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.
** Tags added: latest-bios-1.52
** No longer affects: gnome-settings-daemon (Ubuntu)
** Changed in: linux (Ubuntu)
Importance: Undecided => Low
** Changed in: linux (Ubuntu)
Status: Confirmed => Incomplete
** Summary changed:
- Backlight brightness control regressed from logarithmic scale to linear scale
+ [Lenovo ThinkPad T510] Backlight brightness control regressed from
logarithmic scale to linear scale
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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:
Incomplete
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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