Linux Device Drivers

Interrupt Management

During 2.1 development, some of the Linux internals were changed. New kernels offer good management of internal locks; race conditions are avoided using several fine-grained locks instead of global ones, thus obtaining better performance--especially with SMP configurations.

One of the outcomes of finer locking is that intr_count no longer exists. Version 2.1.34 got rid of the global variable, and the boolean function in_interrupt can be used instead (this function has existed since 2.1.30). Currently, in_interrupt is a macro declared in the header <asm/hardirq.h>, which in turn is included by <linux/interrupt.h>. The header sysdep-2.1.h conditionally defines in_interrupt in terms of intr_count to achieve backward portability to 2.0.

Note that while in_interrupt is an integer, intr_count was unsigned long, so if you want to print the value and be portable across 2.0 and 2.1, you should cast the value to an explicit type and specify the suitable format in calling printk.

Another difference in interrupt management was introduced in 2.1.37: fast and slow interrupt handlers don’t exist any more. The flag SA_INTERRUPT isn’t used by the new version of request_irq, but it still controls whether interrupts are enabled before the handler is executed. If several handlers share the interrupt line, each can be of a different ``type.'' Interrupts are enabled or not according to the type of the first handler being called. Bottom halves are always executed when the interrupt ...