Day79 - IRQ Subsystem Fundamentals¶

Objective¶

Understand the architecture of the Linux Generic IRQ Subsystem and how interrupts are dispatched from IRQ controllers to driver interrupt service routines.

This lab extends the GPIO IRQ simulation framework introduced in Day78.

Lab1 - irq_desc and irq_action¶

Goal¶

Separate interrupt handler information from irq_desc.

Before Day79:

irq_desc
 ├─ handler
 ├─ dev_id
 └─ name

After Day79:

irq_desc
    ↓
irq_action
        ├─ handler
        ├─ dev_id
        └─ name

irq_action¶

struct irq_action {
    irq_handler_t handler;
    const char *name;
    void *dev_id;
};

Represents a driver registered interrupt handler.

irq_desc¶

struct irq_desc {
    int irq;
    enum irq_trigger_type trigger;
    struct irq_action *action;
};

Represents a Linux IRQ descriptor.

request_irq()¶

Registers a new interrupt action.

request_irq(
        irq,
        trigger,
        handler,
        name,
        dev_id);

Responsibilities:

Allocate irq_action
Store ISR information
Attach action to irq_desc

free_irq()¶

Removes the registered action.

free_irq(
        irq,
        dev_id);

Responsibilities:

Locate matching action
Release action memory
Clear irq_desc ownership

Lab2 - IRQ Dispatch Flow¶

Goal¶

Introduce a Generic IRQ dispatch layer.

Instead of calling ISR directly:

irq_trigger()
        ↓
ISR

use:

irq_trigger()
        ↓
irq_dispatch()
        ↓
ISR

irq_dispatch()¶

static irqreturn_t irq_dispatch(
        struct irq_desc *desc);

Responsibilities:

Validate descriptor
Locate irq_action
Invoke ISR

Test¶

Trigger IRQ twice.

Expected output:

[TRIGGER] irq=100
[DISPATCH] irq=100 (button)
[ISR] irq=100 count=1

[TRIGGER] irq=100
[DISPATCH] irq=100 (button)
[ISR] irq=100 count=2

Lab3 - irq_chip Simulation¶

Goal¶

Introduce IRQ controller abstraction.

Linux drivers do not communicate directly with hardware interrupt controllers.

Linux uses:

irq_chip

to abstract controller-specific operations.

irq_chip¶

struct irq_chip {
    const char *name;
    void (*irq_ack)(int irq);
    void (*irq_eoi)(int irq);
};

ACK and EOI¶

Acknowledge interrupt:

irq_ack()

End of interrupt:

irq_eoi()

Simulation:

static void sim_irq_ack(int irq);
static void sim_irq_eoi(int irq);

Updated Dispatch Flow¶

irq_trigger()
        ↓
irq_dispatch()
        ↓
irq_chip->irq_ack()
        ↓
ISR
        ↓
irq_chip->irq_eoi()

Test¶

Expected output:

[TRIGGER] irq=100
[DISPATCH] irq=100 (button)

[CHIP] ack irq=100

[ISR] irq=100 count=1

[CHIP] eoi irq=100

Lab4 - irqreturn_t¶

Goal¶

Understand Linux interrupt handler return values.

irqreturn_t¶

typedef enum {
    IRQ_NONE = 0,
    IRQ_HANDLED,
    IRQ_WAKE_THREAD,
} irqreturn_t;

IRQ_NONE¶

Interrupt does not belong to this handler.

return IRQ_NONE;

Output:

[IRQ] not handled

IRQ_HANDLED¶

Interrupt successfully handled.

return IRQ_HANDLED;

Output:

[IRQ] handled

IRQ_WAKE_THREAD¶

Used by threaded IRQs.

return IRQ_WAKE_THREAD;

Will be used in Day80.

Final Architecture¶

After completing all labs:

IRQ Controller
        ↓
irq_chip
        ↓
irq_desc
        ↓
irq_action
        ↓
ISR

Key Learning Points¶

Linux IRQ Layering¶

Linux separates:

IRQ controller operations
IRQ descriptor management
Driver interrupt handlers

This improves portability and scalability.

Generic IRQ Dispatch¶

Interrupt dispatch is managed by the Generic IRQ Subsystem.

irq_trigger()
        ↓
irq_dispatch()
        ↓
irq_action
        ↓
ISR

Interrupt Result Reporting¶

ISRs communicate handling results through:

IRQ_NONE
IRQ_HANDLED
IRQ_WAKE_THREAD

rather than traditional error codes.

Preparation for Day80¶

The current framework will be extended with:

Top Half
        ↓
IRQ_WAKE_THREAD
        ↓
Bottom Half

Workqueue

Threaded IRQ

in the next lesson.