lucck filed a duplicate report at bug #291742 where he did some good
debugging that I'd like to share here.
"Afer power up display backlight is always set to 100% light,
independent of brightness settings in previous session. I detected that
problem is in acpi kernel driver video.ko and gnome-power-manager. I use
ubuntu 8.10 with kernel 2.6.27-7-generic on 64-bit platform. This is a
regression bug, because in previous version ubuntu 8.04LTS display
backlight works ok.
Aditional comments:
I detected that acpi/video.ko kernel 2.6.24 driver in ubuntu 8.04 didn't
work and backlight was controlled only by ACPI bios, because
/proc/bus/acpi/video/*/backlights always return that backlight is not
supported. In kernel 2.6.27 driver return correct backlight values:
cat /proc/acpi/video/C085/C144/brightness
levels: 100 51 30 37 44 51 58 65 72 79 86 93 100
current: 65
New video driver trying to get current backlight level from ACPI BQI
object but it is optional ACPI function. When ACPI does not support BQI
object driver will set back-light to maximum value. Pls. see code
video.c bellow:
if (device->cap._BQC)
device->backlight->props.brightness =
acpi_video_get_brightness(device->backlight);
else
device->backlight->props.brightness =
device->backlight->props.max_brightness;
backlight_update_status(device->backlight);
When ACPI BQI is not supported video.ko or gnome-power-manager should
save and restore current backlight value from config file.
Workaround:
Currently I workaround this problem by change kernel driver video.ko to
set 50% backlight when BQI object is not found (Pls see patch bellow). I
also changed gnome-power-manager settings by run gconf-editor and set
following configuration values in /apps/gnome-power/manager/backlight by
changing keys values to:
brightness_ac = 50
brightness_battery = 30
brightness_dim_battery = 30
but it is obvious workaround."
--
Laptop TFT monitor - brightness level is not saved
https://bugs.launchpad.net/bugs/35223
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