ehmry/genode
ehmry/genode
1
0
Fork 0
Code Issues Pull Requests Releases Wiki Activity

timer 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 18:38:24 +02:00 committed by Christian Helmuth
parent 16745946e0
commit 652187b25e
1 changed files with 29 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

35
repos/os/src/drivers/timer/pit/time_source.cc
View File

@ -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;