drivers: timer: nrf: refactor for speed and correctness
The existing implementation of z_clock_set_timeout() calculates the
compare value based on a complex series of operations including an
unconditional integer division and multiplication intended to ensure the
compare value is aligned to a tick boundary. On the nRF51 this division
requires a call to an outline function with a data-dependent execution
time.
In the common case where the timeout is set less than one tick past the
last observed tick the devision can be elided, as can several extra
operations intended to deal with fractional ticks.
The code also failed to account for a ticks-per-cycle that violated the
minimum delay required to guarantee a compare value would result in a
match without wrapping. The minimum delay was also unreasonably long
(about 1 ms). Reduce it to a more reasonable value to allow for a
higher ticks-per-second, and diagnose attempts to set the tick frequency
above the supported maximum (8192 Hz).
Finally, move the parts of the compare calculation that are not
dependent on the live counter value out of the locked region.
Prior to this change the observed time between the irq_lock() and
irq_unlock() in z_clock_set_timeout() on the nRF51 ranged between 5 us
and 8 us.
With the revised algorithm the observed lock duration is between 2.16 us
(1024 Hz) and 2.88 us (100 Hz) in the common case that the compare is
set within the current tick. If the compare is set late the duration
will be higher, but no greater than the previous implementation.
Signed-off-by: Peter A. Bigot <pab@pabigot.com>
1 file changed
