Ch10 Interrupt Handling

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Installing an Interrupt Handler

Interrupt handler is similar to userspace signal handler.

Driver install and uninstall interrupt handler by request_irq() and free_irq(). (include/linux/interrupt.h)

  • request_irq()

    • flags

      • SA_INTERRUPT: fast interrupt

      • SA_SHIRQ: interrupt can be shared between devices

      • SA_SAMPLE_RANDOM: interrupt can contribute the entropy poll for /dev/random and /dev/urandom

    • void *dev_id: an unique id to index interrupt handler, use the pointer to driver’s own private_data area. For example, network drivers usually use struct netdev * as dev_id.

      It is used for shared interrupt lines. dev_id can be NULL if interrupt is not shared.

      Interrupt handler will get dev_id as parameter, we can pass device context to handler by it.

  • /proc/interrupts and /proc/stat

  • Autodetecting the IRQ number

    How to get IRQ number of device?

Implementing a Handler

Interrupt handler

  • clear “interrupt pending” bit of interface board. Sometimes handler does it at last instead of first. Some devices don’t need it.

  • awake processes sleeping on the device.

  • interrupt handler should execute in a minimum amount of time.

    • defer a long computation to tasklet or workqueue to schedule computation at safer time.

  • disable interrupt in the handler??

Some restriction in the interrupt handler. They are same as kernel timer.

  • not execute in process context => can’t transfer data to or from userspace.

  • can’t sleep. like calling wait_event(), allocate memory without GFP_ATOMIC, and locking semaphore.

  • can’t call schedule().

Interrupt handler parameters and return value

  • parameters: irq number, dev_id, and cpu registers

    • dev_id: device own private_data pointer, passed by request_irq().

  • return value: whether there was actually an interrupt to handle. IRQ_HANDLED or IRQ_NONE

Enable and Disable Interrupt

  • disable single interrupt

    void disable_irq(int irq);
void disable_irq_nosync(int irq);
void enable_irq(int irq);

    • disable interrupt for specific interrupt line.

    • can’t disable shared interrupt lines. (SA_SHIRQ)

    • mask IRQ on PIC(Programmable Interrupt Controller), thus disable this IRQ on all processors.

    • nested call: calling disable() 2 times needs 2 enable() call to reenable it.

    • disable_irq_nosync():

      disable_irq() should wait interrupt controller complete, and then disable interrupt. It may deadlock if the thread calling disable_irq() hold the resource interrupt handler needs. disable_irq_nosync() shouldn’t wait. but it may cause race condition.

  • disable all interrupt:

    # disable interrupt but save their previous state. restore the previous interrupt state.
void local_irq_save(unsigned long flags);
void local_irq_restore(unsigned long flags);

# disable and enable interrupt on the current processor
void local_irq_disable(void);
void local_irq_enable(void);

Top and Bottom Halves

workqueue

把 work 放進 workqueue 後, worker pool 就會安排 worker thread 一個一個執行 work. work 不使用 user context, 而用自己獨立的 workqueue context, 所以不能跟 userspace 交換資料. Linux 上 kthread 之中的 kworker 就是 workqueue 裡的 worker thread.

跟 multi-thread 的程式之中, 把運算 offload 到 thread pool 上執行的概念相似. 只是 worker pool 跟 thread pool allocate cpu 的 policy 不同. (e.g. http server 收到 connection 後, 用 condition variable 叫醒 thread 幫忙 handle request.)

Interrupt Sharing

Interrupt-Driven I/O