Covers the basic of interrupt handling, Interrupt Request Number (IRQ), registering the interrupts handler in Linux device driver, registering the bottom halves, tasklets, work queues, threaded irq handler

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Interrupt Management

2. @ 2021-21 Embitude Trainings <info@embitude.in>
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What to Expect
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Need for interrupts
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IRQ numbers
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Interrupt Registration
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SoftIRQ
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Tasklets
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Work Queues
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Threaded Irq handling

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Need for interrupts
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Interrupt is the intervention
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Interrupt is the mechanism to get the immediate
CPU attention
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Speed mismatch between CPU & the periperal
devices
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Avoiding the polling for better effieciency

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How interrupts work?
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Whenever there is a change in the hardware
state, CPU is interrupted
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The ISR is invoked to service the interrupts
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Typically process has very few interrupts
– So, the processor is occompanied with interrupt
controller to service various interrupts

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Interrupt Request (IRQ)
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Number for which the ISR is registered
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Depends on the SOC & connections on the board
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So, its part of Board Support Packages (BSP)
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Linux uses the Virtual IRQ numbers

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Interrupt Registration API
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int request_irq(unsigned int irq, irq_handler_t
handler, unsigned long flags, const char *name,
void *dev_id);
– typedef irqreturn_t (*irq_handler_t)(int, void *);
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void free_irq(unsigned int irq, void *dev_id);
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Flags
– IRQF_TRIGGER_RISING, ..., IRQF_TRIGGER_HIGH, ...
– IRQF_SAMPLE_RANDOM
– IRQF_SHARED, ...

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Soft IRQ
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Triggered by Sotware
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Basic bottom half
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Also, known as synchronous interrupts
– HI_SOFTIRQ=0,
– TIMER_SOFTIRQ,
– NET_TX_SOFTIRQ,
– NET_RX_SOFTIRQ,
– BLOCK_SOFTIRQ,
– IRQ_POLL_SOFTIRQ,
– TASKLET_SOFTIRQ,
– SCHED_SOFTIRQ,
– HRTIMER_SOFTIRQ,
– RCU_SOFTIRQ,
– NR_SOFTIRQS

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Tasklets
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One of the bottom halves
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Takes up the remaining portion of the
processing left over by ISR
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Executes in the software interrupt context
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Sleeping is not allowed

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Tasklet Data Structures
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struct tasklet_struct
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void tasklet_init(struct tasklet_struct *t, void (*func)
(unsigned long), unsigned long data);
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DECLARE_TASKLET(name, func, data);
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DECLARE_TASKLET_DISABLED(name, func, data);
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tasklet_enable(t); tasklet_disable(t);
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tasklet_disable_nosync(t);
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tasklet_[hi_]schedule(t);
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tasklet_kill(t);

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Work Queues
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Bottom half, similar to tasklet
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Does the remaining portion of task
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Executes in the special kernel thread
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Can sleep or block

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Work Queue APIs
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struct workqueue_struct, struct work_struct
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Create Work Queue
– q = create_workqueue(name);
– q = create_singlethread_workqueue(name);
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Destroy Work Queue
– flush_workqueue(q);
– destroy_workqueue(q);

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Work Queue APIs
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Initializing the Work
– DECLARE_WORK(w, void (*function)(struct work_struct *));
– INIT_WORK(w, void (*function)(struct work_struct *));
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Scheduling the work
– int queue_work(q, &w);
– int queue_delayed_work(q, &w, d);
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Cancelling the work
– int cancel_delayed_work(&w);
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Shared Work queue
– schedule_work(&w)

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Threaded IRQ
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Reduce the time spent in ISR
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Similar to Tasklets & Work Queues, but
simplifies the registration & scheduling
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Runs in kernel thread with pre-emption enabled
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Suitable for scenarios where long processing
needs to be done
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API
– Int request_threaded_irq(unsigned int irq,
irq_handler_t thread_fn, unsigned long flags, const
char *name, void *dev)

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What all did we learn?
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Need for interrupts
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IRQ numbers
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Interrupt Registration
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SoftIRQ
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Tasklets
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Work Queues
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Threaded Irq handling