https://www.kernel.org/doc/Documentation/power/runtime_pm.txt
https://www.kernel.org/doc/html/v4.12/driver-api/pm/devices.html
- "Devices may be put into low-power states while the system is running, independently of other power management activity in principle. However, devices are not generally independent of each other (for example, a parent device cannot be suspended unless all of its child devices have been suspended)."