Re: [PATCH RFC RFT 2/8] drm/amd/display: Interpolate between backlight LUT entries

[Ray Wu <ray.wu@amd.com>]

Reviewed-by: Ray Wu <ray.wu@amd.com>

On 11/4/2026 5:55 PM, Hugo Sales wrote:
> backlight_millipercent_to_pwm_legacy() takes the floor index into
> backlight_lut, which has one entry per percent of luminance. Any request
> between two whole percents gets the PWM of the lower one, so a PWM panel
> has at most 101 brightness levels however fine the request is
>
> Interpolate linearly between the entries on each side of the request.
> Whole percents still give the LUT entry. The result never decreases as
> the request grows, since validate_ext_backlight_caps() only accepts a
> curve whose signals and luminances strictly increase, so the LUT never
> decreases either. The linear curve bypass and the 100% case are
> unchanged
>
> Signed-off-by: Hugo Sales <hugo@hsal.es>
> ---
> amdgpu_dm only hands the power module whole percents until patch 4, so
> on its own this patch changes no PWM for it. Coming first keeps patch 4
> from dropping a level to the whole percent below
>
> With patch 4 too, on the GV301QH curve with minimum 12, the 62452 levels
> from user_min to user_max reach the panel as 57821 different PWM values.
> Without the series they give 99, and patch 4 without this patch would
> give 100. The bottom tenth of the scale goes from 18 PWM values to 5434
>
> BACKLIGHT_MILLIPERCENT_MAX is defined at the top of power_abm.c as
> 100 * 1000. mod_power_create() sets num_backlight_levels to at least
> 101 and allocates backlight_lut with that many entries, and nothing
> changes it afterwards, so index + 1 stays within the LUT. The function
> still returns early with fewer than 2 levels, as
> backlight_pwm_to_millipercent_legacy() already does
>
> KUnit: dm_test_pwm_lut_between_percents, dm_test_pwm_lut_monotonic and
> dm_test_pwm_lut_min_input_signal from patch 6 fail without this patch
> and pass with it
> ---
>   .../gpu/drm/amd/display/modules/power/power_abm.c  | 29 ++++++++++++++++++----
>   1 file changed, 24 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/display/modules/power/power_abm.c b/drivers/gpu/drm/amd/display/modules/power/power_abm.c
> index 5e86889eaa841..d9c1f1401b604 100644
> --- a/drivers/gpu/drm/amd/display/modules/power/power_abm.c
> +++ b/drivers/gpu/drm/amd/display/modules/power/power_abm.c
> @@ -524,6 +524,9 @@ static unsigned int backlight_millipercent_to_pwm_legacy(
>   {
>   	unsigned int pwm = (unsigned int)-1;
>   	unsigned int index = 0;
> +	unsigned int lower, upper;
> +	unsigned long long scaled;
> +	unsigned int remainder;
>   
>   	if (core_power == NULL)
>   		return 0;
> @@ -541,17 +544,33 @@ static unsigned int backlight_millipercent_to_pwm_legacy(
>   		return pwm;
>   	}
>   
> -	if (millipercent >= (100 * 1000))
> +	if (millipercent >= BACKLIGHT_MILLIPERCENT_MAX)
>   		return core_power->bl_prop[inst].backlight_lut[core_power->bl_prop[inst].num_backlight_levels - 1];
>   
> -	/* This will give the floor index. */
> -	index = ((core_power->bl_prop[inst].num_backlight_levels - 1) *
> -						millipercent) / 100000;
>   	/* Null check otherwise eDP doesn't lightup when connected to DP1 */
>   	if (core_power->bl_prop[inst].backlight_lut == NULL)
>   		return pwm;
>   
> -	pwm = core_power->bl_prop[inst].backlight_lut[index];
> +	if (core_power->bl_prop[inst].num_backlight_levels < 2)
> +		return pwm;
> +
> +	/*
> +	 * Interpolate between the two LUT entries around millipercent.
> +	 * backlight_lut has num_backlight_levels entries, and millipercent is
> +	 * below BACKLIGHT_MILLIPERCENT_MAX here, so index + 1 stays within it
> +	 */
> +	scaled = (unsigned long long)(core_power->bl_prop[inst].num_backlight_levels - 1) *
> +		 millipercent;
> +	index = (unsigned int)div_u64_rem(scaled, BACKLIGHT_MILLIPERCENT_MAX, &remainder);
> +	lower = core_power->bl_prop[inst].backlight_lut[index];
> +	upper = core_power->bl_prop[inst].backlight_lut[index + 1];
> +
> +	if (upper <= lower)
> +		return lower;
> +
> +	pwm = lower + (unsigned int)div_u64((unsigned long long)(upper - lower) * remainder +
> +					    BACKLIGHT_MILLIPERCENT_MAX / 2,
> +					    BACKLIGHT_MILLIPERCENT_MAX);
>   
>   	return pwm;
>   }
>