Interrupts

An interrupt service routine (ISR) is a function that executes asychronously in response to a hardware or software interrupt. An ISR normally preempts the execution of the current thread, allowing the response to occur with very low overhead. Thread execution resumes only once all ISR work has been completed.

Concepts

Any number of ISRs can be defined, subject to the constraints imposed by underlying hardware.

An ISR has the following key properties:

  • An interrupt request (IRQ) signal that triggers the ISR.
  • A priority level associated with the IRQ.
  • An interrupt handler function that is invoked to handle the interrupt.
  • An argument value that is passed to that function.

An IDT or a vector table is used to associate a given interrupt source with a given ISR. Only a single ISR can be associated with a specific IRQ at any given time.

Multiple ISRs can utilize the same function to process interrupts, allowing a single function to service a device that generates multiple types of interrupts or to service multiple devices (usually of the same type). The argument value passed to an ISR’s function allows the function to determine which interrupt has been signaled.

The kernel provides a default ISR for all unused IDT entries. This ISR generates a fatal system error if an unexpected interrupt is signaled.

The kernel supports interrupt nesting. This allows an ISR to be preempted in mid-execution if a higher priority interrupt is signaled. The lower priority ISR resumes execution once the higher priority ISR has completed its processing.

An ISR’s interrupt handler function executes in the kernel’s interrupt context. This context has its own dedicated stack area (or, on some architectures, stack areas). The size of the interrupt context stack must be capable of handling the execution of multiple concurrent ISRs if interrupt nesting support is enabled.

Important

Many kernel APIs can be used only by threads, and not by ISRs. In cases where a routine may be invoked by both threads and ISRs the kernel provides the k_is_in_isr() API to allow the routine to alter its behavior depending on whether it is executing as part of a thread or as part of an ISR.

Preventing Interruptions

In certain situations it may be necessary for the current thread to prevent ISRs from executing while it is performing time-sensitive or critical section operations.

A thread may temporarily prevent all IRQ handling in the system using an IRQ lock. This lock can be applied even when it is already in effect, so routines can use it without having to know if it is already in effect. The thread must unlock its IRQ lock the same number of times it was locked before interrupts can be once again processed by the kernel while the thread is running.

Important

The IRQ lock is thread-specific. If thread A locks out interrupts then performs an operation that allows thread B to run (e.g. giving a semaphore or sleeping for N milliseconds), the thread’s IRQ lock no longer applies once thread A is swapped out. This means that interrupts can be processed while thread B is running unless thread B has also locked out interrupts using its own IRQ lock. (Whether interrupts can be processed while the kernel is switching between two threads that are using the IRQ lock is architecture-specific.)

When thread A eventually becomes the current thread once again, the kernel re-establishes thread A’s IRQ lock. This ensures thread A won’t be interrupted until it has explicitly unlocked its IRQ lock.

Alternatively, a thread may temporarily disable a specified IRQ so its associated ISR does not execute when the IRQ is signalled. The IRQ must be subsequently enabled to permit the ISR to execute.

Important

Disabling an IRQ prevents all threads in the system from being preempted by the associated ISR, not just the thread that disabled the IRQ.

Offloading ISR Work

An ISR should execute quickly to ensure predictable system operation. If time consuming processing is required the ISR should offload some or all processing to a thread, thereby restoring the kernel’s ability to respond to other interrupts.

The kernel supports several mechanisms for offloading interrupt-related processing to a thread.

  • An ISR can signal a helper thread to do interrupt-related processing using a kernel object, such as a fifo, lifo, or semaphore.
  • An ISR can signal an alert which causes the system workqueue thread to execute an associated alert handler function. (See Alerts.)
  • An ISR can instruct the system workqueue thread to execute a work item. (See TBD.)

When an ISR offloads work to a thread, there is typically a single context switch to that thread when the ISR completes, allowing interrupt-related processing to continue almost immediately. However, depending on the priority of the thread handling the offload, it is possible that the currently executing cooperative thread or other higher-priority threads may execute before the thread handling the offload is scheduled.

Implementation

Defining an ISR

An ISR is defined at run-time by calling IRQ_CONNECT. It must then be enabled by calling irq_enable().

Important

IRQ_CONNECT() is not a C function and does some inline assembly magic behind the scenes. All its arguments must be known at build time. Drivers that have multiple instances may need to define per-instance config functions to configure each instance of the interrupt.

The following code defines and enables an ISR.

#define MY_DEV_IRQ  24       /* device uses IRQ 24 */
#define MY_DEV_PRIO  2       /* device uses interrupt priority 2 */
/* argument passed to my_isr(), in this case a pointer to the device */
#define MY_ISR_ARG  DEVICE_GET(my_device)
#define MY_IRQ_FLAGS 0       /* IRQ flags. Unused on non-x86 */

void my_isr(void *arg)
{
   ... /* ISR code */
}

void my_isr_installer(void)
{
   ...
   IRQ_CONNECT(MY_DEV_IRQ, MY_DEV_PRIO, my_isr, MY_ISR_ARG, MY_IRQ_FLAGS);
   irq_enable(MY_DEV_IRQ);
   ...
}

Suggested Uses

Use an ISR to perform interrupt processing that requires a very rapid response, and can be done quickly without blocking.

Note

Interrupt processing that is time consuming, or involves blocking, should be handed off to a thread. See Offloading ISR Work for a description of various techniques that can be used in an application.

Configuration Options

Related configuration options:

Additional architecture-specific and device-specific configuration options also exist.

APIs

The following interrupt-related APIs are provided by irq.h:

  • IRQ_CONNECT
  • irq_lock()
  • irq_unlock()
  • irq_enable()
  • irq_disable()
  • irq_is_enabled()

The following interrupt-related APIs are provided by kernel.h: