[Bug 221865] New: amdgpu: brightness 100% blacks out screen

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=221865

            Bug ID: 221865
           Summary: amdgpu: brightness 100% blacks out screen
           Product: Drivers
           Version: 2.5
          Hardware: AMD
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri@kernel-bugs.osdl.org
          Reporter: me@metantesan.com
        Regression: No

Hardware:
- ASUS ROG laptop, AMD Phoenix1 APU (DCN 3.1.4), eDP panel BOE NE134WUM-N82
(EISA BOE0B4A)
- Kernel 7.1.6 (regression: 7.1.5 works fine)
Symptom: Setting backlight above ~98.4% makes the internal display go black.
Setting it back down recovers instantly.
Diagnosis via amdgpu_dm_brightness tracepoint (/sys/kernel/tracing):
requested=64200 converted=65302  (works)
requested=64300 converted=65535  (true max, works)
requested=64500 converted=65535  (true max, works)
requested=64540 converted=65768  (> 65535, BLACK)
requested=65535 converted=66702  (> 65535, BLACK)
Root cause: convert_custom_brightness() in
drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:
*user_brightness = scale_fw_to_input(min, max, DIV_ROUND_CLOSEST(lum *
brightness, 101));
The panel's ATIF QUERY_BRIGHTNESS_TRANSFER_CHARACTERISTICS (from ASUS BIOS)
returns luminance data points >100 (~103). So lum * brightness / 101 exceeds
65535, and the result is passed as backlight_pwm_u16_16 (u16) to
dc_link_set_backlight_level(), truncating to ~232 → black screen.
Regression: 7.1.6 changed amdgpu_dm_register_backlight_device() from
props.max_brightness = max - min / (max-min)*level/100 to max / max*level/100,
exposing the overflow zone to userspace. 7.1.5's 100% mapped below the overflow
threshold.
Suggested fix: clamp the converted level to max (this is what the hardware
accepts — 65535 is valid full brightness, the plateau at requested 64300–64500
proves it):
static u32 convert_brightness_from_user(const struct amdgpu_dm_backlight_caps
*caps, uint32_t brightness)
{
    unsigned int min, max;

    if (!get_brightness_range(caps, &min, &max))
        return brightness;

    convert_custom_brightness(caps, min, max, &brightness);

    return min_t(u32, min + DIV_ROUND_CLOSEST_ULL((u64)(max - min) *
brightness, max), max);
}

-- 
You may reply to this email to add a comment.

You are receiving this mail because:
You are watching the assignee of the bug.