Comment 29 for bug 257827

@apw: per our discussion, I tried both separately blacklisting the video module and adding the brightness_enable=1 option to the thinkpad_acpi module.

Blacklisting the video module had no effect; adjusting brightness was broken entirely and even worse, the brightness was set at the lowest setting. g-p-m locked up entirely when trying to adjust the screen brightness.

Adding the brightness_enable=1 option to the thinkpad_acpi module options caused brightness settings to start working; g-p-m was able to adjust the brightness settings. Also, with that option, the directory /sys/devices/virtual/backlight/thinkpad_screen/ showed up, and writing values to the brightness file in that directory changed the display brightness.

Writing values to /sys/devices/virtual/backlight/acpi_video0/brightness had no effect in any of the kernels I booted (with the video module blacklisted, that path did not exist). When the -10.20 kernel is booted, there's also a /sys/devices/virtual/backlight/acpi_video1/ directory; writing values to the brightness file in that directory also adjusted the screen's brightness.