Re: [PATCH] drm/amd/display: Scale custom brightness curve from full range

[Mario Limonciello <superm1@kernel.org>]

On 8/13/26 12:09, Akhmed Zhitaev wrote:
> Custom brightness curves use an 8-bit input signal.  After exporting the
> full PWM range to userspace, the curve normalizer still divides requests
> by the physical PWM span.  On panels with a nonzero minimum PWM level,
> this can produce a curve input greater than 255 and send an invalid
> backlight level to DC.
> 
> Scale the userspace [0..max] range to the curve's [0..255] range
> instead.  This retains the full advertised range and keeps the reverse
> readback conversion unchanged.
> 
> Fixes: 8dbd72cb7900 ("drm/amd/display: Export full brightness range to userspace")
> Cc: stable@vger.kernel.org
> Signed-off-by: Akhmed Zhitaev <zhitaevakh@gmail.com>

Thanks for this.

We have moved the code in amd-staging-drm-next

https://gitlab.freedesktop.org/agd5f/linux/-/commit/6c2b46e9774f13e45985f6da9f7d5f8458767b98

As your changes are trivial, I'll adjust while commiting.

Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>

> ---
>   drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> 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 40d82a3ee..533c69f0b 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> @@ -5131,10 +5131,10 @@ static int get_brightness_range(const struct amdgpu_dm_backlight_caps *caps,
>   	return 1;
>   }
>   
> -/* Rescale from [min..max] to [0..AMDGPU_MAX_BL_LEVEL] */
> -static inline u32 scale_input_to_fw(int min, int max, u64 input)
> +/* Rescale userspace [0..max] to the firmware curve's [0..255]. */
> +static inline u32 scale_input_to_fw(int max, u64 input)
>   {
> -	return DIV_ROUND_CLOSEST_ULL(input * AMDGPU_MAX_BL_LEVEL, max - min);
> +	return DIV_ROUND_CLOSEST_ULL(input * AMDGPU_MAX_BL_LEVEL, max);
>   }
>   
>   /* Rescale from [0..AMDGPU_MAX_BL_LEVEL] to [min..max] */
> @@ -5147,7 +5147,7 @@ static void convert_custom_brightness(const struct amdgpu_dm_backlight_caps *cap
>   				      unsigned int min, unsigned int max,
>   				      uint32_t *user_brightness)
>   {
> -	u32 brightness = scale_input_to_fw(min, max, *user_brightness);
> +	u32 brightness = scale_input_to_fw(max, *user_brightness);
>   	u8 lower_signal, upper_signal, upper_lum, lower_lum, lum;
>   	int left, right;
>