Re: [PATCH] drm/amdgpu: Clamp the ATIF backlight curve to its 99 points

[Mario Limonciello <superm1@kernel.org>]

On 10/31/26 10:39, Hugo Sales wrote:
> amdgpu_atif_query_backlight_caps() stores the firmware's number_of_points
> as data_points but copies only the 99 points that luminance_data[] holds.
> number_of_points is a u8, so a firmware table that claims more makes the
> custom curve code in amdgpu_dm index past the array, and
> dm_query_extended_brightness_caps() copy past the end of the
> data_points[] buffer it fills
> 
> Clamp the count to ATIF_QBTC_MAX_DATA_POINTS and warn, since a table
> that claims more is a firmware bug
> 
> Fixes: f729e63743f0 ("drm/amd: Pass luminance data to amdgpu_dm_backlight_caps")
> Signed-off-by: Hugo Sales <hugo@hsal.es>
> ---
> The other ATIF checks log at debug level because they make the call fail
> and the driver falls back to its defaults. Here the driver keeps going
> with a truncated curve, so this one warns
> 
>   drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
> index 63088c072087a..4874ee864b807 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
> @@ -406,7 +406,11 @@ int amdgpu_atif_query_backlight_caps(struct amdgpu_atif *atif)
>   			characteristics.max_input_signal;
>   	atif->backlight_caps.ac_level = characteristics.ac_level;
>   	atif->backlight_caps.dc_level = characteristics.dc_level;
> -	atif->backlight_caps.data_points = characteristics.number_of_points;
> +	if (characteristics.number_of_points > ATIF_QBTC_MAX_DATA_POINTS)
> +		pr_warn(FW_BUG "ATIF: backlight curve has %u points, using the first %u\n",
> +			characteristics.number_of_points, ATIF_QBTC_MAX_DATA_POINTS);
> +	atif->backlight_caps.data_points = min_t(u8, characteristics.number_of_points,
> +						 ATIF_QBTC_MAX_DATA_POINTS);
>   	memcpy(atif->backlight_caps.luminance_data,
>   	       characteristics.data_points,
>   	       sizeof(atif->backlight_caps.luminance_data));

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

Thanks; this makes sense.  I'll get this applied to amd-staging-drm-next.

I would like to know though; did you see this in the wild?  If so I'd 
like to hear more about the system that had it and the system firmware 
on it.

What model?
What APU?
What system firmware version?
What AGESA version?

Some systems will publish this to SMBIOS and you'll see it from "dmesg | 
grep AGESA".

If it's a problem in AMD AGESA I would like to get it fixed for a future 
AGESA release.