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.
Where dependent-phys-path is the device that is kept powered up, such as the CD-ROM drive. phys-path represents the device whose power state is to be depended on, such as the frame buffer.
The following syntax enables you to indicate dependency in a general fashion:
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.
For more information, see the removable-media(9P) man page.