Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Writing interrupt handlers
• InterruptMask()
• InterruptUnmask()
• InterruptLock()
• InterruptUnlock()
• InterruptDisable()
• InterruptEnable()
• in*() and out*()
Basically, the rule of thumb is, “Don’t use anything that’s going to take a huge amount
of stack space or time, and don’t use anything that issues kernel calls.” The stack
space requirement stems from the fact that ISRs have very limited stacks.
The list of interrupt-safe functions makes sense — you might want to move some
memory around, in which case the mem*() and str*() functions are a good choice.
You’ll most likely want to read data registers from the hardware (in order to save
transitory data variables and/or clear the source of the interrupt), so you’ll want to use
the in*() and out*() functions.
What about the bewildering choice of Interrupt*() functions? Let’s examine them in
pairs:
InterruptMask() and InterruptUnmask()
These functions are responsible for masking the interrupt source at the PIC
level; this keeps them from being passed on to the CPU. Generally, you’d use
this if you want to perform further work in the thread and can’t clear the source
of the interrupt in the ISR itself. In this case, the ISR would issue
InterruptMask(), and the thread would issue InterruptUnmask() when it had
completed whatever operation it was invoked to do.
Keep in mind that InterruptMask() and InterruptUnmask() are counting — you
must “unmask” the same number of times that you’ve “masked” in order for the
interrupt source to be able to interrupt you again.
By the way, note that the InterruptAttachEvent() performs the InterruptMask()
for you (in the kernel) — therefore you must call InterruptUnmask() from your
interrupt-handling thread.
InterruptLock() and InterruptUnlock()
These functions are used to disable (InterruptLock()) and enable
(InterruptUnlock()) interrupts on a single or multiprocessor system. You’d want
to disable interrupts if you needed to protect the thread from the ISR (or
additionally, on an SMP system, the ISR from a thread). Once you’ve done your
critical data manipulation, you’d then enable interrupts. Note that these
functions are recommended over the “old” InterruptDisable() and
InterruptEnable() functions as they will operate properly on an SMP system.
April 30, 2009 Chapter 4 • Interrupts 187