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[Anders Kaseorg <andersk@xxxxxxx>]

http://marc.info/?l=linux-acpi&m=140600387808371&w=2

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https://bugs.launchpad.net/bugs/1341108

Title:
  [Lenovo ThinkPad T510] Backlight brightness control regressed from
  logarithmic scale to linear scale

Status in “linux” package in Ubuntu:
  Triaged

Bug description:
  After upgrading from kernel 3.15.0-6 to kernel 3.16.0-3, the backlight
  became very difficult to control near the low end of the scale. The
  lowest setting now turns the backlight completely off, but the second-
  lowest setting is too bright. Meanwhile, the difference between the
  highest several settings is nearly imperceptible.

  This happened because /sys/class/backlight/acpi_video0/brightness has
  disappeared in kernel 3.16 in favor of
  /sys/class/intel_backlight/brightness. But the relationship between
  acpi_video0 and intel_backlight is not linear.  As acpi_video varies
  [0,1,2,…,13,14,15], intel_backlight varies as
  [52,87,139,…,2557,3358,4437].

  I measured the exact relationship between the two scales on kernel
  3.15 and plotted it with the intel_backlight values on a logarithmic
  scale (see attachment). You can see that the function intel_backlight
  = 60*(4/3)^acpi_video0 is a good fit.

  It’s well known that perceived brightness varies logarithmically with
  physical intensity (Fechner’s law), so it’s no surprise that the
  acpi_video0 scale was more useful. Either the kernel or GNOME should
  adjust the intel_backlight scale to logarithmic.

  (I’m running utopic on a Lenovo Thinkpad T510.)

  ProblemType: Bug
  DistroRelease: Ubuntu 14.10
  Package: linux-image-3.16.0-3-generic 3.16.0-3.8
  ProcVersionSignature: Ubuntu 3.16.0-3.8-generic 3.16.0-rc4
  Uname: Linux 3.16.0-3-generic x86_64
  NonfreeKernelModules: openafs
  ApportVersion: 2.14.4-0ubuntu2
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC0:  anders     2179 F.... pulseaudio
  CurrentDesktop: GNOME
  Date: Sat Jul 12 07:06:12 2014
  EcryptfsInUse: Yes
  HibernationDevice: RESUME=UUID=fd305e7c-c58c-4061-8105-5cda63c38849
  InstallationDate: Installed on 2010-12-05 (1315 days ago)
  InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Alpha amd64 (20101202)
  MachineType: LENOVO 4313CTO
  ProcFB: 0 inteldrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.16.0-3-generic root=/dev/mapper/fdisk-ubuntu ro init=/lib/systemd/systemd quiet splash crashkernel=384M-:128M crashkernel=384M-:128M vt.handoff=7
  RelatedPackageVersions:
   linux-restricted-modules-3.16.0-3-generic N/A
   linux-backports-modules-3.16.0-3-generic  N/A
   linux-firmware                            1.132
  SourcePackage: linux
  UpgradeStatus: Upgraded to utopic on 2013-10-31 (254 days ago)
  dmi.bios.date: 09/26/2012
  dmi.bios.vendor: LENOVO
  dmi.bios.version: 6MET92WW (1.52 )
  dmi.board.name: 4313CTO
  dmi.board.vendor: LENOVO
  dmi.board.version: Not Available
  dmi.chassis.asset.tag: No Asset Information
  dmi.chassis.type: 10
  dmi.chassis.vendor: LENOVO
  dmi.chassis.version: Not Available
  dmi.modalias: dmi:bvnLENOVO:bvr6MET92WW(1.52):bd09/26/2012:svnLENOVO:pn4313CTO:pvrThinkPadT510:rvnLENOVO:rn4313CTO:rvrNotAvailable:cvnLENOVO:ct10:cvrNotAvailable:
  dmi.product.name: 4313CTO
  dmi.product.version: ThinkPad T510
  dmi.sys.vendor: LENOVO

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