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The bug appears from Ubuntu 18.04 and persists in Ubuntu 20.04 and
20.10.
The hardware:
00:02.0 VGA compatible controller: Intel Corporation Atom Processor
Z36xxx/Z37xxx Series Graphics & Display (rev 11)
The bug appears only if both LVDS-Outputs are connected to TFT panels at
the same time.
3) Expected (works at Ubuntu 16.04):
The graphic card is properly initialized and the intel_backlight links appear:
ll /sys/class/backlight/
lrwxrwxrwx 1 root root 0 Feb 11 2016 intel_backlight ->
../../devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight/
lrwxrwxrwx 1 root root 0 Feb 11 2016 intel_backlight
../../devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-2/intel_backlight/
Both TFT panels connected via LVDS are on and are showing the desktop.
4) What happens instead:
The the initialisation of the graphic card by the i915 kernel driver fails:
dmesg | grep dup
[ 2.951765] sysfs: cannot create duplicate filename
'/class/backlight/intel_backlight'
[ 2.951792] sysfs_warn_dup.cold+0x17/0x35
The /sys/class/backlight/ directory is empty.
Modesetting fails:
xrandr shows only one eDP-1 output and the disconnected outputs HDMI-1 and
VGA-1 are gone.
The TFT-Panels are off.
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Affects: debian
Importance: Undecided
Status: New
** Tags: graphics i915 intel kernel-bug ubuntu
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i915 kerner driver can't initialize two connected LVDS outputs
https://bugs.launchpad.net/bugs/1903841
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