Only one issue related to backlight on kernel 5.8:
$ ls -la /sys/class/backlight/
total 0
drwxr-xr-x 2 root root 0 Aug 16 18:20 .
drwxr-xr-x 70 root root 0 Aug 16 18:20 ..
lrwxrwxrwx 1 root root 0 Aug 16 18:20 amdgpu_bl0 -> ../../devices/pci0000:00/0000:00:08.1/0000:04:00.0/backlight/amdgpu_bl0
$ systemctl status systemd-backlight@backlight\:amdgpu_bl0.service
● systemd-backlight@backlight:amdgpu_bl0.service - Load/Save Screen Backlight Brightness of backlight:amdgpu_bl0
Loaded: loaded (/usr/lib/systemd/system/systemd-backlight@.service; static; vendor preset: disabled)
Active: failed (Result: exit-code) since Sun 2020-08-16 18:20:20 BST; 13min ago
Docs: man:systemd-backlight@.service(8)
Process: 824 ExecStart=/usr/lib/systemd/systemd-backlight load backlight:amdgpu_bl0 (code=exited, status=1/FAILURE)
Main PID: 824 (code=exited, status=1/FAILURE)
Aug 16 18:20:20 my-pc systemd[1]: Starting Load/Save Screen Backlight Brightness of backlight:amdgpu_bl0...
Aug 16 18:20:20 my-pc systemd-backlight[824]: amdgpu_bl0: Saved brightness 64000 too high; decreasing to 255.
Aug 16 18:20:20 my-pc systemd-backlight[824]: amdgpu_bl0: Failed to write system 'brightness' attribute: No such device or address
Aug 16 18:20:20 my-pc systemd[1]: systemd-backlight@backlight:amdgpu_bl0.service: Main process exited, code=exited, status=1/FAILURE
Aug 16 18:20:20 my-pc systemd[1]: systemd-backlight@backlight:amdgpu_bl0.service: Failed with result 'exit-code'.
Aug 16 18:20:20 my-pc systemd[1]: Failed to start Load/Save Screen Backlight Brightness of backlight:amdgpu_bl0.
$ journalctl -b _PID=824
-- Logs begin at Tue 2019-11-12 20:32:42 GMT, end at Sun 2020-08-16 18:35:44 BST. --
Aug 16 18:20:20 my-pc systemd-backlight[824]: amdgpu_bl0: Saved brightness 64000 too high; decreasing to 255.
Aug 16 18:20:20 my-pc systemd-backlight[824]: amdgpu_bl0: Failed to write system 'brightness' attribute: No such device or address
As a consequence this message appears during startup…
Failed to start Load/Save Screen Backlight Brightness of backlight:amdgpu_bl0.
… and backlight is set to maximum on login.
System: Kernel: 5.8.0-2-MANJARO x86_64 bits: 64 compiler: N/A Desktop: Xfce 4.14.2 tk: Gtk 3.24.20 info: xfce4-panel
wm: xfwm4 dm: LightDM 1.30.0 Distro: Manjaro Linux
Machine: Type: Laptop System: HP product: HP EliteBook 755 G5 v: N/A serial: <filter> Chassis: type: 10 serial: <filter>
Mobo: HP model: 83D5 v: KBC Version 08.58.00 serial: <filter> UEFI: HP v: Q81 Ver. 01.10.03 date: 03/24/2020
Battery: ID-1: BAT0 charge: 50.4 Wh condition: 50.6/50.6 Wh (100%) volts: 12.8/11.6 model: Hewlett-Packard Primary
type: Li-ion serial: <filter> status: Unknown
CPU: Topology: Quad Core model: AMD Ryzen 7 PRO 2700U w/ Radeon Vega Mobile Gfx bits: 64 type: MT MCP arch: Zen
L2 cache: 2048 KiB
flags: avx avx2 lm nx pae sse sse2 sse3 sse4_1 sse4_2 sse4a ssse3 svm bogomips: 35147
Speed: 1223 MHz min/max: 1600/2200 MHz boost: enabled Core speeds (MHz): 1: 1223 2: 1277 3: 1224 4: 1224 5: 1269
6: 1391 7: 1221 8: 1220
Graphics: Device-1: AMD Raven Ridge [Radeon Vega Series / Radeon Vega Mobile Series] vendor: Hewlett-Packard driver: amdgpu
v: kernel bus ID: 04:00.0 chip ID: 1002:15dd
Device-2: Chicony HP HD Camera type: USB driver: uvcvideo bus ID: 4-2:3 chip ID: 04f2:b5ee
Display: x11 server: X.Org 1.20.8 driver: amdgpu,ati unloaded: modesetting alternate: fbdev,vesa
resolution: 1920x1080~60Hz s-dpi: 96
OpenGL: renderer: AMD RAVEN (DRM 3.38.0 5.8.0-2-MANJARO LLVM 10.0.1) v: 4.6 Mesa 20.1.5 direct render: Yes
Audio: Device-1: Advanced Micro Devices [AMD/ATI] Raven/Raven2/Fenghuang HDMI/DP Audio vendor: Hewlett-Packard
driver: snd_hda_intel v: kernel bus ID: 04:00.1 chip ID: 1002:15de
Device-2: Advanced Micro Devices [AMD] Family 17h HD Audio vendor: Hewlett-Packard driver: snd_hda_intel v: kernel
bus ID: 04:00.6 chip ID: 1022:15e3
Sound Server: ALSA v: k5.8.0-2-MANJARO
Network: Device-1: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet vendor: Hewlett-Packard driver: r8168
v: 8.048.03-NAPI port: 3300 bus ID: 01:00.0 chip ID: 10ec:8168
IF: enp1s0f0 state: down mac: <filter>
Device-2: Intel Wireless 8265 / 8275 driver: iwlwifi v: kernel port: 3000 bus ID: 02:00.0 chip ID: 8086:24fd
IF: wlp2s0 state: up mac: <filter>
IF-ID-1: docker0 state: down mac: <filter>
Drives: Local Storage: total: 238.47 GiB used: 357.29 GiB (149.8%)
ID-1: /dev/nvme0n1 vendor: Toshiba model: N/A size: 238.47 GiB speed: 15.8 Gb/s lanes: 2 serial: <filter>
rev: ADHA0101 scheme: GPT
Partition: ID-1: / size: 233.43 GiB used: 178.63 GiB (76.5%) fs: ext4 dev: /dev/nvme0n1p2
Swap: Alert: No Swap data was found.
Sensors: System Temperatures: cpu: 62.9 C mobo: 46.0 C gpu: amdgpu temp: 62 C
Fan Speeds (RPM): N/A
Info: Processes: 270 Uptime: 24m Memory: 30.38 GiB used: 1.94 GiB (6.4%) Init: systemd v: 245 Compilers: gcc: 10.1.0
clang: 10.0.1 Packages: pacman: 1436 Shell: Bash v: 5.0.18 running in: server inxi: 3.1.05