Re: [PATCH 1/3] drm: New function to get luminance range based on static hdr metadata

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

On Tue, 26 Apr 2022, Jouni Högander <jouni.hogander@intel.com> wrote:
> Split luminance min/max calculation using static hdr metadata from
> drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:update_connector_ext_caps
> into drm/drm_edid.c.

IMO all of this should be computed at EDID parsing time once and stored
in the metadata. Maybe in drm_parse_hdr_metadata_block().

Over time we've added a bunch of functions to do this type of stuff, and
all drivers call them at slightly different times and different ways,
and it just grows complicated.

(Also, I think basically everything that comes out of the EDID or
DisplayID should be stored in connector->display_info instead of being
spread around like we currently do. But that's for another patch series,
another time.)

BR,
Jani.

>
> Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
> Cc: Harry Wentland <harry.wentland@amd.com>
> Cc: Lyude Paul <lyude@redhat.com>
> Cc: Mika Kahola <mika.kahola@intel.com>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
> ---
>  drivers/gpu/drm/drm_edid.c | 55 ++++++++++++++++++++++++++++++++++++++
>  include/drm/drm_edid.h     |  4 +++
>  2 files changed, 59 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> index 7a8482b75071..1cb886debbbe 100644
> --- a/drivers/gpu/drm/drm_edid.c
> +++ b/drivers/gpu/drm/drm_edid.c
> @@ -4005,6 +4005,61 @@ drm_display_mode_from_cea_vic(struct drm_device *dev,
>  }
>  EXPORT_SYMBOL(drm_display_mode_from_cea_vic);
>  
> +/**
> + * drm_luminance_range_from_static_hdr_metadata() - luminance range from static hdr
> + * metadata
> + * @connector: connector we're calculating for
> + * @min: Calculated min value
> + * @max: Calculated max value
> + *
> + * Calculates backlight min and max as described in CTA-861-G
> + *
> + * Returns: True when calculation succeeds.
> + */
> +bool
> +drm_luminance_range_from_static_hdr_metadata(struct drm_connector *connector,
> +					     u32 *min, u32 *max)
> +{
> +	struct hdr_sink_metadata *hdr_metadata = &connector->hdr_sink_metadata;
> +	static const u8 pre_computed_values[] = {
> +		50, 51, 52, 53, 55, 56, 57, 58, 59, 61, 62, 63, 65, 66, 68, 69,
> +		71, 72, 74, 75, 77, 79, 81, 82, 84, 86, 88, 90, 92, 94, 96, 98};
> +	u32 min_cll, max_cll, q, r;
> +
> +	if (!(hdr_metadata->hdmi_type1.metadata_type &
> +	      BIT(HDMI_STATIC_METADATA_TYPE1)))
> +		return false;
> +
> +	max_cll = hdr_metadata->hdmi_type1.max_cll;
> +	min_cll = hdr_metadata->hdmi_type1.min_cll;
> +
> +	/* From the specification (CTA-861-G), for calculating the maximum
> +	 * luminance we need to use:
> +	 *	Luminance = 50*2**(CV/32)
> +	 * Where CV is a one-byte value.
> +	 * For calculating this expression we may need float point precision;
> +	 * to avoid this complexity level, we take advantage that CV is divided
> +	 * by a constant. From the Euclids division algorithm, we know that CV
> +	 * can be written as: CV = 32*q + r. Next, we replace CV in the
> +	 * Luminance expression and get 50*(2**q)*(2**(r/32)), hence we just
> +	 * need to pre-compute the value of r/32. For pre-computing the values
> +	 * We just used the following Ruby line:
> +	 *	(0...32).each {|cv| puts (50*2**(cv/32.0)).round}
> +	 * The results of the above expressions can be verified at
> +	 * pre_computed_values.
> +	 */
> +	q = max_cll >> 5;
> +	r = max_cll % 32;
> +	*max = (1 << q) * pre_computed_values[r];
> +
> +	/* min luminance: maxLum * (CV/255)^2 / 100 */
> +	q = DIV_ROUND_CLOSEST(min_cll, 255);
> +	*min = *max * DIV_ROUND_CLOSEST((q * q), 100);
> +
> +	return true;
> +}
> +EXPORT_SYMBOL(drm_luminance_range_from_static_hdr_metadata);
> +
>  static int
>  do_cea_modes(struct drm_connector *connector, const u8 *db, u8 len)
>  {
> diff --git a/include/drm/drm_edid.h b/include/drm/drm_edid.h
> index c3204a58fb09..63e441c84d72 100644
> --- a/include/drm/drm_edid.h
> +++ b/include/drm/drm_edid.h
> @@ -406,6 +406,10 @@ drm_hdmi_avi_infoframe_quant_range(struct hdmi_avi_infoframe *frame,
>  				   const struct drm_display_mode *mode,
>  				   enum hdmi_quantization_range rgb_quant_range);
>  
> +bool
> +drm_luminance_range_from_static_hdr_metadata(struct drm_connector *connector,
> +					     u32 *min, u32 *max);
> +
>  /**
>   * drm_eld_mnl - Get ELD monitor name length in bytes.
>   * @eld: pointer to an eld memory structure with mnl set

-- 
Jani Nikula, Intel Open Source Graphics Center