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 ?
** Changed in: linux (Ubuntu)
Status: Confirmed => Incomplete
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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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