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

[Anders Kaseorg]

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

[Brad Figg (brad-figg)]

This change was made by a bot.

[Rik Mills (rikmills)]

Seems to be same issue on Toshiba Satellite L870 with Intel sandybridge HD3000 graphics running under KDE.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[penalvch (penalvch)]

Anders Kaseorg, could you please test the latest upstream kernel available from the very top line at the top of the page (not the daily folder) following https://wiki.ubuntu.com/KernelMainlineBuilds ? It will allow additional upstream developers to examine the issue. Once you've tested the upstream kernel, please comment on which kernel version specifically you tested. If this bug is fixed in the mainline kernel, please add the following tags:
kernel-fixed-upstream
kernel-fixed-upstream-VERSION-NUMBER

where VERSION-NUMBER is the version number of the kernel you tested. For example:
kernel-fixed-upstream-3.16-rc6

This can be done by clicking on the yellow circle with a black pencil icon next to the word Tags located at the bottom of the bug description.

If the mainline kernel does not fix this bug, please add the following tags:
kernel-bug-exists-upstream
kernel-bug-exists-upstream-VERSION-NUMBER

Once testing of the upstream kernel is complete, please mark this bug's Status as Confirmed. Please let us know your results. Thank you for your understanding.

[penalvch (penalvch)]

Anders Kaseorg, the next step is to fully commit bisect the kernel in order to identify the offending commit. Could you please do this following https://wiki.ubuntu.com/Kernel/KernelBisection ?

[Anders Kaseorg (andersk)]

See commit 751109aad5834375ca9ed0dcfcd85a00cbf872b5. Setting video.use_native_backlight=0 is a workaround because it makes /sys/class/backlight/acpi_video0/brightness come back.

[penalvch (penalvch)]

Anders Kaseorg, the issue you are reporting is an upstream one. Could you please report this problem through the appropriate channel by following the instructions _verbatim_ at https://wiki.ubuntu.com/Bugs/Upstream/kernel ?

Please provide a direct URL to your e-mail to the mailing list once you have made it so that it may be tracked.

Thank you for your understanding.

[Anders Kaseorg (andersk)]

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