <jani.nikula@intel.com>]

On Wed, 20 May 2026, Chenyu Chen <chen-yu.chen@amd.com> wrote:
> Parse the Display Parameters Data Block (tag 0x21) defined in
> DisplayID v2.1a Section 4.2.6. Extract the Display Device Technology
> field from payload byte 27 bits [6:4], which indicates whether the
> panel uses LCD (001b) or OLED (010b) technology.
>
> Add a did_panel_type field to struct drm_display_info and populate it
> during DisplayID iteration so downstream drivers can use it for
> panel-type-dependent behavior. Add DRM_MODE_PANEL_TYPE_LCD to the
> UAPI constants for use as an internal communication value between
> DRM core and drivers.
>
> Assisted-by: Copilot:Claude-Opus-4.6
> Signed-off-by: Chenyu Chen <chen-yu.chen@amd.com>
> Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
> ---
>  drivers/gpu/drm/drm_displayid_internal.h | 25 ++++++++++++++
>  drivers/gpu/drm/drm_edid.c               | 44 ++++++++++++++++++++++++
>  include/drm/drm_connector.h              |  6 ++++
>  include/uapi/drm/drm_mode.h              |  1 +
>  4 files changed, 76 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_displayid_internal.h b/drivers/gpu/drm/drm_displayid_internal.h
> index 5b1b32f73516..e0f7c54d2987 100644
> --- a/drivers/gpu/drm/drm_displayid_internal.h
> +++ b/drivers/gpu/drm/drm_displayid_internal.h
> @@ -142,6 +142,31 @@ struct displayid_formula_timing_block {
>  	struct displayid_formula_timings_9 timings[];
>  } __packed;
>  
> +/*
> + * DisplayID v2.x Display Parameters Data Block (tag 0x21).
> + *
> + * Per VESA DisplayID v2.1a, Section 4.2.6, Table 4-14:
> + * Offset 0x1E (payload byte 27) contains Native Color Depth and
> + * Display Device Technology fields.
> + *   bits [2:0] = Native Color Depth
> + *   bit  [3]   = RESERVED
> + *   bits [6:4] = Display Device Technology
> + *     000b = not specified, 001b = LCD, 010b = OLED, others reserved
> + *   bit  [7]   = Display Device Theme Preference
> + */

Please drop the comment...

> +#define DISPLAYID_DISPLAY_PARAMS_DEVICE_TECH	GENMASK(6, 4)
> +
> +struct displayid_display_params_block {
> +	struct displayid_block base;
> +	u8 payload[27];
> +	u8 device_tech_byte; /* bits [6:4] = Display Device Technology */
> +	u8 reserved;

...and actually define the information here.

Sure, you're only interested in one thing, but don't leave the rest for
someone who comes after you, since you clearly have access to the spec,
and whoever reviews this also needs to have access to the spec.

> +} __packed;
> +
> +#define DISPLAYID_DISPLAY_PARAMS_MIN_LEN	\
> +	(sizeof(struct displayid_display_params_block) -	\
> +	 sizeof(struct displayid_block))

Is this helpful? Do you suggest we should add the define for all of the
blocks? What about when there's a minimum, and you can optionally have
more?

> +
>  #define DISPLAYID_VESA_MSO_OVERLAP	GENMASK(3, 0)
>  #define DISPLAYID_VESA_MSO_MODE		GENMASK(6, 5)
>  
> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> index 04878478ab78..d1de1a398677 100644
> --- a/drivers/gpu/drm/drm_edid.c
> +++ b/drivers/gpu/drm/drm_edid.c
> @@ -6713,6 +6713,8 @@ static void drm_reset_display_info(struct drm_connector *connector)
>  
>  	info->source_physical_address = CEC_PHYS_ADDR_INVALID;
>  	memset(&info->amd_vsdb, 0, sizeof(info->amd_vsdb));
> +
> +	info->did_panel_type = DRM_MODE_PANEL_TYPE_UNKNOWN;
>  }
>  
>  static void drm_displayid_process_section_header(struct drm_connector *connector,
> @@ -6731,6 +6733,44 @@ static void drm_displayid_process_section_header(struct drm_connector *connector
>  		info->non_desktop = true;
>  }
>  
> +static void
> +drm_displayid_parse_display_params(struct drm_connector *connector,
> +				   const struct displayid_block *block)
> +{
> +	struct drm_display_info *info = &connector->display_info;
> +	const struct displayid_display_params_block *params =
> +		(const struct displayid_display_params_block *)block;
> +

