Hi Marc / Daniel / Jason,
I had a couple of questions about percpu irq API, hopefully you can help answer.
On ARM, how do u handle requesting per cpu IRQs - specifically usage of
request_percpu_irq() / enable_percpu_irq() API.
It seems, for using them, we obviously need to explicitly set irq as percpu and
as a consequence explicitly enable autoen (since former disables that). See
arch/arc/kernel/irq.c: arc_request_percpu_irq() called by ARC per cpu timer
setup.
if (!cpu) {
irq_set_percpu_devid() <--- disables AUTOEN
irq_modify_status(IRQ_NOAUTOEN) <-- to undo side-effect of above
request_percpu_irq
}
enable_percpu_irq
I don't see pattern in general for drivers/clocksource/ and/or arm_arch_timer.c
for PPI case.
Further there is an ordering requirement as in request_percpu_irq() needs to be
called only for the first calling core, and enable_percpu_irq() on each one. If
enable is done ahead of request it obviously fails.
For ARC I've historically used a wrapper arc_request_percpu_irq() [pseudo code
above] - which has an inherent assumption (now realize fragile) that it will be
called on core0 first thus guaranteeing the ordering above. This is true for
timer, IPI etc but not for other late probed peripherals - specially perf.
Infact ARC perf probe open codes on_each_cpu() to ensure irq request is done
locally first.
But this all falls apart, when perf probe happens on coreX (not core0), causing
enable to be called ahead of request anyways. This is what I'm running into now.
I think the solution is to call request_percpu_irq() on whatever core hits
first and call enable_percpu_irq() from a cpu up notifier. But I think the
notifier won't run on boot cpu ? Or is there a better way to clean up all this
mess.
FWIW, I see this issue on 3.18 kernel but not latest 4.4-rcX because in 3.18
arc perf probe invariably happens on coreX (due to init task migration right
after clocksource switch - something which doesn't happen on 4.4 likely due to
recent timer core changes).
Thx,
-Vineet
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