Some devices should be powered down only when other devices are also powered down. For example, if a CD-ROM drive is allowed to power down, necessary functions, such as the ability to eject a CD, might be lost.
To prevent a device from powering down independently, you can make that device dependent on another device that is likely to remain powered on. Typically, a device is made dependent upon a frame buffer, because a monitor is generally on whenever a user is utilizing a system.
The power.conf(4)file specifies the dependencies among devices. (A parent node in the device tree implicitly depends upon its children. This dependency is handled automatically by the power management framework.) You can specify a particular dependency with a power.conf(4) entry of this form:
device-dependency dependent-phys-path phys-path
device-dependency-property property phys-path
Such an entry mandates that any device that exports the property property must be dependent upon the device named by phys-path. Because this dependency applies especially to removable-media devices, /etc/power.conf includes the following line by default:
device_dependent-property removable-media /dev/fb
With this syntax, no device that exports the removable-media property can be powered down unless the console frame buffer is also powered down.