question about periodic clocks

[Jeremy Fitzhardinge]

How does the clock period get set on periodic timers?  In my clock
driver, I'm seeing a call to ->set_mode(CLOCK_EVT_MODE_PERIODIC, evt),
but then... nothing.  I was expecting a call to set_next_event to set
the timer period.

The calltrace is:

#0  xen_new_set_mode (mode=CLOCK_EVT_MODE_PERIODIC, evt=0xc10a2ac0)
    at arch/i386/xen/time.c:275
#1  0xc01323da in clockevents_set_mode (dev=0xc10a2ac0, 
    mode=CLOCK_EVT_MODE_PERIODIC) at kernel/time/clockevents.c:64
#2  0xc0132854 in tick_setup_periodic (dev=0xc10a2ac0, 
    broadcast=<value optimized out>) at kernel/time/tick-common.c:111

and tick_setup_periodic does just call clockevents_set_mode, but nothing
to set a period.

Am I supposed to assume some default period?  HZ?  (That's what hpet
seems to do.)

Is set_next_event only ever called if the timer is in ONESHOT mode?

Thanks,
    J
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