Bug#950609: systemd: backlight service messes with screen brightness

[Valerio Passini]

Package: systemd
Version: 244.1-2
Severity: normal
Tags: patch

Dear Maintainer,

Brightness controls on keyboard (and any other mean except xrandr) have not 
been working reliably since the day I installed Debian on my laptop: sometime 
it was possible to tune brightness after a fresh boot, sometime only after a 
S2R and resume.
Trying to find a solution, I have discovered that masking the backlight 
service fixes the problem:

sudo systemctl mask systemd-backlight at backlight:nvidia_0.service

I have double checked that enabling the service again restores the issue.
The relevant information on my system are:
Laptop model: HP Omen 15-dh0002nl, GPU Nvidia
Software: Debian/Unstable Xorg Nvidia proprietary drivers (don't mind about my
custom kernel, the issue is the same with Debian stock kernel)



-- Package-specific info:

-- System Information:
Debian Release: bullseye/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (500, 'testing'), (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 5.4.17-hpomen (SMP w/12 CPU cores; PREEMPT)
Kernel taint flags: TAINT_PROPRIETARY_MODULE, TAINT_OOT_MODULE, 
TAINT_UNSIGNED_MODULE
Locale: LANG=it_IT.UTF-8, LC_CTYPE=it_IT.UTF-8 (charmap=UTF-8), 
LANGUAGE=en_US:it (charmap=UTF-8)
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages systemd depends on:
ii  adduser          3.118
ii  libacl1          2.2.53-5
ii  libapparmor1     2.13.3-7
ii  libaudit1        1:2.8.5-2+b1
ii  libblkid1        2.34-0.1
ii  libc6            2.29-9
ii  libcap2          1:2.27-1
ii  libcryptsetup12  2:2.2.2-2
ii  libgcrypt20      1.8.5-3
ii  libgnutls30      3.6.11.1-2
ii  libgpg-error0    1.36-7
ii  libidn2-0        2.2.0-2
ii  libip4tc2        1.8.4-2
ii  libkmod2         26+20191223-1
ii  liblz4-1         1.9.2-2
ii  liblzma5         5.2.4-1+b1
ii  libmount1        2.34-0.1
ii  libpam0g         1.3.1-5
ii  libpcre2-8-0     10.34-7
ii  libseccomp2      2.4.2-2
ii  libselinux1      3.0-1
ii  libsystemd0      244.1-2
ii  mount            2.34-0.1
ii  util-linux       2.34-0.1

Versions of packages systemd recommends:
ii  dbus  1.12.16-2

Versions of packages systemd suggests:
ii  policykit-1        0.105-26
pn  systemd-container  <none>

Versions of packages systemd is related to:
pn  dracut           <none>
ii  initramfs-tools  0.136
ii  libnss-systemd   244.1-2
ii  libpam-systemd   244.1-2
ii  udev             244.1-2

-- no debconf information