kernel-packages team mailing list archive

[Anders Kaseorg <andersk@xxxxxxx>]

Public bug reported:

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

** Affects: gnome-settings-daemon (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: linux (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug regression-release utopic

** Attachment added: "intel_backlight vs. acpi_video0 brightness scales"
   https://bugs.launchpad.net/bugs/1341108/+attachment/4151081/+files/backlight-scale.png

** Summary changed:

- Backlight control regressed from logarithmic scale to linear scale
+ Backlight brightness control regressed from logarithmic scale to linear scale

** Also affects: gnome-settings-daemon (Ubuntu)
   Importance: Undecided
       Status: New

-- 
You received this bug notification because you are a member of Kernel
Packages, which is subscribed to linux in Ubuntu.
https://bugs.launchpad.net/bugs/1341108

Title:
  Backlight brightness control regressed from logarithmic scale to
  linear scale

Status in “gnome-settings-daemon” package in Ubuntu:
  New
Status in “linux” package in Ubuntu:
  New

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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