drm/amd/display: fix initial backlight brightness calculation

[REGRESSION] [PATCH v2] drm/amd/display: fix initial backlight brightness calculation

Posted by Lauri Tirkkonen 2 months, 2 weeks ago

DIV_ROUND_CLOSEST(x, 100) returns either 0 or 1 if 0<x<=100, so the
division needs to be performed after the multiplication and not the
other way around, to properly scale the value.

Fixes: 8b5f3a229a70 ("drm/amd/display: Fix default DC and AC levels")
Signed-off-by: Lauri Tirkkonen <lauri@hacktheplanet.fi>
Cc: stable@vger.kernel.org
---
 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 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 f58fa5da7fe5..8a5b5dfad1ab 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -4941,9 +4941,9 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
 	caps = &dm->backlight_caps[aconnector->bl_idx];
 	if (get_brightness_range(caps, &min, &max)) {
 		if (power_supply_is_system_supplied() > 0)
-			props.brightness = (max - min) * DIV_ROUND_CLOSEST(caps->ac_level, 100);
+			props.brightness = DIV_ROUND_CLOSEST((max - min) * caps->ac_level, 100);
 		else
-			props.brightness = (max - min) * DIV_ROUND_CLOSEST(caps->dc_level, 100);
+			props.brightness = DIV_ROUND_CLOSEST((max - min) * caps->dc_level, 100);
 		/* min is zero, so max needs to be adjusted */
 		props.max_brightness = max - min;
 		drm_dbg(drm, "Backlight caps: min: %d, max: %d, ac %d, dc %d\n", min, max,
-- 
2.50.1

-- 
Lauri Tirkkonen | lotheac @ IRCnet

Re: [REGRESSION] [PATCH v2] drm/amd/display: fix initial backlight brightness calculation

Posted by Mario Limonciello 2 months, 2 weeks ago


On 7/20/25 7:59 PM, Lauri Tirkkonen wrote:
> DIV_ROUND_CLOSEST(x, 100) returns either 0 or 1 if 0<x<=100, so the
> division needs to be performed after the multiplication and not the
> other way around, to properly scale the value.
> 
> Fixes: 8b5f3a229a70 ("drm/amd/display: Fix default DC and AC levels")
> Signed-off-by: Lauri Tirkkonen <lauri@hacktheplanet.fi>
> Cc: stable@vger.kernel.org

Thanks so much!

Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>

> ---
>   drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 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 f58fa5da7fe5..8a5b5dfad1ab 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> @@ -4941,9 +4941,9 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
>   	caps = &dm->backlight_caps[aconnector->bl_idx];
>   	if (get_brightness_range(caps, &min, &max)) {
>   		if (power_supply_is_system_supplied() > 0)
> -			props.brightness = (max - min) * DIV_ROUND_CLOSEST(caps->ac_level, 100);
> +			props.brightness = DIV_ROUND_CLOSEST((max - min) * caps->ac_level, 100);
>   		else
> -			props.brightness = (max - min) * DIV_ROUND_CLOSEST(caps->dc_level, 100);
> +			props.brightness = DIV_ROUND_CLOSEST((max - min) * caps->dc_level, 100);
>   		/* min is zero, so max needs to be adjusted */
>   		props.max_brightness = max - min;
>   		drm_dbg(drm, "Backlight caps: min: %d, max: %d, ac %d, dc %d\n", min, max,