Kernel Korner - Dynamic Interrupt Request Allocation for Device Drivers

A computer cannot meet its requirements unless it communicates with its external devices. An interrupt is a communication gateway between the device and a processor. The allocation of an interrupt request line for a device and how the interrupt is handled play vital roles in device driver development. As the number of interrupt request lines in a system is limited, sharing an interrupt between devices is a must to access more devices. Any attempt to allocate an interrupt already in use, however, eventually crashes the system. This article explains the basics of the interrupt and the fundamentals of interrupt handling and includes an implementation of an interrupt request (IRQ) allocation for a character device.

The purpose of any device is to do some useful job, and to do so it should communicate with the microprocessor. When a processor wants to communicate with a device, it sends instructions to the device controller. A device controller controls the operation of a device. Similarly, if a device wants to reply to a processor that says new data is ready to be retrieved, the devices generate an interrupt to capture the processor's attention. An interrupt is a hardware mechanism that enables a device to communicate with a processor.

Until version 2.6, Linux had been non-preemptive, meaning that when a process is running in kernel mode, if any higher-priority process arrives in the ready-to-run queue, the lower-priority process cannot be preempted until it returns to user mode. But, an interrupt is allowed to divert CPU attention even though it is executing a process in kernel mode. This helps to improve the throughput of a system. When an interrupt occurs, the CPU suspends the current task and executes some other code, which responds to whatever event caused the interrupt.

Each device in a computer has a device controller, and it has a hardware pin that is used to assert when the device requires CPU service. This pin is attached to the corresponding interrupt pin in the CPU, which facilitates communication. The pin in the processor connected to the controller is called the interrupt request line. A CPU has several such pins so that many devices can be serviced by the processor. In a modern operating system, a programmable interrupt controller (PIC) is used to manage the IRQ lines between the processor and the various device controllers. The number of free IRQs in a system is restricted, but Linux has a mechanism to allow many pieces of hardware to share the same interrupts.

Interrupt servicing can be compared to a programmer's job. The programmer opens a mailbox and does his routine programming work. When new mail arrives, he is interrupted by a beep or by some other notification at the corner of the screen. Immediately, he saves the program and switches over to the mailbox. He then reads the mail, sends an acknowledgement and resumes his earlier work. A detailed reply listing the steps he has taken is sent later.

Similarly, when a CPU executes a process, a device can send an interrupt to the CPU regarding some task, for example, data is ready for transfer. When an interrupt comes, the CPU instantly saves the current value of the program counter in the kernel mode stack and executes the corresponding interrupt service routine (ISR). An ISR is a function situated in the kernel that determines the nature of the interrupt and performs whatever actions are needed, such as moving a block of data from hard disk to main memory. After executing the ISR, the CPU resumes the earlier process and executes.

A device driver is a software module in the kernel that waits for requests from the application program. Whenever an application wants to read data from a device, the corresponding device driver is invoked immediately, and the respective device is open for reading. If the system is waiting for slow hardware, it cannot do any useful job. One of the prime aims of kernel developers is to utilize system resources effectively. To avoid waiting for data from the hardware, the kernel gives this job to the device controller and resumes the stopped process. When reading completes, the device notifies the CPU through an interrupt. The processor then executes the corresponding ISR.

Interrupts are divided into two broad categories, synchronous and asynchronous. Synchronous interrupts are generated by the CPU control unit when it is executing an instruction. The control unit issues an interrupt after terminating the instructions, hence the name synchronous interrupt. Asynchronous interrupts are created by hardware devices at random times with respect to the CPU clock. In the Intel context, the first one is called exceptions and the second is interrupts. Interrupt is identified by an unsigned one-byte integer called a vector. The vector ranges between 0 to 255. The first 32 (0–31) vectors are exceptions and non-maskable interrupts, which was explained in my article “Linux Signals for the Application Programmer”, LJ, March 2003. The range from 32–47 is assigned to maskable interrupts and is generated by IRQs (0–15 IRQ line numbers). The last range, from 48–255, is used to identify software interrupts; an example of this is interrupt 128 (int 0X80 assembly instructions), which is used to implement system calls.