Re: [PATCH RESEND 2/2] drm/amd/display: Keep backlight precision in millipercent

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

Hi Mathew,

Same as your other patch, a similar change has already been merged into 
asdn DC 3.2.400 release.
     commit: drm/amd/display: map PWM brightness through custom 
backlight curve

Please give it a try, and if you still see any other issues, feel free 
to send an updated patch.

Thanks,
Ray

On 9/17/2026 3:11 AM, Matthew Schwartz wrote:
> The power module takes the backlight level in millipercent, but DM
> rounds the 16-bit level to a whole percent before multiplying by 1000,
> collapsing the range to 101 steps. It also divides the absolute level,
> which already includes the panel minimum, by the min to max span, so
> panels with a nonzero minimum PWM overshoot the top of the range.
>
> Scale the offset from the minimum to millipercent directly so the full
> resolution reaches the power module and the minimum is only added once.
>
> Fixes: 3c108046e1d6 ("drm/amd/display: Add power module on Linux")
> Signed-off-by: Matthew Schwartz <matthew.schwartz@linux.dev>
> ---
>   drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_backlight.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_backlight.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_backlight.c
> index 45e1732a5b04..079277d47b3f 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_backlight.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_backlight.c
> @@ -326,7 +326,8 @@ void amdgpu_dm_backlight_set_level(struct amdgpu_display_manager *dm,
>   	} else {
>   		/* power module uses millipercent */
>   		get_brightness_range(caps, &min, &max);
> -		brightness = DIV_ROUND_CLOSEST(brightness * 100, (max - min)) * 1000;
> +		brightness = DIV_ROUND_CLOSEST_ULL((u64)(brightness - min) * 100000,
> +						   max - min);
>   		rc = mod_power_set_backlight_percent(dm->power_module, stream,
>   						     brightness, 0, false);
>   	}