Linux generic IRQ handling¶

Copyright:	© 2005-2010: Thomas Gleixner
Copyright:	© 2005-2006: Ingo Molnar

Introduction¶

The generic interrupt handling layer is designed to provide a complete abstraction of interrupt handling for device drivers. It is able to handle all the different types of interrupt controller hardware. Device drivers use generic API functions to request, enable, disable and free interrupts. The drivers do not have to know anything about interrupt hardware details, so they can be used on different platforms without code changes.

This documentation is provided to developers who want to implement an interrupt subsystem based for their architecture, with the help of the generic IRQ handling layer.

Rationale¶

The original implementation of interrupt handling in Linux uses the __do_IRQ() super-handler, which is able to deal with every type of interrupt logic.

Originally, Russell King identified different types of handlers to build a quite universal set for the ARM interrupt handler implementation in Linux 2.5/2.6. He distinguished between:

Level type
Edge type
Simple type

During the implementation we identified another type:

Fast EOI type

In the SMP world of the __do_IRQ() super-handler another type was identified:

Per CPU type

This split implementation of high-level IRQ handlers allows us to optimize the flow of the interrupt handling for each specific interrupt type. This reduces complexity in that particular code path and allows the optimized handling of a given type.

The original general IRQ implementation used hw_interrupt_type structures and their ->ack, ->end [etc.] callbacks to differentiate the flow control in the super-handler. This leads to a mix of flow logic and low-level hardware logic, and it also leads to unnecessary code duplication: for example in i386, there is an ioapic_level_irq and an ioapic_edge_irq IRQ-type which share many of the low-level details but have different flow handling.

A more natural abstraction is the clean separation of the ‘irq flow’ and the ‘chip details’.

Analysing a couple of architecture’s IRQ subsystem implementations reveals that most of them can use a generic set of ‘irq flow’ methods and only need to add the chip-level specific code. The separation is also valuable for (sub)architectures which need specific quirks in the IRQ flow itself but not in the chip details - and thus provides a more transparent IRQ subsystem design.

Each interrupt descriptor is assigned its own high-level flow handler, which is normally one of the generic implementations. (This high-level flow handler implementation also makes it simple to provide demultiplexing handlers which can be found in embedded platforms on various architectures.)

The separation makes the generic interrupt handling layer more flexible and extensible. For example, an (sub)architecture can use a generic IRQ-flow implementation for ‘level type’ interrupts and add a (sub)architecture specific ‘edge type’ implementation.

To make the transition to the new model easier and prevent the breakage of existing implementations, the __do_IRQ() super-handler is still available. This leads to a kind of duality for the time being. Over time the new model should be used in more and more architectures, as it enables smaller and cleaner IRQ subsystems. It’s deprecated for three years now and about to be removed.

Known Bugs And Assumptions¶

None (knock on wood).

Abstraction layers¶

There are three main levels of abstraction in the interrupt code:

High-level driver API
High-level IRQ flow handlers
Chip-level hardware encapsulation

Interrupt control flow¶

Each interrupt is described by an interrupt descriptor structure irq_desc. The interrupt is referenced by an ‘unsigned int’ numeric value which selects the corresponding interrupt description structure in the descriptor structures array. The descriptor structure contains status information and pointers to the interrupt flow method and the interrupt chip structure which are assigned to this interrupt.

Whenever an interrupt triggers, the low-level architecture code calls into the generic interrupt code by calling desc->handle_irq(). This high-level IRQ handling function only uses desc->irq_data.chip primitives referenced by the assigned chip descriptor structure.

High-level Driver API¶

The high-level Driver API consists of following functions:

See the autogenerated function documentation for details.

High-level IRQ flow handlers¶

The generic layer provides a set of pre-defined irq-flow methods:

handle_level_irq()
handle_edge_irq()
handle_fasteoi_irq()
handle_simple_irq()
handle_percpu_irq()
handle_edge_eoi_irq()
handle_bad_irq()

The interrupt flow handlers (either pre-defined or architecture specific) are assigned to specific interrupts by the architecture either during bootup or during device initialization.

Default flow implementations¶

Helper functions¶

The helper functions call the chip primitives and are used by the default flow implementations. The following helper functions are implemented (simplified excerpt):

default_enable(struct irq_data *data)
{
    desc->irq_data.chip->irq_unmask(data);
}

default_disable(struct irq_data *data)
{
    if (!delay_disable(data))
        desc->irq_data.chip->irq_mask(data);
}

default_ack(struct irq_data *data)
{
    chip->irq_ack(data);
}

default_mask_ack(struct irq_data *data)
{
    if (chip->irq_mask_ack) {
        chip->irq_mask_ack(data);
    } else {
        chip->irq_mask(data);
        chip->irq_ack(data);
    }
}

noop(struct irq_data *data))
{
}