Software control of display illumination on hardware level?

Question

I'm using an old ThinkPad R60 as my daily workhorse. It works like a charm, has IMHO the best keyboard in the world, and is rugged as hell thanks to its magnesium endoskeleton. Only the display is a too dark to work comfortably in bright daylight (the successor model T60 improved on this). The display brightness can be increased by Fn + Home and decreased by Fn + End, but only within the preset range. Is it possible to access the hardware controller of the backlight and increase the maximum brightness value?

Inside the directory /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/acpi_video0 I found the following files (contents in parentheses):

-rw-r--r-- 1 root root 4096 bl_power ('0')
-rw-r--r-- 1 root root 4096 brightness ('7')
-r--r--r-- 1 root root 4096 actual_brightness ('7')
-r--r--r-- 1 root root 4096 max_brightness ('7')
-r--r--r-- 1 root root 4096 type ('firmware')
-rw-r--r-- 1 root root 4096 uevent (empty)

brightness seems to adjust the current setting, the value changes from 0 to 7 according to the brightness and, conversely, changing the brightness changes the value. What is the meaning of the other files and why can't I write to max_brightness even as root?

I tried echo 10 | sudo tee /sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/acpi_video0/max_brightness both as regular user and root (doing sudo su), but I keep getting:

tee: '/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/acpi_video0/max_brightness': Permission denied

UPDATE: I've changed the file permissions, now I'm getting an I/O error:

tee: '/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/acpi_video0/max_brightness': Input/output error

Is there a way to hack the backlight controller and adjust the actual drive current?

Answer 1

The reason that you're unable to write to max_brightness is that it is read only (-r--r--r--), so it's not modifiable by anyone. This is not an issue with the permissions, the ACPI firmware is simply exposing the value that the firmware says is the maximum possible brightness that can be set (you'll note that type reports firmware).

The files in this directory are exposed by ACPI to provide user interaction with the device, and you can find additional information in the thinkpad-acpi documentation and in the acpi backlight sysfs documentation.

Changing the max_brightness value likely requires being able to modify and load modified firmware (I don't think it would be possible by modifying the DSDT alone, but you could investigate that too), and probably the device datasheet.