asus-oled-brightness

中文摘要：ASUS ROG OLED 笔记本（GU605MV，三星 ATNA60DL01-0 面板）在 Linux 混合显卡模式下屏幕亮度不可调、低亮度频闪严重。根因：i915 因面板 EDID 缺 HDR 元数据而放弃面板的 IHDR 背光接口，落到一条坏路径上。内核参数 i915.enable_dpcd_backlight=3 强制走 IHDR 后，全量程可调、最低亮度无频闪。本仓库 = 修复说明 + dpcd_read.py（读 eDP DPCD 寄存器的小工具，用于验证）。

TL;DR — On ASUS ROG OLED laptops under Linux hybrid graphics, screen brightness is uncontrollable because i915 rejects the panel's IHDR backlight interface (EDID lacks HDR static metadata). Force it with the kernel parameter i915.enable_dpcd_backlight=3.

# /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash i915.enable_dpcd_backlight=3"
sudo update-grub && sudo reboot

Problem

Tested on an ASUS ROG Zephyrus G16 GU605MV (Samsung ATNA60DL01-0 OLED, 2560x1600 @ 240 Hz) running Ubuntu 22.04 / kernel 6.8.

With hybrid graphics (gpu_mux_mode=1, internal panel driven by the Intel iGPU):

• Fn brightness keys and the GNOME slider move, sysfs values change -- the panel ignores them.
• The panel can get stuck near 1% brightness with heavy flicker.
• Windows works fine (it uses the ACPI _BCM path).

Root cause

The kernel log says it outright:

i915 ...: Panel is missing HDR static metadata. ... If your backlight
controls don't work try booting with i915.enable_dpcd_backlight=3

The panel is IHDR-capable -- DPCD 0x340-0x342 reads 01 77 01 (TCON v1, BRIGHTNESS_NITS_CAP, SDR_uses_AUX) -- and it does honor IHDR writes (verified by hand: 4-byte nits to 0x354, then enable bit 4 of 0x344). But i915's capability check refuses the panel because its EDID lacks CTA-861 HDR static metadata (the brightness data lives in a DisplayID block instead). The panel ends up on the old, broken path.

Dead ends (don't bother)

Path	Result
acpi_backlight=video (ACPI _BCM/_BQC)	writes accepted, values read back, panel ignores
nvidia_wmi_ec_backlight (forced load)	same -- silent no-op
old intel PWM path	broken in hybrid mode; DPCD 0x721 shows preset-mode

The fix

i915.enable_dpcd_backlight semantics: 0 = disable, 1 = enable with capability check (default), 2 = force VESA interface, 3 = force Intel HDR (IHDR) interface. Option 3 skips the metadata check that is failing.

Add to the kernel command line (e.g. /etc/default/grub GRUB_CMDLINE_LINUX_DEFAULT), run sudo update-grub, reboot.

Verify it worked

• /sys/class/backlight/intel_backlight exists and max_brightness is 512 (the IHDR path; the old path exposed a different range).
• DPCD 0x344 bit 4 is set and 0x354 tracks the sysfs brightness 1:1:

$ sudo ./dpcd_read.py
== /dev/drm_dp_aux0
0x340 (4B): 01 77 01 00  # IHDR caps -- expect '01 77 01' ...
0x344 (1B): 10  [IHDR backlight ENABLED (bit4)]
0x354 (4B): 7a 01 00 00  [brightness = 378 nits]
...

• Fn keys / slider now visibly change the panel; no flicker at minimum brightness (nits-based DC dimming, not PWM).

dpcd_read.py

Tiny dependency-free reader for DPCD registers over native AUX.

# backlight preset (IHDR caps / enable / nits + VESA registers)
sudo ./dpcd_read.py

# another AUX device
sudo ./dpcd_read.py -d /dev/drm_dp_aux1

# arbitrary addresses, addr[:len]
sudo ./dpcd_read.py 0x700 0x701:4 0x721

Notes:

• Must run as root. pkexec ./dpcd_read.py gives a GUI password prompt (handy over SSH-less desktop sessions).
• /dev/drm_dp_aux* are native AUX transactions (lseek + read). The i2c AUX x/DDI x adapters are DDC/EDID semantics -- they will not return DPCD registers.
• On Intel hybrid graphics the internal eDP panel is usually /dev/drm_dp_aux0; if the output looks wrong, try the other indices.

Register cheat sheet

Address	Meaning
0x340-0x342	IHDR caps: 01 77 01 = TCON v1 + nits cap + SDR via AUX
0x344 bit 4	IHDR backlight enable
0x354-0x357	IHDR brightness, 16-bit little-endian nits
0x700	eDP revision (0x05 = eDP 1.4b)
0x701	VESA backlight caps (bit 2 = AUX enable cap; may be 0)
0x720-0x723	VESA backlight mode set / 16-bit brightness

Related

• asusctl #352 -- the same silent no-op signature (writes accepted, no error, panel ignores) in the opposite MUX mode: dGPU drives the panel and the firmware's ACPI _BCM stub turns out to be an empty function. Cross-check for our acpi_backlight=video dead end: on this firmware family, "value accepted" proves nothing.

Tested on

• ASUS ROG Zephyrus G16 GU605MV, Samsung ATNA60DL01-0 OLED
• Ubuntu 22.04, kernel 6.8.0-138, hybrid graphics (gpu_mux_mode=1)

License

MIT