Bug#846882: libdrm2: wrong scale for backlight with Xfce

[Alain <rpnpif@trob.eu>]

Package: libdrm2
Version: 2.4.73-1~bpo8+1
Severity: normal

The scale of change of the intensity of the backlight is wrong.
Changing this is done with the hot keys to increase or decrease.

The minimum get a black screen.
cat /sys/class/backlight/eeepc/brigthness
0
cat /sys/class/backlight/intel_backlight/brigthness
0

1st press on the hot key : get also a black screen.
cat /sys/class/backlight/eeepc/brigthness
1
cat /sys/class/backlight/intel_backlight/brigthness
79787

2nd press : get a medium intensity.
cat /sys/class/backlight/eeepc/brigthness
2
cat /sys/class/backlight/intel_backlight/brigthness
159374

3rd to 12th press get a normal change of the backlight.
To : 796875
But the intensity of the 12th is less than the 11th.

Expected :
The minimum should not light off the screen.
/sys/class/backlight/intel_backlight/brigthness
159374
>From /sys/class/backlight/eeepc/brigthness=1 to 10 or 12, the backlight 
should change linearly without light off.

This buggy behaviour was seen only with Xfce, not with lightdm or the Linux 
console.

The older libdrm and xserver-xorg work fine.

-- System Information:
Debian Release: 8.6
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable')
Architecture: i386 (i686)

Kernel: Linux 4.7.0-0.bpo.1-686-pae (SMP w/2 CPU cores)
Locale: LANG=fr_FR.utf8, LC_CTYPE=fr_FR.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages libdrm2 depends on:
ii  libc6              2.19-18+deb8u6
ii  multiarch-support  2.19-18+deb8u6

libdrm2 recommends no packages.

libdrm2 suggests no packages.

-- no debconf information