Interrupt Overview

Interrupts are sent by hardware or software to indicate an event +has occurred. Interrupts typically cause an Interrupt Service Routine +(ISR) to be executed. The Interrupt platform service specifies the +interface APIs for setting up the ISRs and connecting them to specific +Interrupt IDs.

Interrupts can be managed using the GPIO platform +service when an interrupt source is connected through GPIO pins.

The Interrupt platform service is used by the device drivers that +use an interrupt source routed directly to the interrupt controller +hardware. For more information on how to use GPIO to manage interrupts, +see GPIO Design +Considerations.

Not all hardware events are indicated by interrupts; hardware may +indicate a status change by setting a value in a register or sending +a command/signal over a bus.

What +is the Interrupt platform service

The Interrupt platform +service specifies certain APIs for attaching an Interrupt Service +Routine to an API. It is the low level interface between hardware/software +interrupts, and the kernel and kernel level device drivers.

Required +background

You should be familiar with the following:

Writing a C++ +function that is either global or is in a static class so that the +function address can be used for the ISR.
The specification +for your hardware, including determining interrupt IDs.

Key +concepts and terms

The following concepts are relevant +to this component:

Interrupt ID: An identifier which says which interrupt has occurred. Typically +an unsigned integer.
Interrupt Service Routine (ISR): An ISR is a static function which is called when an interrupt +is received. It should be very short, allocate no memory on the heap, +store any volatile information that needs to be captured for that +event, and, if required, queue a DFC for further processing.
Bind: Connects an Interrupt ID to an Interrupt Service Routine. Unbind +removes the connection.
Enable: When an interrupt occurs, execute the associated Interrupt +Service Routine.

The +Interrupt class

The interrupt interface is provided by +functions of the Interrupt class. The class is +defined in the os/kernelhwsrv/kernel/eka/include/kernel/arm/assp.h file.

Key +users

The key users of the Interrupt platform service are +kernel and kernel-level device driver writers, and anyone who needs +to specify code to be run when a system event occurs.

Limitations

The ISR needs to complete in short and finite time to prevent +excessive interrupt latency.
To keep ISRs short, most of the event processing should be +delegated to a DFC. The DFC will execute in the context of a DFC thread +allocated to itself by the device driver.
It is safe to access the device driver objects using appropriate +synchronization techniques such as spin-locks.
The ISR should store critical data which may not be available +later.
Interrupts can happen in the middle of updating kernel heap +free list or other non-atomic actions. ISRs must be coded with that +in mind.

This means that ISRs cannot:

access user process memory. This includes completing an asynchronous +request.
perform any allocation / de-allocation on heaps. The current +interrupt might have occurred when a heap operation is already in +progress.

It is therefore necessary that ISRs must only use pre-allocated +objects on the kernel heap.