The flow of brightness hotkey is as follows

key pressed -> ec triggers events -> bios/os performs a series of
following functions -> ... -> ACPI EV5 -> ACPI BRT6 -> ACPI Notify LCD
0x86/0x87 -> ACPI VGA driver generates keycodes -> userspace write to
/sys/class/backlight/intel_backlight/brightness

It is possible to manually perform functions in ACPI EV5 and later. EV5
and later does not freeze screen, hardware and BIOS are likely to be
blamed; otherwise, it is possible to further narrow down which parts
break and identify the responsible drivers.

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https://bugs.launchpad.net/bugs/1891032

Title:
  Brightness key freeze screen

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