Kernel Korner - Dynamic Interrupt Request Allocation for Device Drivers
A snapshot of interrupts already in use on the system is stored in the /proc directory. The $cat /proc/interrupt command displays the data related to the interrupts. The following output was displayed on my machine:
CPU0 0: 82821789 XT-PIC timer 1: 122 XT-PIC i8042 2: 0 XT-PIC cascade 8: 1 XT-PIC rtc 10: 154190 XT-PIC eth0 12: 100 XT-PIC i8042 14: 21578 XT-PIC ide0 15: 18 XT-PIC ide1 NMI: 0 ERR: 0
The first column is the IRQ line (vector ranges from 32–47), and the next column is the number of times the interrupts are delivered in the CPU after booting the system. The third column is related to the PIC, and the last column is the list of the device names that have registered handlers for the corresponding interrupt.
The simplest way to load a device driver dynamically is first to find the unused IRQ line in the system. A request_irq function is used to allocate a specified IRQ line number for a device. The syntax for the request_irq follows and is declared in linux/sched.h:
int request_irq (unsigned int irq, void (*handler) (int, void *, struct pt_regs *), unsigned long flags, const char *device, void *dev_id);
The details of the arguments in this function are:
unsigned int irq: interrupt number, which we want to request from the system.
void (*handler) (int, void *, struct pt_regs *): whenever an interrupt is generated, we have to write ISRs to handle the interrupt; otherwise, the processor simply acknowledges it and does nothing else for that interrupt. This argument is the pointer to the handler function. The syntax for the handler function is:
void handler (int irq, void *dev_id, struct pt_regs *regs);
The first argument is the IRQ number, which we already have mentioned in the request_irq function. The second argument is a device identifier, using major and minor numbers to identify which device is in charge of the current interrupt event. The third argument is used to save the process' context in the kernel stack before the processor starts executing the interrupt handler function. This structure is used when the system resumes the execution of the earlier process. Normally, device driver writers need not worry about this argument.
unsigned long flags: the flags variable is used for interrupt management. The SA_INTERRUPT flag is set for fast interrupt handler, and it disables all the maskable interrupt. SA_SHIRQ is set when we want to share the irq with more than one device, SA_PROBE is set if we are interested in probing a hardware device using the IRQ line, and SA_RANDOM is used to seed the kernel random number generator. For more details of this flag, see /usr/src/linux/drivers/char/random.c.
constant char *device: a device name that holds the IRQ.
void *dev_id: the device identifier—it's a pointer to the device structure. When the interrupt is shared, this field points to the particular device.
The request_irq function returns 0 on success and -EBUSY when the allocation has failed. EBUSY is the error number of 16, which is described in the /usr/src/linux/include/asm/errno.h file. The free_irq function releases the IRQ number from the device. The syntax for this function is:
free_irq (unsigned int irq, void *dev_id);
The explanation for the arguments is the same as above.
An ISR is invoked whenever an interrupt occurs. The operations to be performed on the cause of the interrupt are described in the ISR. The kernel maintains a table in memory, which contains the addresses of the interrupt routines (interrupt vectors). When an interrupt occurs, the processor checks the address of the ISR in the interrupt vector table and then executes. The task of the ISR is to react to the device according to the nature of the interrupt, such as read or write data. Typically, the ISR wakes up sleeping processes on the device if the interrupt signals the event for which they are waiting.
The amount of time the processor takes to respond to an interrupt is called interrupt latency. Interrupt latency is composed of hardware propagation time, register saving time and software propagation time. Interrupt latency should be minimal to improve the system's performance; for this reason, the ISR should be short and disable interrupts only for a brief time. Other interrupts can occur while interrupts are disabled, but the processor does not allow them until interrupts are re-enabled. If more than one interrupt is blocked, the processor allows them in priority order when it is ready for interrupt service.
Device driver developers should disable interrupts in driver code only when necessary, because the system does not update the system timers, transfer network packets to and from buffers and so on during the interrupt disabling. Driver developers should write ISRs to release the processor for other tasks. In real-world scenarios, however, ISRs handle lengthy tasks. In such situations, the ISR can do only the time-critical communication with the hardware to disable the interrupt and use the tasklet to perform most of the actual data transfer processing. The tasklet is the advanced feature in the latest Linux kernel that does certain operations related to the interrupt during safe times. The tasklet is the software interrupt, and it can be interrupted by other interrupts. The internals of the interrupts have been explained in detail by Bovet and Cesati (see the on-line Resources), and the implementation of the interrupts in device driver perspective is presented by Rubini and Corbet (see Resources).
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