Driver Context

The driver context refers to the condition under which a driver is currently operating. The context limits the operations that a driver can perform. The driver context depends on the executing code that is invoked. Driver code executes in four contexts:

• User context. A driver entry point has user context when invoked by a user thread in a synchronous fashion. That is, the user thread waits for the system to return from the entry point that was invoked. For example, the read(9E) entry point of the driver has user context when invoked by a read(2) system call. In this case, the driver has access to the user area for copying data into and out of the user thread.

• Kernel context. A driver function has kernel context when invoked by some part of the kernel. In a block device driver, the strategy(9E) entry point can be called by the pageout daemon to write pages to the device. Because the page daemon has no relation to the current user thread, strategy(9E) has kernel context in this case.

• Interrupt context.Interrupt context is a more restrictive form of kernel context. Interrupt context is invoked as a result of the servicing of an interrupt. Driver interrupt routines operate in interrupt context with an associated interrupt level. Callback routines also operate in an interrupt context. See Chapter 8, Interrupt Handlers for more information.

• High-level interrupt context.High-level interrupt context is a more restricted form of interrupt context. If ddi_intr_hilevel(9F) indicates that an interrupt is high level, the driver interrupt handler runs in high-level interrupt context. See Chapter 8, Interrupt Handlers for more information.

The manual pages in section 9F document the allowable contexts for each function. For example, in kernel context the driver must not call copyin(9F).