Superfluous blank line.

> +	u8 tech;
> +
> +	if (block->num_bytes < DISPLAYID_DISPLAY_PARAMS_MIN_LEN) {
> +		drm_dbg_kms(connector->dev,
> +			    "[CONNECTOR:%d:%s] DisplayID Display Parameters block too short (%u < %zu)\n",
> +			    connector->base.id, connector->name,
> +			    block->num_bytes,
> +			    DISPLAYID_DISPLAY_PARAMS_MIN_LEN);
> +		return;
> +	}
> +
> +	tech = FIELD_GET(DISPLAYID_DISPLAY_PARAMS_DEVICE_TECH,
> +			 params->device_tech_byte);
> +
> +	drm_dbg_kms(connector->dev,
> +		    "[CONNECTOR:%d:%s] DisplayID Display Parameters: device technology %u\n",
> +		    connector->base.id, connector->name, tech);

It would be more useful to print LCD or OLED, not the number.

> +
> +	switch (tech) {
> +	case 1: /* LCD */
> +		info->did_panel_type = DRM_MODE_PANEL_TYPE_LCD;
> +		break;
> +	case 2: /* OLED */

The comments above are useless.

> +		info->did_panel_type = DRM_MODE_PANEL_TYPE_OLED;
> +		break;
> +	default:
> +		break;
> +	}
> +}
> +
>  static void update_displayid_info(struct drm_connector *connector,
>  				  const struct drm_edid *drm_edid)
>  {
> @@ -6744,6 +6784,10 @@ static void update_displayid_info(struct drm_connector *connector,
>  			drm_displayid_process_section_header(connector, &iter);
>  			header_processed = true;
>  		}
> +
> +		if (displayid_version(&iter) == DISPLAY_ID_STRUCTURE_VER_20 &&
> +		    block->tag == DATA_BLOCK_2_DISPLAY_PARAMETERS)
> +			drm_displayid_parse_display_params(connector, block);
>  	}
>  	displayid_iter_end(&iter);
>  }
> diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> index c398dbc68bbc..b95aec34ddb7 100644
> --- a/include/drm/drm_connector.h
> +++ b/include/drm/drm_connector.h
> @@ -899,6 +899,12 @@ struct drm_display_info {
>  	 * @amd_vsdb: AMD-specific VSDB information.
>  	 */
>  	struct drm_amd_vsdb_info amd_vsdb;
> +
> +	/**
> +	 * @did_panel_type: Panel type from DisplayID Display Parameters
> +	 * Data Block (tag 0x21). Uses DRM_MODE_PANEL_TYPE_* constants.
> +	 */
> +	u8 did_panel_type;

What does the did_ prefix give us?

>  };
>  
>  int drm_display_info_set_bus_formats(struct drm_display_info *info,
> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index 3693d82b5279..d7ca1040b92e 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -169,6 +169,7 @@ extern "C" {
>  /* Panel type property */
>  #define DRM_MODE_PANEL_TYPE_UNKNOWN	0
>  #define DRM_MODE_PANEL_TYPE_OLED	1
> +#define DRM_MODE_PANEL_TYPE_LCD		2

This is a UABI header.

Should we add this to the panel type property too?


BR,
Jani.


>  
>  /*
>   * DRM_MODE_ROTATE_<degrees>

-- 
Jani Nikula, Intel