i915 kerner driver can't initialize two connected LVDS outputs

[Marcin Fabisz]

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.

[Paul White (paulw2u)]

If you've reported the bug directly to Debian please provide details of the Debian bug #

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is missing log files that will aid in diagnosing the problem. While running an Ubuntu kernel (not a mainline or third-party kernel) please enter the following command in a terminal window:

apport-collect 1903841

and then change the status of the bug to 'Confirmed'.

If, due to the nature of the issue you have encountered, you are unable to run this command, please add a comment stating that fact and change the bug status to 'Confirmed'.

This change has been made by an automated script, maintained by the Ubuntu Kernel Team.

[Marcin Fabisz (mfabisz)]

I was asked to report this bug to the Intel driver Github:

https://gitlab.freedesktop.org/drm/intel/-/issues/2794

I have also provided logs on this bug.

This issue appears also on other projects, where it is not possible to use multiple same devices on a system because of this bug. Here an example:

https://bugzilla.redhat.com/show_bug.cgi?id=1443678

The issue was not fixed back then. Some people made hotfixes by themselves:

https://github.com/pruiz/aic94xx-hotfix