Why is there no spin-lock variant of IOLockSleep()? Is it by design?
(perhaps because what I want to do is frowned upon?). Or is it just an
accidental omission from the API and I should file an enhancement
request?
Here is some background:
Many of my device interrupts are done to deliver latency critical
wakeups to a blocked userspace thread. The sleep/interrupt/wakeup
process currently add over 10us of end-to-end latency over a simple
polling approach (which itself is 3.5us). If possible, I'd like to
issue these wakeups from a filter interrupt, rather than from my
workloop, so as to avoid the extra context switch into my workloop.
I currently use IOLockSleep() to block the userspace thread. This
boils down to the Mach lck_mtx_sleep() function. Unfortunately,
to avoid missed wakeup races, it requires that the wakeup be
issued while holding the lock. Since this is a sleep lock,
that means the wakeup can not be issued from a hardware interrupt
context.
The good news is that there is a corresponding Mach variant of
lck_mtx_sleep() called lck_spin_sleep() that I could use. If
I understand things correctly, it is OK to grab a spinlock
(IOSimpleLock) from a hardware interrupt context. It would
have to be OK to be able to schedule workloops...
My worry is that there is no IOSimpleLockSleep(), and I would have to
call a Mach function directly. Since I've been burned by going around
IOKit in the past, I thought should I ask why the IOKit wrapper is not
there..
Thanks,
Drew
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