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Linux kernel will, as documented in drivers/video/backlight/backlight.c, report changes to a backlights brightness as a uevent (ACTION=change). systemd-udev will consume the uevent, match on this rule and try to activate the systemd-backlight service for the backlight. BUT when systemd is not compiled with backlight support, this will lead to failure that is reported in the journal. Since the failure to activate systemd-backlight and subsequent failure log entry happens on every backlight brightness change, we found the resulting logspam during regular operation excessive and came up with this patch to mitigate it. The conditional is also extended to "*kbd_backlight" match, since even though we did not investigate to see if the logspam would be similar, the unconditional match to activate systemd-backlight here would also not make sense when the feature is not compiled in. Signed-off-by: Simon Braunschmidt <simon.braunschmidt@iba-group.com>
Files in this directory contain configuration for systemd-udevd.service, a daemon that manages symlinks to device nodes, permissions of devices nodes, emits device events for userspace, and renames network interfaces. See man:udev(7) for an overview of the configuration file format, and man:systemd-udevd.service(8) for a description of service itself. Use 'systemd-analyze cat-config udev/rules.d' to display the effective config.