Sleeping in an ISR is not fundamentally impossible - I could design a multitasker that permitted it - but has significant problems, and most multitaskers, including Linux, forbid it. The first problem is the scheduler. "Sleeping" is actually a call into the scheduler to choose another process to run. There are times - so-called critical sections - when the scheduler can't be called. If an interrupt can call the scheduler, then every criticial section has to disable interrupts. Otherwise, an interrupt might arrive and end up calling the scheduler. This increases interrupt latency. If interrupts are forbidden to sleep, then there's no need to disable interrupts in critical sections, so interrupts can be responded to faster. Most multitaskers find this worth the price. The second problem is shared interrupts. You want to sleep until something happens. The processor hears about that event via an interrupt. Inside an ISR, interrupts are disabled. You have to somehow enable the interrupt that will wake up the sleeping ISR without enabling the interrupt that the ISR is in the middle of handling (or the handler will start a second time and make a mess). This is complicated and prone to error. And, in the case of shared interrupts (as allowed by PCI), it's possible that the the interrupt you need to wait for is exactly the same interrupt as what you're in the middle of handling. So it might be impossible! The third problem is that you're obviously increasing the latency of the interrupt whose handler you're sleeping in. Finally, if you're even *thinking* of wanting to sleep in an ISR, you probably have a deadlock waiting to happen. - To unsubscribe from this list: send the line "unsubscribe linux-newbie" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.linux-learn.org/faqs
