Chapter 2. Rationale
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## Chapter 2. Rationale

The original implementation of interrupt handling in Linux is using 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

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

- Per CPU type

This split implementation of highlevel IRQ handlers allows us to optimize the flow of the interrupt handling for each specific interrupt type. This reduces complexity in that particular codepath 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 lowlevel hardware logic, and it also leads to unnecessary code duplication: for example in i386, there is a ioapic_level_irq and a ioapic_edge_irq irq-type which share many of the lowlevel 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 highlevel flow handler, which is normally one of the generic implementations. (This highlevel 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.

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