Interrupt programming

Function	Description
request_irq ( unsigned int irq, irq_handler_t handler, unsigned long flags, const char *name, void *dev_id)	Register an IRQ, the parameters are as follows:  irq: IRQ number to allocate.  handler: This is Interrupt handler function.This function will be invoked whenever the operating system receives the interrupt.The data type of return is irq_handler_t, if its return value is IRQ_HANDLED, it indicates that the processing is completed successfully, but if the return value is IRQ_NONE, the processing fails.  flags: can be either zero or a bit mask of one or more of the flags defined in linux/interrupt.h. The most important of these flags are:  IRQF_DISABLED  IRQF_SAMPLE_RANDOM IRQF_SHARED IRQF_TIMER (Explained after this table) name: Used to identify the device name using this IRQ, for example, cat / proc / interrupts will list the IRQ number and device name.  dev_id: IRQ shared by many devices. When an interrupt handler is freed, dev provides a unique cookie to enable the removal of only the desired interrupt handler from the interrupt line. Without this parameter, it would be impossible for the kernel to know which handler to remove on a given interrupt line. You can pass NULL here if the line is not shared, but you must pass a unique cookie if your interrupt line is shared. This pointer is also passed into the interrupt handler on each invocation. A common practice is to pass the driver's device structure. This pointer is unique and might be useful to have within the handlers. Return returns zero on success and nonzero value indicates an error. request_irq() cannot be called from interrupt context (other situations where code cannot block), because it can block.
free_irq( unsigned int irq, void *dev_id)	Release an IRQ registered by request_irq() with the following parameters:  irq: IRQ number.  dev_id: is the last parameter of request_irq. If the specified interrupt line is not shared, this function removes the handler and disables the line. If the interrupt line is shared, the handler identified via dev_id is removed, but the interrupt line is disabled only when the last handler is removed. With shared interrupt lines, a unique cookie is required to differentiate between the multiple handlers that can exist on a single line and enable free_irq() to remove only the correct handler. In either case (shared or unshared), if dev_id is non-NULL, it must match the desired handler. A call to free_irq() must be made from process context.
enable_irq(unsigned int irq)	Re-enable interrupt disabled by disable_irq or disable_irq_nosync.
disable_irq(unsigned int irq)	Disable an IRQ from issuing an interrupt.
disable_irq_nosync(unsigned int irq)	Disable an IRQ from issuing an interrupt, but wait until there is an interrupt handler being executed.
in_irq()	returns true when in interrupt handler
in_interrupt()	returns true when in interrupt handler or bottom half

Interrupts Flags

These are the second parameter of the  function. It has several flags. Here I explained important flags.

• IRQF_DISABLED.
  • When set, this flag instructs the kernel to disable all interrupts when executing this interrupt handler.
  • When unset, interrupt handlers run with all interrupts except their own enabled.
  Most interrupt handlers do not set this flag, as disabling all interrupts is bad form. Its use is reserved for performance-sensitive interrupts that execute quickly. This flag is the current manifestation of the SA_INTERRUPT flag, which in the past distinguished between “fast” and “slow” interrupts.
• IRQF_SAMPLE_RANDOM. This flag specifies that interrupts generated by this device should contribute to the kernel entropy pool. The kernel entropy pool provides truly random numbers derived from various random events. If this flag is specified, the timing of interrupts from this device are fed to the pool as entropy. Do not set this if your device issues interrupts at a predictable rate (e.g. the system timer) or can be influenced by external attackers (e.g. a networking device). On the other hand, most other hardware generates interrupts at non deterministic times and is therefore a good source of entropy.
• IRQF_TIMER. This flag specifies that this handler processes interrupts for the system timer.
• IRQF_SHARED. This flag specifies that the interrupt line can be shared among multiple interrupt handlers. Each handler registered on a given line must specify this flag; otherwise, only one handler can exist per line.

Registering an Interrupt Handler

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#define IRQ_NO 11   if (request_irq(IRQ_NO, irq_handler, IRQF_SHARED, "etx_device", (void *)(irq_handler))) {             printk(KERN_INFO "my_device: cannot register IRQ ");                     goto irq;         }

Freeing an Interrupt Handler

1	free_irq(IRQ_NO,(void *)(irq_handler));

Interrupt Handler

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static irqreturn_t irq_handler(int irq,void *dev_id) {   printk(KERN_INFO "Shared IRQ: Interrupt Occurred");   return IRQ_HANDLED; }