ExternalVendorCode/genode
1
0
Fork 0
mirror of https://github.com/genodelabs/genode.git synced 2025-03-23 04:25:21 +00:00
Code Issues Actions Packages Projects Releases Wiki Activity

hw pit: fix precision reduction to milliseconds

Browse Source 
Due to the simplicity of the algorithm that translated from timer ticks
to time, we lost microseconds precision although the timer allows for it.

Ref #2400
This commit is contained in:
Martin Stein 2017-08-23 18:19:32 +02:00 committed by Christian Helmuth
parent 02bbb2efaf
commit 16745946e0
1 changed files with 24 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

25
repos/base-hw/src/core/spec/x86_64/timer.cc
View File

@ -64,6 +64,7 @@ Timer_driver::Timer_driver(unsigned)
/* Calculate timer frequency */
ticks_per_ms = pit_calc_timer_freq();
assert(ticks_per_ms >= 1000);
}
@ -84,7 +85,29 @@ void Timer::_start_one_shot(time_t const ticks) {
time_t Timer::_ticks_to_us(time_t const ticks) const {
return (ticks / _driver.ticks_per_ms) * 1000; }
/*
* If we would do the translation with one division and
* multiplication over the whole argument, we would loose
* microseconds granularity although the timer frequency would
* allow for such granularity. Thus, we treat the most significant
* half and the least significant half of the argument separate.
* Each half is shifted to the best bit position for the
* translation, then translated, and then shifted back.
*/
enum {
HALF_WIDTH = (sizeof(time_t) << 2),
MSB_MASK = ~0UL << HALF_WIDTH,
LSB_MASK = ~0UL >> HALF_WIDTH,
MSB_RSHIFT = 10,
LSB_LSHIFT = HALF_WIDTH - MSB_RSHIFT,
};
time_t const msb = ((((ticks & MSB_MASK) >> MSB_RSHIFT)
* 1000) / _driver.ticks_per_ms) << MSB_RSHIFT;
time_t const lsb = ((((ticks & LSB_MASK) << LSB_LSHIFT)
* 1000) / _driver.ticks_per_ms) >> LSB_LSHIFT;
return msb + lsb;
}
time_t Timer::us_to_ticks(time_t const us) const {