Re: Buster breaks backlight adjustment

[David Farrier <farrier@iglou.com>]

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?