On devices that possess backlit keyboards, powerd is responsible for adjusting the backlight brightness.
For most devices, the backlight is turned on in response to the user activity (including keyboard presses, touchpad events, or plugging/unplugging the device into AC) instead. After user activity stops, the backlight remains on for a period of time (the duration is supplied by the keyboard_backlight_keep_on_ms preference, which defaults to 30 seconds), and then fades to off.
A small number of devices have sensors capable of detecting when the user‘s hands are hovering over it. For such devices, the backlight turns on when the user’s hands are hovering over the device, and then remains on for a further period of time, again controlled by the keyboard_backlight_keep_on_ms preference.
When full screen video is detected, powerd turns the keyboard backlight off more quickly so as to not distract the user. This time is configured by the keyboard_backlight_keep_on_during_video_ms preference, which defaults to three seconds.
Powerd reads raw percentages from keyboard_backlight_user_steps preference, scales the first step as 0%, second step as 10% and last step as 100%, and calculates the rest of the scaled percentages linearly.
If an ambient light sensor is present, powerd uses its readings to determine the keyboard backlight brightness level. In a well-lit environment, the backlight is turned off. In a dark environment, the backlight is turned on at a moderate level (pursuant to user activity, as described below). The ambient light ranges and corresponding backlight brightness percentages are read from the keyboard_backlight_als_steps preference. The percentages in this preference should be scaled percentages.
If no ambient light sensor is present, powerd reads a single brightness percentage from the keyboard_backlight_no_als_brightness preference and uses that instead when the backlight is turned on. The percentage in this preference should be scaled percentage.
The user is able to adjust the keyboard backlight brightness by holding Alt while pressing the Brightness Up or Brightness Down keys. The brightness moves between the raw percentage steps in the keyboard_backlight_user_steps preference. On the UI, the keyboard backlight brightness controller bar moves between the scaled percentage steps. Once the user has manually adjusted the brightness, powerd refrains from making any automated adjustments until the system reboots. The backlight will still be dimmed or off for extended periods of inactivity, but this becomes based on the longer timeouts used to dim the display, and not the shorter timeouts used by default.
On devices that have it on their keyboard, pressing the keyboard backlight toggle key turns the keyboard backlight on/off. Toggling the keyboard backlight on/off is functionally the same as forcing it on/off, with two differences. First, if a user-initiated brightness adjustment, e.g. an increase or decrease, is made while we're toggled off, we are no longer toggled off. Second, a brightness change signal is emitted any time the user changes the toggle state, even if the brightness percentage has not changed.
Prior to M52 (mid 2016), for devices without a hover sensor, the keyboard backlight used to turn on and off in lock-step with the display. https://crrev.com/c/340927 changed the behavior to turn off the backlight more quickly after user activity ceased to reduce power consumption.