12.1. Interrupt Handlers

12.1.1. 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 (InterruptRe quests)[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. [2]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. Usually there is a certain number of IRQs available. How many IRQs there are is hardware-dependent. 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_work() mark_bh(BH_IMMEDIATE) to schedule the bottom half.

12.1.2. 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).

Example 12-1. intrpt.c

/* a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html intrpt.c - An interrupt handler. a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html Copyright (C) 2001 by Peter Jay Salzman figures/ /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html The necessary header files figures/ /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html Standard in kernel modules figures/ #include <linux/kernel.h> /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var We're doing kernel work figures/ #include <linux/module.h> /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var Specifically, a module figures/ #include <linux/sched.h> #include <linux/workqueue.h> #include <linux/interrupt.h> /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var We want an interrupt figures/ #include <asm/io.h> #define MY_WORK_QUEUE_NAME "WQsched.c" static struct workqueue_struct *my_workqueue; /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html This will get called by the kernel as soon as it's safe a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html to do everything normally allowed by kernel modules. figures/ static void got_char(void *scancode) { printk(KERN_INFO "Scan Code %x %s.\n", (int)*((char *)scancode) & 0x7F, *((char *)scancode) & 0x80 ? "Released" : "Pressed"); } /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html This function services keyboard interrupts. It reads the relevant a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html information from the keyboard and then puts the non time critical a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html part into the work queue. This will be run when the kernel considers it safe. figures/ irqreturn_t irq_handler(int irq, void *dev_id, struct pt_regs *regs) { /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html This variables are static because they need to be a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html accessible (through pointers) to the bottom half routine. figures/ static int initialised = 0; static unsigned char scancode; static struct work_struct task; unsigned char status; /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html Read keyboard status figures/ 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; } /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html Initialize the module - register the IRQ handler figures/ int init_module() { my_workqueue = create_workqueue(MY_WORK_QUEUE_NAME); /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html Since the keyboard handler won't co-exist with another handler, a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html such as us, we have to disable it (free its IRQ) before we do a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html anything. Since we don't know where it is, there's no way to a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html reinstate it later - so the computer will have to be rebooted a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html when we're done. figures/ free_irq(1, NULL); /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html Request IRQ 1, the keyboard IRQ, to go to our irq_handler. a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html SA_SHIRQ means we're willing to have othe handlers on this IRQ. a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html SA_INTERRUPT can be used to make the handler into a fast interrupt. figures/ return request_irq(1, /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var The number of the keyboard IRQ on PCs figures/ irq_handler, /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var our handler figures/ SA_SHIRQ, "test_keyboard_irq_handler", (void *)(irq_handler)); } /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html Cleanup figures/ void cleanup_module() { /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html This is only here for completeness. It's totally irrelevant, since a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html we don't have a way to restore the normal keyboard interrupt so the a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html computer is completely useless and has to be rebooted. figures/ free_irq(1, NULL); } /apps /backup /bin /boot /data /dev /etc /home /lib /lost+found /media /misc /mnt /net /opt /proc /root /sbin /selinux /srv /sys /tftpboot /tmp /usr /var a1387.htm a1387.html a1403.htm a1403.html book1.htm c1050.htm c1050.html c1159.htm c1159.html c119.htm c119.html c1209.htm c1209.html c1254.htm c1324.htm c1324.html c1350.htm c1350.html c38.htm c38.html c425.htm c425.html c567.htm c567.html c708.htm c708.html c885.htm c885.html c890.htm c890.html c976.htm c976.html doc-index.html f25.htm f25.html figures hello2.html i1413.htm index.html interrupthandlers.html ln16.html TXT_lkmpg.html x1052.html x1161.html x1194.htm x1194.html x1211.html x121.html x1256.html x1326.html x1352.html x1389.html x1405.html x181.htm x181.html x217.htm x245.htm x245.html x279.htm x279.html x27.html x30.htm x30.html x323.htm x323.html x351.htm x351.html x35.htm x35.html x380.htm x380.html x40.html x427.html x44.htm x44.html x569.html x710.html x769.htm x769.html x810.htm x810.html x861.htm x861.html x887.html x892.html x978.html some work_queue related functions are just available to GPL licensed Modules figures/ MODULE_LICENSE("GPL"); 

Notes

[1]

This is standard nomencalture on the Intel architecture where Linux originated.

[2]

In practice IRQ handling can be a bit more complex. Hardware is often designed in a way that chains two interrupt controllers, so that all the IRQs from interrupt controller B are cascaded to a certain IRQ from interrupt controller A. Of course that requires that the kernel finds out which IRQ it really was afterwards and that adds overhead. Other architectures offer some special, very low overhead, so called "fast IRQ" or FIQs. To take advantage of them requires handlers to be written in assembler, so they do not really fit into the kernel. They can be made to work similar to the others, but after that procedure, they're no longer any faster than "common" IRQs. SMP enabled kernels running on systems with more than one processor need to solve another truckload of problems. It's not enough to know if a certain IRQs has happend, it's also important for what CPU(s) it was for. People still interested in more details, might want to do a web search for "APIC" now ;)