Hello all!
We're currently working with David Brownell and Remy Bohmer on implementation
of clocksource/events on the at91rm9200 processor (arm arch).
While debugging our implementation, we came about a rounding problem, in the
setting of min_delta_ns field, and I think it can be in almost all
clock_event_device.
Almost all clock_event_device initialize the field min_delta_ns this way:
clkevt32k.mult = div_sc(CLOCK_TICK_RATE, NSEC_PER_SEC, clkevt32k.shift);
clkevt32k.max_delta_ns = clockevent_delta2ns(0x000fffff, &clkevt32k);
clkevt32k.min_delta_ns = clockevent_delta2ns(1, &clkevt32k);
The clockevent_delta2ns function rounds the result down, so the rounded
min_delta_ns
can be smaller than the real min_delta_ns.
When clockevents_program_event is called, this rounding problem shows up:
...
if (delta < dev->min_delta_ns)
delta = dev->min_delta_ns;
clc = delta * dev->mult;
clc >>= dev->shift;
return dev->set_next_event((unsigned long) clc, dev)
When delta is to small, min_delta_ns will be used, and with a wrong rounding,
will call set_next_event with a value smaller than the expected, and
set_next_event will fail.
This will result in an infinite loop when clockevents_program_event is called
from tick_program_event with a too short delay.
In our driver, we fixed this problem using that:
- clkevt32k.min_delta_ns = clockevent_delta2ns(1, &clkevt32k);
+ clkevt32k.min_delta_ns = clockevent_delta2ns(1, &clkevt32k)+1;
I think other clock_event_device can have the same problem.
Regards
Marc
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