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bsps/i386: Update calibration of TSC to be more accurate

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Closes #4456
This commit is contained in:
Jan Sommer
2021-06-11 09:35:30 +02:00
parent 8d4382ba34
commit 4925ab4f52
1 changed files with 37 additions and 32 deletions
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69
bsps/i386/pc386/clock/ckinit.c
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@@ -104,48 +104,60 @@ static uint32_t pc386_get_timecount_i8254(struct timecounter *tc)
/*
* Calibrate CPU cycles per tick. Interrupts should be disabled.
* Will also set the PIT, so call this before registering the
* periodic timer for rtems tick generation
*/
static void calibrate_tsc(void)
{
uint64_t begin_time;
uint8_t then_lsb, then_msb, now_lsb, now_msb;
uint32_t i;
uint8_t lsb, msb;
uint32_t max_timer_value;
uint32_t last_tick, cur_tick;
int32_t diff, remaining;
/*
* We just reset the timer, so we know we're at the beginning of a tick.
*/
/*
* Count cycles. Watching the timer introduces a several microsecond
* uncertaintity, so let it cook for a while and divide by the number of
* ticks actually executed.
*/
/* Set the timer to free running mode */
outport_byte(TIMER_MODE, TIMER_SEL0 | TIMER_16BIT | TIMER_INTTC);
/* Reset the 16 timer reload value, first LSB, then MSB */
outport_byte(TIMER_CNTR0, 0);
outport_byte(TIMER_CNTR0, 0);
/* We use the full 16 bit */
max_timer_value = 0xffff;
/* Calibrate for 1s, i.e. TIMER_TICK PIT ticks */
remaining = TIMER_TICK;
begin_time = rdtsc();
for (i = rtems_clock_get_ticks_per_second() * pc386_isrs_per_tick;
i != 0; --i ) {
/* We know we've just completed a tick when timer goes from low to high */
then_lsb = then_msb = 0xff;
do {
READ_8254(now_lsb, now_msb);
if ((then_msb < now_msb) ||
((then_msb == now_msb) && (then_lsb < now_lsb)))
break;
then_lsb = now_lsb;
then_msb = now_msb;
} while (1);
READ_8254(lsb, msb);
last_tick = (msb << 8) | lsb;
while(remaining > 0) {
READ_8254(lsb, msb);
cur_tick = (msb << 8) | lsb;
/* PIT counts down, so subtract cur from last */
diff = last_tick - cur_tick;
last_tick = cur_tick;
if (diff < 0) {
diff += max_timer_value;
}
remaining -= diff;
}
pc586_tsc_frequency = rdtsc() - begin_time;
#if 0
printk( "CPU clock at %u MHz\n", (uint32_t)(pc586_tsc_frequency / 1000000));
printk( "CPU clock at %u Hz\n", (uint32_t)(pc586_tsc_frequency ));
#endif
}
static void clockOn(void)
{
/*
* First calibrate the TSC. Do this every time we
* turn the clock on in case the CPU clock speed has changed.
*/
if ( x86_has_tsc() ) {
calibrate_tsc();
}
rtems_interrupt_lock_context lock_context;
pc386_isrs_per_tick = 1;
pc386_microseconds_per_isr = rtems_configuration_get_microseconds_per_tick();
@@ -171,13 +183,6 @@ static void clockOn(void)
rtems_interrupt_lock_release(&rtems_i386_i8254_access_lock, &lock_context);
bsp_interrupt_vector_enable( BSP_PERIODIC_TIMER );
/*
* Now calibrate cycles per tick. Do this every time we
* turn the clock on in case the CPU clock speed has changed.
*/
if ( x86_has_tsc() )
calibrate_tsc();
}
bool Clock_isr_enabled = false;