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timer pit: fix precision reduction to milliseconds

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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 18:38:24 +02:00 committed by Christian Helmuth
parent 16745946e0
commit 652187b25e
1 changed files with 29 additions and 6 deletions
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35
repos/os/src/drivers/timer/pit/time_source.cc
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@ -68,7 +68,7 @@ void Timer::Time_source::schedule_timeout(Microseconds duration,
Duration Timer::Time_source::curr_time()
{
uint32_t passed_ticks;
uint32_t ticks;
/* read PIT count and status */
bool wrapped;
@ -76,19 +76,42 @@ Duration Timer::Time_source::curr_time()
/* determine the time since we looked at the counter */
if (wrapped && !_handled_wrap) {
passed_ticks = _counter_init_value;
ticks = _counter_init_value;
/* the counter really wrapped around */
if (curr_counter)
passed_ticks += PIT_MAX_COUNT + 1 - curr_counter;
ticks += PIT_MAX_COUNT + 1 - curr_counter;
_handled_wrap = true;
} else {
if (_counter_init_value)
passed_ticks = _counter_init_value - curr_counter;
ticks = _counter_init_value - curr_counter;
else
passed_ticks = PIT_MAX_COUNT + 1 - curr_counter;
ticks = PIT_MAX_COUNT + 1 - curr_counter;
}
_curr_time_us += (passed_ticks*1000)/PIT_TICKS_PER_MSEC;
/*
* 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.
*/
using time_t = unsigned long;
static_assert(PIT_TICKS_PER_MSEC >= 1000, "Bad TICS_PER_MS value");
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) / PIT_TICKS_PER_MSEC) << MSB_RSHIFT;
time_t const lsb = ((((ticks & LSB_MASK) << LSB_LSHIFT)
* 1000) / PIT_TICKS_PER_MSEC) >> LSB_LSHIFT;
_curr_time_us += (msb + lsb);
/* use current counter as the reference for the next update */
_counter_init_value = curr_counter;