Getting Started with QNX Neutrino - QNX Software Systems

Summary
© 2009, QNX Software Systems GmbH & Co. KG.
There’s an additional cost over the “old” functions to perform the check on an
SMP system, but in a single processor system it’s negligible, which is why I’m
recommending that you always use InterruptLock() and InterruptUnlock().
InterruptDisable() and InterruptEnable()
These functions shouldn’t be used in new designs. Historically, they were used
to invoke the x86 processor instructions cli and sti when Neutrino was
x86-only. They’ve since been upgraded to handle all supported processors, but
you should use InterruptLock() and InterruptUnlock() (to keep SMP systems
happy).
The one thing that bears repeating is that on an SMP system, it is possible to have both
the interrupt service routine and another thread running at the same time.
Summary
Keep the following things in mind when dealing with interrupts:
• Don’t take too long in an ISR — perform the minimum amount of work you can
get away with. This helps minimize interrupt latency and debugging.
• Use InterruptAttach() when you need to access the hardware as soon as the
interrupt occurs; otherwise, avoid it.
• Use InterruptAttachEvent() at all other times. The kernel will schedule a thread
(based on the event that you passed) to handle the interrupt.
• Protect variables used by both the interrupt service routine (if using
InterruptAttach()) and threads by calling InterruptLock() and InterruptUnlock().
• Declare variables that are going to be used between the thread and the ISR as
volatile so that the compiler isn’t caching “stale” values that have been changed
by the ISR.
188 Chapter 4 • Interrupts April 30, 2009