Interrupt Handlers

Except for the last chapter, everything we did in the kernel so far we've done as a response to a process asking for it, either by dealing with a special file, sending an ioctl(), or issuing a system call. But the job of the kernel isn't just to respond to process requests. Another job, which is every bit as important, is to speak to the hardware connected to the machine.

There are two types of interaction between the CPU and the rest of the computer's hardware. The first type is when the CPU gives orders to the hardware, the other is when the hardware needs to tell the CPU something. The second, called interrupts, is much harder to implement because it has to be dealt with when convenient for the hardware, not the CPU. Hardware devices typically have a very small amount of RAM, and if you don't read their information when available, it is lost.

Under Linux, hardware interrupts are called IRQ's (Interrupt Requests)[1]. There are two types of IRQ's, short and long. A short IRQ is one which is expected to take a very short period of time, during which the rest of the machine will be blocked and no other interrupts will be handled. A long IRQ is one which can take longer, and during which other interrupts may occur (but not interrupts from the same device). If at all possible, it's better to declare an interrupt handler to be long.

When the CPU receives an interrupt, it stops whatever it's doing (unless it's processing a more important interrupt, in which case it will deal with this one only when the more important one is done), saves certain parameters on the stack and calls the interrupt handler. This means that certain things are not allowed in the interrupt handler itself, because the system is in an unknown state. The solution to this problem is for the interrupt handler to do what needs to be done immediately, usually read something from the hardware or send something to the hardware, and then schedule the handling of the new information at a later time (this is called the "bottom half") and return. The kernel is then guaranteed to call the bottom half as soon as possible -- and when it does, everything allowed in kernel modules will be allowed.

The way to implement this is to call request_irq() to get your interrupt handler called when the relevant IRQ is received (there are 15 of them, plus 1 which is used to cascade the interrupt controllers, on Intel platforms). This function receives the IRQ number, the name of the function, flags, a name for /proc/interrupts and a parameter to pass to the interrupt handler. The flags can include SA_SHIRQ to indicate you're willing to share the IRQ with other interrupt handlers (usually because a number of hardware devices sit on the same IRQ) and SA_INTERRUPT to indicate this is a fast interrupt. This function will only succeed if there isn't already a handler on this IRQ, or if you're both willing to share.

Then, from within the interrupt handler, we communicate with the hardware and then use queue_task_irq() with tq_immediate() and mark_bh(BH_IMMEDIATE) to schedule the bottom half. The reason we can't use the standard queue_task in version 2.0 is that the interrupt might happen right in the middle of somebody else's queue_task[2]. We need mark_bh because earlier versions of Linux only had an array of 32 bottom halves, and now one of them (BH_IMMEDIATE) is used for the linked list of bottom halves for drivers which didn't get a bottom half entry assigned to them.

Keyboards on the Intel Architecture

The rest of this chapter is completely Intel specific. If you're not running on an Intel platform, it will not work. Don't even try to compile the code here.

I had a problem with writing the sample code for this chapter. On one hand, for an example to be useful it has to run on everybody's computer with meaningful results. On the other hand, the kernel already includes device drivers for all of the common devices, and those device drivers won't coexist with what I'm going to write. The solution I've found was to write something for the keyboard interrupt, and disable the regular keyboard interrupt handler first. Since it is defined as a static symbol in the kernel source files (specifically, drivers/char/keyboard.c), there is no way to restore it. Before insmod'ing this code, do on another terminal sleep 120 ; reboot if you value your file system.

This code binds itself to IRQ 1, which is the IRQ of the keyboard controlled under Intel architectures. Then, when it receives a keyboard interrupt, it reads the keyboard's status (that's the purpose of the inb(0x64)) and the scan code, which is the value returned by the keyboard. Then, as soon as the kernel thinks it's feasible, it runs got_char which gives the code of the key used (the first seven bits of the scan code) and whether it has been pressed (if the 8th bit is zero) or released (if it's one).

/*
 *  intrpt.c - An interrupt handler.
 *
 *  Copyright (C) 2001 by Peter Jay Salzman
 */

/* 
 * The necessary header files 
 */

/* 
 * Standard in kernel modules 
 */
#include <linux/kernel.h>	/* We're doing kernel work */
#include <linux/module.h>	/* Specifically, a module */
#include <linux/sched.h>
#include <linux/workqueue.h>
#include <linux/interrupt.h>	/* We want an interrupt */
#include <asm/io.h>

#define MY_WORK_QUEUE_NAME "WQsched.c"

static struct workqueue_struct *my_workqueue;

/* 
 * This will get called by the kernel as soon as it's safe
 * to do everything normally allowed by kernel modules.
 */
static void got_char(void *scancode)
{
	printk("Scan Code %x %s.\n",
	       (int)*((char *)scancode) & 0x7F,
	       *((char *)scancode) & 0x80 ? "Released" : "Pressed");
}

/* 
 * This function services keyboard interrupts. It reads the relevant
 * information from the keyboard and then puts the non time critical
 * part into the work queue. This will be run when the kernel considers it safe.
 */
irqreturn_t irq_handler(int irq, void *dev_id, struct pt_regs *regs)
{
	/* 
	 * This variables are static because they need to be
	 * accessible (through pointers) to the bottom half routine.
	 */
	static int initialised = 0;
	static unsigned char scancode;
	static struct work_struct task;
	unsigned char status;

	/* 
	 * Read keyboard status
	 */
	status = inb(0x64);
	scancode = inb(0x60);

	if (initialised == 0) {
		INIT_WORK(&task, got_char, &scancode);
		initialised = 1;
	} else {
		PREPARE_WORK(&task, got_char, &scancode);
	}

	queue_work(my_workqueue, &task);

	return IRQ_HANDLED;
}

/* 
 * Initialize the module - register the IRQ handler 
 */
int init_module()
{
	my_workqueue = create_workqueue(MY_WORK_QUEUE_NAME);

	/* 
	 * Since the keyboard handler won't co-exist with another handler,
	 * such as us, we have to disable it (free its IRQ) before we do
	 * anything.  Since we don't know where it is, there's no way to
	 * reinstate it later - so the computer will have to be rebooted
	 * when we're done.
	 */
	free_irq(1, NULL);

	/* 
	 * Request IRQ 1, the keyboard IRQ, to go to our irq_handler.
	 * SA_SHIRQ means we're willing to have othe handlers on this IRQ.
	 * SA_INTERRUPT can be used to make the handler into a fast interrupt.
	 */
	return request_irq(1,	/* The number of the keyboard IRQ on PCs */
			   irq_handler,	/* our handler */
			   SA_SHIRQ, "test_keyboard_irq_handler",
			   (void *)(irq_handler));
}

/* 
 * Cleanup 
 */
void cleanup_module()
{
	/* 
	 * This is only here for completeness. It's totally irrelevant, since
	 * we don't have a way to restore the normal keyboard interrupt so the
	 * computer is completely useless and has to be rebooted.
	 */
	free_irq(1, NULL);
}

/* 
 * some work_queue related functions are just available to GPL licensed Modules
 */
MODULE_LICENSE("GPL");