[Bug 1865346] Re: Changing backlight no longer works with Linux 5.4

Please boot with kernel parameter "drm.debug=0xe log_buf_len=16M" and
attach dmesg here.

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https://bugs.launchpad.net/bugs/1865346

Title:
Changing backlight no longer works with Linux 5.4

Status in linux package in Ubuntu:
Confirmed

Bug description:
As of upgrading to Ubuntu 20.04 (and with it Linux 5.4), adjusting
brightness no longer works. `/sys/class/backlight/intel_backlight/`
exists, but writing to
`/sys/class/backlight/intel_backlight/brightness` doesn't have any
effect. If I boot into Linux 5.3 instead, changing brightness works
again. Complete dmesg output is attached, but I don't see anything
interesting in it.

My hardware is a Dell Inc. XPS 15 9575, with an Intel Core i7-8705G.

More info:

`lsb_release -rd`:
Description: Ubuntu Focal Fossa (development branch)
Release: 20.04

`apt-cache policy linux-generic`:
linux-generic:
Installed: 5.4.0.14.17
Candidate: 5.4.0.14.17
Version table:
*** 5.4.0.14.17 500
500 http://no.archive.ubuntu.com/ubuntu focal/main amd64 Packages
100 /var/lib/dpkg/status

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