Suspend-Safe and Suspend-Unsafe Devices

suspend-safe: a suspend-safe device is one that does not access memory or interrupt the system while the operating system is in quiescence. A driver is suspend-safe if it supports operating system quiescence (suspend/resume). It also guarantees that when a suspend request is successfully completed, the device that the driver manages will not attempt to access memory, even if the device is open when the suspend request is made.

suspend-unsafe: a suspend-unsafe device is one that allows a memory access or a system interruption while the operating system is in quiescence.

Suspend-safe drivers provide the ability to:

• Stop user threads

• Execute the DDI_SUSPEND call in each device driver

• Stop the clock

• Stop the CPUs

The operating system refuses a quiescence request if a suspend-unsafe device is open. To manually suspend the device, you may have to close the device by killing the processes that have it open, asking users not to use the device, or disconnecting the cables. For example, if a device that allows asynchronous unsolicited input is open, you can disconnect its cables prior to activating operating system quiescence and reconnect them after the operating system resumes. This action prevents traffic from arriving at the device and, thus, the device has no reason to access the backplane.