Linux – How does keyboard interrupt ends up as process signal

1. All modern operating systems support multitasking. This means that the system is able to execute multiple processes at the same time; either in pseudo-parallel (when only one CPU is available) or nowadays with multi-core CPUs being common in parallel (one task/core).

   Let's take the simpler case of only one CPU being available. This means that if you execute at the same time two different processes (let's say a web browser and a music player) the system is not really able to execute them at the same time. What happens is that the CPU is switching from one process to the other all the time; but this is happening extremely fast, thus you never notice it.

   Now let's assume that while those two processes are executing, you press the reset button (bad boy). The CPU will immediately stop whatever is doing and reboot the system. Congratulations: you generated an interrupt.

   The case is similar when you are programming and want to ask for a service from the CPU. The difference is that in this case you execute software code -- usually library procedures that are executing system calls (for example fopen for opening a file).

   Thus, 1 describes two different ways of getting attention from the CPU.

2. Most modern operating systems support two execution modes: user mode and kernel mode. By default an operating system runs in user mode. User mode is very limited. For example, all I/O is forbidden; thus, you are not allowed to open a file from your hard disk. Of course this never happens in real, because when you open a file the operating system switches from user to kernel mode transparently. In kernel mode you have total control of the hardware.

   If you are wondering why those two modes exist, the simplest answer is for protection. Microkernel-based operating systems (for example MINIX 3) have most of their services running in user mode, which makes them less harmful. Monolithic kernels (like Linux) have almost all their services running in kernel mode. Thus a driver that crashes in MINIX 3 is unlikely to bring down the whole system, while this is not unusual in Linux.

   System calls are the primitive used in monolithic kernels (shared data model) for switching from user to kernel mode. Message passing is the primitive used in microkernels (client/server model). To be more precise, in a message passing system programmers also use system calls to get attention from the CPU. Message passing is visible only to the operating system developers. Monolithic kernels using system calls are faster but less reliable, while microkernels using message passing are slower but have better fault isolation.

   Thus, 2 mentions two different ways of switching from user to kernel mode.

3. To revise, the most common way of creating a software interrupt, aka trap, is by executing a system call. Interrupts on the other hand are generated purely by hardware.

   When we interrupt the CPU (either by software or by hardware) it needs to save somewhere its current state -- the process that it executes and at which point it did stop -- otherwise it will not be able to resume the process when switching back. That is called a context switch and it makes sense: Before you switch off your computer to do something else, you first need to make sure that you saved all your programs/documents, etc so that you can resume from the point where you stopped the next time you'll turn it on :)

   Thus, 3 explains what needs to be done after executing a trap or an interrupt and how similar the two cases are.

This is covered in chapter 10 of Linux Device Drivers, 3rd edition, by Corbet et al. It is available for free online, or you may toss some shekels O'Reilly's way for dead tree or ebook forms. The part relevant to your question begins on page 278 in the first link.

For what it's worth, here is my attempt to paraphrase those three pages, plus other bits I've Googled up:

• When you register a shared IRQ handler, the kernel checks that either:

  a. no other handler exists for that interrupt, or

  b. all of those previously registered also requested interrupt sharing

  If either case applies, it then checks that your dev_id parameter is unique, so that the kernel can differentiate the multiple handlers, e.g. during handler removal.

• When a PCI¹ hardware device raises the IRQ line, the kernel's low-level interrupt handler is called, and it in turn calls all of the registered interrupt handlers, passing each back the dev_id you used to register the handler via request_irq().

  The dev_id value needs to be machine-unique. The common way to do that is to pass a pointer to the per-device struct your driver uses to manage that device. Since this pointer must be within your driver's memory space for it to be useful to the driver, it is ipso facto unique to that driver.²

  If there are multiple drivers registered for a given interrupt, they will all be called when any of the devices raises that shared interrupt line. If it wasn't your driver's device that did this, your driver's interrupt handler will be passed a dev_id value that doesn't belong to it. Your driver's interrupt handler must immediately return when this happens.

  Another case is that your driver is managing multiple devices. The driver's interrupt handler will get one of the dev_id values known to the driver. Your code is supposed to poll each device to find out which one raised the interrupt.

  The example Corbet et al. give is that of a PC parallel port. When it asserts the interrupt line, it also sets the top bit in its first device register. (That is, inb(0x378) & 0x80 == true, assuming standard I/O port numbering.) When your handler detects this, it is supposed to do its work, then clear the IRQ by writing the value read from the I/O port back to the port with the top bit cleared.

  I don't see any reason that particular mechanism is special. A different hardware device could choose a different mechanism. The only important thing is that for a device to allow shared interrupts, it has to have some way for the driver to read the interrupt status of the device, and some way to clear the interrupt. You'll have to read your device's datasheet or programming manual to find out what mechanism your particular device uses.

• When your interrupt handler tells the kernel it handled the interrupt, that doesn't stop the kernel from continuing to call any other handlers registered for that same interrupt. This is unavoidable if you are to share an interrupt line when using level-triggered interrupts.

  Imagine two devices assert the same interrupt line at the same time. (Or at least, so close in time that the kernel doesn't have time to call an interrupt handler to clear the line and thereby see the second assertion as separate.) The kernel must call all handlers for that interrupt line, to give each a chance to query its associated hardware to see if it needs attention. It is quite possible for two different drivers to successfully handle an interrupt within the same pass through the handler list for a given interrupt.

  Because of this, it is imperative that your driver tell the device it is managing to clear its interrupt assertion sometime before the interrupt handler returns. It's not clear to me what happens otherwise. The continuously-asserted interrupt line will either result in the kernel continuously calling the shared interrupt handlers, or it will mask the kernel's ability to see new interrupts so the handlers are never called. Either way, disaster.

Footnotes:

1. I specified PCI above because all of the above assumes level-triggered interrupts, as used in the original PCI spec. ISA used edge-triggered interrupts, which made sharing tricky at best, and possible even then only when supported by the hardware. PCIe uses message-signalled interrupts; the interrupt message contains a unique value the kernel can use to avoid the round-robin guessing game required with PCI interrupt sharing. PCIe may eliminate the very need for interrupt sharing. (I don't know if it actually does, just that it has the potential to.)

2. Linux kernel drivers all share the same memory space, but an unrelated driver isn't supposed to be mucking around in another's memory space. Unless you pass that pointer around, you can be pretty sure another driver isn't going to come up with that same value accidentally on its own.