Ubuntu
gnome-power-manager package

backlight brightness control won't work in 2011 iMac 27"

Bug #818826 reported by JC
22
This bug affects 5 people
Affects Status Importance Assigned to Milestone
gnome-power-manager (Ubuntu)
Confirmed
Undecided
Unassigned

Bug Description

When I press the 'Brightness Control(UP, DOWN)' key in Apple Keyboard,
the OSD shows Increasing Backlight(or Decrease) at the right-top corner.
But the Screen Brightness doesn't changed.

Also, I waited 15 minutes, the screen turned black. (Power Setting)
But the Backlight still alive.

This iMac has AMD Radeon HD 6970M.
I think maybe it's because the driver doesn't exist in kernel module.

Sorry about my poor English and few information.

Revision history for this message
RedSingularity (redsingularity) wrote :

Assigning package.

affects: ubuntu → gnome-power-manager (Ubuntu)
Revision history for this message
Launchpad Janitor (janitor) wrote :

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

Changed in gnome-power-manager (Ubuntu):
status: New → Confirmed
Revision history for this message
kapouer (kapouer) wrote :

/sys/class/backlight/acpi_video0/brightness
is present, but does not have any effect when imac is booted in EFI mode.
It does work when booted in legacy BIOS mode.

Revision history for this message
kapouer (kapouer) wrote :

Update : linux 3.2-rc7 boots that imac 12,2 in EFI mode without any patch or trick.
radeon drm is working. Brightness is not.
I dualboot on osx to change the backlight, which is saved and restored upon reboot (or halt).

Revision history for this message
artik (jeff-artik) wrote :

Affected also.

Revision history for this message
kapouer (kapouer) wrote :

ACPI DSDT has actually two paths for changing backlight.
One is simple to understand and is using SMI Command Port B2, but only works when booting with BIOS.
The other is quite complicated (for me acpi noob) and is using a ressource called "smbus-ddcblc".
I don't know if the backlight acpi is failing because it doesn't follow the right code path (in which case acpi
think it's bios where it isn't) or because memory is not mapped as acpi thinks it should be, or... ?

Revision history for this message
kapouer (kapouer) wrote :

Update : the second way is using i2c to control backlight.

Revision history for this message
artik (jeff-artik) wrote :

Did someone tryed this solution : http://www.webupd8.org/2012/08/fix-dell-xps-13-backlight-brightness.html ?
Maybe this works (can't try, I'm under 11.04)

Revision history for this message
Joris Muller (jmuller) wrote :

Found one easy solution (a least on Ubuntu 20.04) to implement manually solution by changing grub configuration.

Edit /etc/default/grub
To the parameter GRUB_CMDLINE_LINUX_DEFAULT add "acpi_backlight=vendor"
The line should be GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

update grub sudo update-grub

reboot

Don't know how to implement this in ubuntu distribution. Is it possible to create specific grub config during the installation process ?

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