[Bug 367309] Re: xrandr backlight control is bizarre on Macbook 945GM

Geir Ove Myhr gomyhr at gmail.com
Mon Apr 27 12:49:12 UTC 2009

** Description changed:

  Binary package hint: xserver-xorg-video-intel
  I'm using Jaunty on a second-generation Macbook ("Macbook2,1" in Apple
  parlance) with an Intel 945GM graphics chip. Since at least Intrepid,
  I've had a strange and puzzling behaviour with the backlight keys.
  Tonight I tried upgrading to version 2.7 of the Intel driver from the
  x-updates PPA in the vain hope it might resolve the issue, but since it
  didn't I sat down and took a good hard look at the problem.
  I tried switching to each of the available BACKLIGHT_CONTROL methods
  with "xrandr --output LVDS --set BACKLIGHT_CONTROL $method", then
  checked the available range with "xrandr --prop" and tried a selection
  of brightness settings with "xrandr --output LVDS --set BACKLIGHT
  When I run "xrandr --prop" the default BACKLIGHT_CONTROL value is
  "native", whose range of backlight values is the unusual range 0..296.
  These numbers map to brightnesses as follows:
   - Even numbers from 0 to 40 turn off the backlight.
   - Even numbers from 42 to 296 uniformly increase the brightness to full strength.
   - Odd numbers from 1 to 253 uniformly decrease the brightness from full to very dim.
   - Odd numbers greater than or equal to 255 turn off the backlight.
  The end result is that trying to change the brightness with F1 and F2
  involves mashing one or the other until you get a brightness level
  you're willing to put up with and hope that nothing changes it.
  The "legacy" BACKLIGHT_CONTROL method has a range of 0..255, where every
  value produces the same mid-level brightness.
  The "combination" BACKLIGHT_CONTROL has a range of 0..148, half the range of the "native" method, and it behaves like half the "native" method too:
   - Numbers from 0 to 20 turn off the backlight.
   - Numbers from 21 to 148 uniformly increase the brightness from very dim to full strength.
  The "kernel" BACKLIGHT_CONTROL has a range of 0..0. Setting the
  backlight value to '0' produces no obvious effect (if the backlight was
  on, it stays on; if the backlight was off it stays off)
  A side note: when I first play with the keyboard brightness buttons,
  they followed the chaotic behaviour of the default, "native"
  BACKLIGHT_CONTROL above. Now that I've been messing with the
  BACKLIGHT_CONTROL value, the brightness buttons control a smooth
  gradient from 'very dim' to 'full strength' regardless of the
  BACKLIGHT_CONTROL, although I can't actually turn the backlight off with
  the keys. I'll ascribe that to some GNOME feature being clever, and
  ignore it until the underlying xrandr feature is fixed.
  I would be happy if the Intel driver defaulted to "combination"
  BACKLIGHT_CONTROL on my hardware, since at least that works in a
  predictable and useful fashion. However, certain patterns make me
  suspect there's some underlying problem to be fixed - like the fact that
  the total 'lit' range in 'combination' mode is 127 brightness levels,
  which seems a far more likely range than 148.
  ProblemType: Bug
  Architecture: amd64
  DistroRelease: Ubuntu 9.04
  Package: xserver-xorg-video-intel 2:2.7.0-1ubuntu1~xup~1
   PATH=(cuusernameom, user)
  ProcVersion: Linux version 2.6.28-11-generic (buildd at creusernameed) (gcc version 4.3.3 (Ubuntu 4.3.3-5ubuntu4) ) #42-Ubuntu SMP Fri Apr 17 01:58:03 UTC 2009
  SourcePackage: xserver-xorg-video-intel
  Uname: Linux 2.6.28-11-generic x86_64
  UnreportableReason: This is not a genuine Ubuntu package
+ [lspci]
+ 00:00.0 Host bridge [0600]: Intel Corporation Mobile 945GM/PM/GMS, 943/940GML and 945GT Express Memory Controller Hub [8086:27a0] (rev 03)
+ 	Subsystem: Intel Corporation Device [8086:7270]
+ 00:02.0 VGA compatible controller [0300]: Intel Corporation Mobile 945GM/GMS, 943/940GML Express Integrated Graphics Controller [8086:27a2] (rev 03)
+ 	Subsystem: Intel Corporation Device [8086:7270]

** Tags added: 945gm backlight intel jaunty xorg

xrandr backlight control is bizarre on Macbook 945GM
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