Writing Device Drivers

Types of Interrupts

There are two common ways in which buses implement interrupts: vectored and polled. Both methods commonly supply a bus-interrupt priority level. However, vectored devices also supply an interrupt vector; polled devices do not.

Vectored Interrupts

Devices that use vectored interrupts are assigned an interrupt vector. This is a number that identifies a particular interrupt handler. This vector may be fixed, configurable (using jumpers or switches), or programmable. In the case of programmable devices, an interrupt device cookie is used to program the device interrupt vector. When the interrupt handler is registered, the kernel saves the vector in a table.

When the device interrupts, the system enters the interrupt acknowledge cycle, asking the interrupting device to identify itself. The device responds with its interrupt vector. The kernel then uses this vector to find the responsible interrupt handler.

The VMEbus supports vectored interrupts.

Polled Interrupts

In polled (or autovectored) devices, the only information the system has about a device interrupt is either the bus interrupt priority level (IPL, on an SBus in a SPARC machine, for example) or the interrupt request number (IRQ on an ISA bus in an x86 machine, for example).

When an interrupt handler is registered, the system adds the handler to a list of potential interrupt handlers for each IPL or IRQ. Once the interrupt occurs, the system must determine which device, of all the devices associated with a given IPL or IRQ, actually interrupted. It does this by calling all the interrupt handlers for the designated IPL or IRQ, until one handler claims the interrupt.

The SBus, ISA, EISA, MCA, and PCI buses are capable of supporting polled interrupts.

Software Interrupts

The Solaris 7 DDI/DKI supports software interrupts, also known as soft interrupts. Soft interrupts are not initiated by a hardware device; they are initiated by software. Handlers for these interrupts must also be added to and removed from the system. Soft interrupt handlers run in interrupt context and therefore can be used to do many of the tasks that belong to an interrupt handler.

Commonly, hardware interrupt handlers are supposed to perform their tasks quickly, since they may suspend other system activity while running. This is particularly true for high-level interrupt handlers, which operate at priority levels greater than that of the system scheduler. High-level interrupt handlers mask the operations of all lower-priority interrupts--including those of the system clock. Consequently, the interrupt handler must avoid involving itself in an activity (such as acquiring a mutex) that might cause it to sleep.

If the handler sleeps, then the system may hang because the clock is masked and incapable of scheduling the sleeping process. For this reason, high-level interrupt handlers normally perform a minimum amount of work at high-priority levels and delegate remaining tasks to software interrupts, which run below the priority level of the high-level interrupt handler. Because software interrupt handlers run below the priority level of the system scheduler, they can do the work that the high-level interrupt handler was incapable of doing. For more information on high-level interrupts, see "Handling High-Level Interrupts".

Note -

Drivers have the option of using a high-level mutex to protect shared data between the high-level interrupt handler and the soft interrupt handler. See "High-level Mutexes".

Software interrupt handlers must not perform as if they have work to do when they run, since (like hardware interrupt handlers) they can run because some other driver triggered a soft interrupt. For this reason, the driver must indicate to the soft interrupt handler that it should do work before triggering the soft interrupt.