Bug#797080: linux: Bug possibly replicated on a Panasonic CF-19

[farrier@iglou.com]

Followup-For: Bug #797080
Source: linux

Dear Maintainer,

   Does this sound like the same bug? I also have a Panasonic Toughbook,
and it too has a nonfunctioning interface
/sys/class/backlight/panasonic
Differences are I have a model CF-19 instead of a C2. My interface
/sys/class/backlight/intel_backlight does work, and I am able to quite
reliably reproduce the malfunction.

Outwardly, my experience has been quite different, but digging deeper, I
see similarities with the subject bug report. My backlight brightness
hotkeys and the GUI backlight controls worked fine until I upgraded to
Debian 10. The CF-19 has two sets of hotkeys. Regarding the GUI, I have
been testing mostly with xfce and its power manager applet. Unfortunately,
after upgrading to 10, neither the hotkeys nor GUI have any effect on
backlight brightness. Digging deeper, there are two entries in
/sys/class/backlight: "intel_backlight" and "panasonic". Unfortunately,
"panasonic" is broken. Writing to
/sys/class/backlight/panasonic/brightness has no effect on the backlight
brightness. What I think is happening, "panasonic" has been broken for a
long time. That was not obvious under Debian 9 because it uses
"intel_backlight" And in my case, "intel_backlight" does work. I do not
know why Debian 10 switched to "panasonic", but I am guessing it is trying
to conform to the kernel documentation which says interfaces of "type"
"platform" should be given higher precedence than ones of type "raw".

I did find a workaround. In the bootloader, add the kernel parameter
acpi_backlight=video
With that, the backlight hotkeys and GUI work again. Digging deeper, there
is a new entry in /sys/class/backlight "acpi_video0". The entry
"panasonic" still doesn't work. What I think is happening, "acpi_video0"
is type "firmware", which is even higher precedence than "platform". So,
Debian 10 ignores "panasonic" and uses "acpi_video0" instead.

-- System Information:
Debian Release: 9.12
  APT prefers oldstable
  APT policy: (500, 'oldstable')
Architecture: amd64 (x86_64)

Kernel: Linux 4.19.0-0.bpo.5-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_US.utf8, LC_CTYPE=en_US.utf8 (charmap=UTF-8),
LANGUAGE=en_US.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)