Please help debug a display backlight problem. When I upgraded my
Panasonic CF-19 to buster, the backlight adjustment keys quit working. I
have downgraded back to stretch. Before I try buster again, I would like
to understand how the backlighting adjustment works. Looking at the
list archive, found info on the kernel end of this process. Also on the
user interface end, although mine is a little different since I am using
xfce. Can anyone tell me what should be happening in between?

On the kernel end of things, that seems to work both in stretch and
buster. For example, I can change the intensity using the command:
# echo 400 > /sys/class/backlight/intel_backlight/brightness
which for the CF-19, '400' results in roughly 10% of full brightness. The CF-19's backlight is really powerful, so 400 is about right for indoor
use.

On the user interface end, the laptop provides two sets of backlight keys:
one pair on the keyboard and one pair on the front edge of the cabinet.
xfce provides a "power manager" plugin that includes a slider so you can
use a mouse to adjust screen brightness. Just like the keys, the slider
works in stretch but has no effect in buster.

In the category of configuration, xfce provides a control panel, also
called "Power Manager". Nearly all the settings have to do with
sleep, but it also provides "Handle display brightness keys". That
defaults to enabled. If I disable it, the keys don't do anything, but
the slider provided by the brightness manager plugin still works. So I
turned the "Handle display brightness keys" back on.

The final user interface feature is a plugin that displays notifications
of various events. Normally, this seems like eye candy. Why do you need a dialog for something obvious like the backlight brightness? But with my problems, maybe the notifications plugin is useful for debugging. The notifications plugin seems to report the brightness level you select with
the keys and slider, as opposed to the actual brightness.

Digging deeper, xfce provides a daemon, also named "power_manager". The documentation on the xfce web site says, for debugging purposes, you can
stop the daemon and then run it as an ordinary command:
# xfce4-power-manager -q
# xfce4-power-manager --no-daemon --debug
The web site doesn't say much about interpreting the debug messages, but
it does describe how to tell if Policy Kit is blocking the daemon. I don't
seem to be having that problem. Anyhow, a typical set of messages for a
button push (either stretch or buster) is:
TRACE[xfpm-button.c:118] xfpm_button_filter_x_events(): Key press:
((XfpmButtonKey) BUTTON_MON_BRIGHTNESS_UP)
TRACE[xfpm-backlight.c:188] xfpm_backlight_show(): Level 584
TRACE[xfpm-manager.c:368] xfpm_manager_button_pressed_cb(): Received
button press event: ((XfpmButtonKey) BUTTON_MON_BRIGHTNESS_UP)

I also tried looking in the system logs:
# journalctl -f
A typical set of messages (stretch or buster) for a button press is:
Aug 09 08:59:34 tough2 pkexec[29498]: pam_unix(polkit-1:session):
session opened for user root by (uid=1000)
Aug 09 08:59:34 tough2 pkexec[29498]: farrier: Executing command
[USER=root] [TTY=unknown] [CWD=/home/farrier]
[COMMAND=/usr/sbin/xfpm-power-backlight-helper --set-brightness 315]

Turns out xfpm-power-backlight-helper is another application provided by
xfce. According to its man page, it's function is to "control the
brightness from the command line." I tried that, hoping to view any error messages. It will let me do things like read the current brightness
setting. However, even as root, I can't control the brighness:
# xfpm-power-backlight-helper --set-brightness 315
This program must only be run through pkexec

I don't know where to go from here. All these messages look similar in
stretch versus buster. I think something is intervening between xfpm-power-backlight-helper and the kernel. But what is it, and how can I examine what it is doing?

I should also mention, upgrading to buster also causes some problems with sleep. Is hard to descibe, usually works okay but not always. There has
been some recent traffic on debian-xfce regarding sleep. So, once I figure
out how to describe that problem, I may post a request on that mailing
list. But I thought I should at least mention the sleep problem here in
case it is related to backlight.

Thank you for your consideration.

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Two days ago, I posted a lengthy post asking why the backlight keys on
my Panasonic Toughbook CF-19 quit working when I upgraded to Debian 10 (buster). Since then, I have done some more poking around, and am now able
to ask a much more concise question. Please feel free to tackle my
previous post, but perhaps today's question is a better starting point:

On 12 August, I said

On the kernel end of things, that seems to work both in stretch and
buster. For example, I can change the intensity using the command:
# echo 400 > /sys/class/backlight/intel_backlight/brightness
which for the CF-19, '400' results in roughly 10% of full brightness. The >CF-19's backlight is really powerful, so 400 is about right for indoor
use.

However, I am now unsure the kernel is handling things properly. There are
two entries under /sys/class/backlight: intel_backlight and panasonic.
Their contents look similar, but they don't behave the same.
# echo 40 > /sys/class/backlight/panasonic/brightness
does not change the screen brightness. Should it?

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