logind: add SetBrightness() bus call for setting brightness of leds/backlight devices associated with a seat

This augments the drm/input device management by adding a single method
call for setting the brightness of an "leds" or "backlight" kernel class
device.

This method call requires no privileges to call, but a caller can only
change the brightness on sessions that are currently active, and they
must own the session.

This does not do enumeration of such class devices, feature or range
probing, chnage notification; it doesn't help associating graphics or
input devices with their backlight or leds devices. For all that clients
should go directly to udev/sysfs. The SetBrightness() call is just for
executing the actual change operation, that is otherwise privileged.

Example line:

   busctl call org.freedesktop.login1 /org/freedesktop/login1/session/self org.freedesktop.login1.Session SetBrightness ssu "backlight" "intel_backlight" 200

The parameter the SetBrightness() call takes are the kernel subsystem
(i.e. "leds" or "backlight"), the device name, and the brightness
value.

On some hw setting the brightness is slow, and implementation and write
access to the sysfs knobs exposes this slowness. Due to this we'll fork
off a writer process in the background so that logind doesn't have to
block. Moreover, write requestes are coalesced: when a write request is
enqueued while one is already being executed it is queued. When another
write reques is then enqueued the earlier one is replaced by the newer
one, so that only one queued write request per device remains at any
time. Method replies are sent as soon as the first write request that
happens after the request was received is completed.

It is recommended that bus clients turn off the "expect_reply" flag on
the dbus messages they send though, that relieves logind from sending
completion notification and is particularly a good idea if clients
implement reactive UI sliders that send a quick secession of write
requests.

Replaces: #12413

0 files changed, 0 insertions, 0 deletions