Arch Linux

And your claim is that /sys/devices/pci0000:00/0000:00:02.0/backlight/acpi_video0/brightness exists? What are the permissions and ownership on this file (ls -l is fine)

Hello Dave,

Yes it exists.

[orschiro@thinkpad ~]$ ls -l /sys/devices/pci0000:00/0000:00:02.0/backlight/acpi_video0/brightness
-rw-r--r-- 1 root root 4096 27. Feb 2013 /sys/devices/pci0000:00/0000:00:02.0/backlight/acpi_video0/brightness



[orschiro@thinkpad ~]$ udevadm info -a -p /sys/class/backlight/acpi_video0

looking at device '/devices/pci0000:00/0000:00:02.0/backlight/acpi_video0':
KERNEL=="acpi_video0"
SUBSYSTEM=="backlight"
DRIVER==""
ATTR{type}=="firmware"
ATTR{brightness}=="15"
ATTR{bl_power}=="0"
ATTR{max_brightness}=="15"
ATTR{actual_brightness}=="15"

looking at parent device '/devices/pci0000:00/0000:00:02.0':
KERNELS=="0000:00:02.0"
SUBSYSTEMS=="pci"
DRIVERS=="i915"
ATTRS{irq}=="47"
ATTRS{subsystem_vendor}=="0x17aa"
ATTRS{broken_parity_status}=="0"
ATTRS{class}=="0x030000"
ATTRS{consistent_dma_mask_bits}=="36"
ATTRS{dma_mask_bits}=="36"
ATTRS{local_cpus}=="00000000,00000003"
ATTRS{device}=="0x2a42"
ATTRS{msi_bus}==""
ATTRS{local_cpulist}=="0-1"
ATTRS{vendor}=="0x8086"
ATTRS{subsystem_device}=="0x20e4"
ATTRS{boot_vga}=="1"
ATTRS{numa_node}=="-1"
ATTRS{d3cold_allowed}=="1"

looking at parent device '/devices/pci0000:00':
KERNELS=="pci0000:00"
SUBSYSTEMS==""
DRIVERS==""

I added OWNER="root" to my rule. Now testing the rule with 'udevadm test /sys/class/acpi_video0/backlight' no longer gives me permission denied but when I unplug the cable it still does not change the brightness to 2.

I added acpi_backlight=vendor to the kernel line and now use intel_backlight. But the error remains. Udev does not write the attribute 2.

Manually writing the value works:

# echo 2 > /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight/brightness


The rule:

[orschiro@thinkpad rules.d]$ cat 50-powersave.rules
ACTION=="add", DEVPATH=="/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight", SUBSYSTEM=="backlight", OWNER="root", ATTR{brightness}="2"

With OWNER="root"

rules contain 196608 bytes tokens (16384 * 12 bytes), 29035 bytes strings
16401 strings (148186 bytes), 13926 de-duplicated (121627 bytes), 2476 trie nodes used
ACTION=add
DEVPATH=/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight
SUBSYSTEM=backlight
USEC_INITIALIZED=1328197320
unload module index


Without OWNER="root"

ATTR '/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight/brightness' writing '2' /etc/udev/rules.d/50-powersave.rules:3
error opening ATTR{/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight/brightness} for writing: Permission denied
ACTION=add
DEVPATH=/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight
SUBSYSTEM=backlight
USEC_INITIALIZED=1241377860
unload module index

Problem solved?

I use another approach now by directly writing into the file through a script rather than a udev rule. As written on the Gentoo forum [1] writing to /sys through an Udev rule is not recommended.

[1] http://forums.gentoo.org/viewtopic-t-934196-start-0.html