Re: Higher resolution timers

[Colin D Bennett <colin@gibibit.com>]

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On Mon, 12 May 2008 09:33:54 -0700
Colin D Bennett <colin@gibibit.com> wrote:

> I think that using the TSC (w/ RDTSC instruction) and calibrating it
> with a quick 2-3 RTC tick loop at startup might be the easiest option.

Just wondered if something like this would be a practical way to go:

/*******************************************************************/
/* How many ticks to use for calibration loop. (>= 1) */
#define CALIBRATION_TICKS 2

/* Output: Number of TSC cycles per second. */
grub_uint64_t tsc_cycles_per_second;

void grub_timer_tsc_calibrate(void)
{
	/* Calibrate to real time (relative, not absolute). */
	grub_uint64_t start_tsc;
	grub_uint64_t end_tsc;
	grub_uint32_t initial_tick;
	grub_uint32_t start_tick;
	grub_uint32_t end_tick;

	/* Wait for the start of the next tick;
	   we'll base out timing off this edge. */
	initial_tick = grub_get_rtc();
	do 
	{
		start_tick = grub_get_rtc();
	}
	while (start_tick == initial_tick);
	start_tsc = rdtsc();

	/* Wait for the start of the next tick.  This will
	   be the end of the 1-tick period. */
	do 
	{
		end_tick = grub_get_rtc();
	}
	while (end_tick - start_tick < CALIBRATION_TICKS);
	end_tsc = rdtsc();

	tsc_cycles_per_second = 
		(end_tsc - start_tsc) / (end_tick - start_tick) 
		* GRUB_TICKS_PER_SEC;
}
/*******************************************************************/


Colin

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