linux-kernel - Re: [PATCH v2] drm/amd/display: Initialise backlight level values from hw

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Message-ID: <d928e0e0-2478-4cdc-9d7d-5a984a8146d2@kernel.org>
Date: Mon, 12 Jan 2026 09:52:20 -0600
From: Mario Limonciello <superm1@...nel.org>
To: Vivek Das Mohapatra <vivek@...labora.com>
Cc: airlied@...il.com, alexander.deucher@....com,
 amd-gfx@...ts.freedesktop.org, christian.koenig@....com,
 dri-devel@...ts.freedesktop.org, harry.wentland@....com,
 kernel@...labora.com, linux-kernel@...r.kernel.org, simona@...ll.ch,
 siqueira@...lia.com, sunpeng.li@....com
Subject: Re: [PATCH v2] drm/amd/display: Initialise backlight level values
 from hw

On 1/12/26 9:28 AM, Vivek Das Mohapatra wrote:
> Internal backlight levels are initialised from ACPI but the values
> are sometimes out of sync with the levels in effect until there has
> been a read from hardware (eg triggered by reading from sysfs).
> 
> This means that the first drm_commit can cause the levels to be set
> to a different value than the actual starting one, which results in
> a sudden change in brightness.
> 
> This path shows the problem (when the values are out of sync):
> 
>     amdgpu_dm_atomic_commit_tail()
>     -> amdgpu_dm_commit_streams()
>     -> amdgpu_dm_backlight_set_level(..., dm->brightness[n])
> 
> This patch calls the backlight ops get_brightness explicitly
> at the end of backlight registration to make sure dm->brightness[n]
> is in sync with the actual hardware levels.
> 
> Signed-off-by: Vivek Das Mohapatra <vivek@...labora.com>

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

> ---
>   .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c  | 18 +++++++++++++++++-
>   1 file changed, 17 insertions(+), 1 deletion(-)
> 
> 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 354e359c4507..9e8cbfeee915 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> @@ -5258,6 +5258,8 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
>   	struct amdgpu_dm_backlight_caps *caps;
>   	char bl_name[16];
>   	int min, max;
> +	int real_brightness;
> +	int init_brightness;
>   
>   	if (aconnector->bl_idx == -1)
>   		return;
> @@ -5282,6 +5284,8 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
>   	} else
>   		props.brightness = props.max_brightness = MAX_BACKLIGHT_LEVEL;
>   
> +	init_brightness = props.brightness;
> +
>   	if (caps->data_points && !(amdgpu_dc_debug_mask & DC_DISABLE_CUSTOM_BRIGHTNESS_CURVE)) {
>   		drm_info(drm, "Using custom brightness curve\n");
>   		props.scale = BACKLIGHT_SCALE_NON_LINEAR;
> @@ -5300,8 +5304,20 @@ amdgpu_dm_register_backlight_device(struct amdgpu_dm_connector *aconnector)
>   	if (IS_ERR(dm->backlight_dev[aconnector->bl_idx])) {
>   		drm_err(drm, "DM: Backlight registration failed!\n");
>   		dm->backlight_dev[aconnector->bl_idx] = NULL;
> -	} else
> +	} else {
> +		/*
> +		 * dm->brightness[x] can be inconsistent just after startup until
> +		 * ops.get_brightness is called.
> +		 */
> +		real_brightness =
> +			amdgpu_dm_backlight_ops.get_brightness(dm->backlight_dev[aconnector->bl_idx]);
> +
> +		if (real_brightness != init_brightness) {
> +			dm->actual_brightness[aconnector->bl_idx] = real_brightness;
> +			dm->brightness[aconnector->bl_idx] = real_brightness;
> +		}
>   		drm_dbg_driver(drm, "DM: Registered Backlight device: %s\n", bl_name);
> +	}
>   }
>   
>   static int initialize_plane(struct amdgpu_display_manager *dm,