Back in the good old days, PC's had random pieces of rock for

frequency generation and what frequency the generated was anyones
guess.

In general the 32.768kHz RTC clock x-tal was the best, because that
was a regular wrist-watch Xtal, whereas the X-tal generating the
ISA bus frequency was much lower quality, often costing as much as
several cents a piece, so it made good sense to check the ISA bus
frequency against the RTC clock.

The other relevant property of those machines, is that they
typically had no more than 16MB RAM.

These days, CPU chips croak if their clocks are not tightly within
specs and all necessary frequencies are derived from the master
crystal by means if PLL's.

Considering that it takes on average 1.5 second to calibrate the
frequency of the i8254 counter, that more likely than not, we will
not actually use the result of the calibration, and as the final
clincher, we seldom use the i8254 for anything besides BEL in
syscons anyway, it has become time to drop the calibration code.

If you need to tell the system what frequency your i8254 runs,
you can do so from the loader using hw.i8254.freq or using the
sysctl kern.timecounter.tc.i8254.frequency.

1 files changed, 0 insertions, 112 deletions

diff --git a/sys/amd64/isa/clock.c b/sys/amd64/isa/clock.c
index 342fe18c5a78..5cc076ee1383 100644
--- a/sys/amd64/isa/clock.c
+++ b/sys/amd64/isa/clock.c
@@ -430,86 +430,6 @@ readrtc(int port)
 	return(bcd2bin(rtcin(port)));
 }
 
-static u_int
-calibrate_clocks(void)
-{
-	u_int count, prev_count, tot_count;
-	int sec, start_sec, timeout;
-
-	if (bootverbose)
-	        printf("Calibrating clock(s) ... ");
-	if (!(rtcin(RTC_STATUSD) & RTCSD_PWR))
-		goto fail;
-	timeout = 100000000;
-
-	/* Read the mc146818A seconds counter. */
-	for (;;) {
-		if (!(rtcin(RTC_STATUSA) & RTCSA_TUP)) {
-			sec = rtcin(RTC_SEC);
-			break;
-		}
-		if (--timeout == 0)
-			goto fail;
-	}
-
-	/* Wait for the mC146818A seconds counter to change. */
-	start_sec = sec;
-	for (;;) {
-		if (!(rtcin(RTC_STATUSA) & RTCSA_TUP)) {
-			sec = rtcin(RTC_SEC);
-			if (sec != start_sec)
-				break;
-		}
-		if (--timeout == 0)
-			goto fail;
-	}
-
-	/* Start keeping track of the i8254 counter. */
-	prev_count = getit();
-	if (prev_count == 0 || prev_count > i8254_max_count)
-		goto fail;
-	tot_count = 0;
-
-	/*
-	 * Wait for the mc146818A seconds counter to change.  Read the i8254
-	 * counter for each iteration since this is convenient and only
-	 * costs a few usec of inaccuracy. The timing of the final reads
-	 * of the counters almost matches the timing of the initial reads,
-	 * so the main cause of inaccuracy is the varying latency from 
-	 * inside getit() or rtcin(RTC_STATUSA) to the beginning of the
-	 * rtcin(RTC_SEC) that returns a changed seconds count.  The
-	 * maximum inaccuracy from this cause is < 10 usec on 486's.
-	 */
-	start_sec = sec;
-	for (;;) {
-		if (!(rtcin(RTC_STATUSA) & RTCSA_TUP))
-			sec = rtcin(RTC_SEC);
-		count = getit();
-		if (count == 0 || count > i8254_max_count)
-			goto fail;
-		if (count > prev_count)
-			tot_count += prev_count - (count - i8254_max_count);
-		else
-			tot_count += prev_count - count;
-		prev_count = count;
-		if (sec != start_sec)
-			break;
-		if (--timeout == 0)
-			goto fail;
-	}
-
-	if (bootverbose) {
-	        printf("i8254 clock: %u Hz\n", tot_count);
-	}
-	return (tot_count);
-
-fail:
-	if (bootverbose)
-	        printf("failed, using default i8254 clock of %u Hz\n",
-		       i8254_freq);
-	return (i8254_freq);
-}
-
 static void
 set_i8254_freq(u_int freq, int intr_freq)
 {
@@ -547,42 +467,10 @@ i8254_init(void)
 void
 startrtclock()
 {
-	u_int delta, freq;
 
 	writertc(RTC_STATUSA, rtc_statusa);
 	writertc(RTC_STATUSB, RTCSB_24HR);
 
-	freq = calibrate_clocks();
-#ifdef CLK_CALIBRATION_LOOP
-	if (bootverbose) {
-		printf(
-		"Press a key on the console to abort clock calibration\n");
-		while (cncheckc() == -1)
-			calibrate_clocks();
-	}
-#endif
-
-	/*
-	 * Use the calibrated i8254 frequency if it seems reasonable.
-	 * Otherwise use the default, and don't use the calibrated i586
-	 * frequency.
-	 */
-	delta = freq > i8254_freq ? freq - i8254_freq : i8254_freq - freq;
-	if (delta < i8254_freq / 100) {
-#ifndef CLK_USE_I8254_CALIBRATION
-		if (bootverbose)
-			printf(
-"CLK_USE_I8254_CALIBRATION not specified - using default frequency\n");
-		freq = i8254_freq;
-#endif
-		i8254_freq = freq;
-	} else {
-		if (bootverbose)
-			printf(
-		    "%d Hz differs from default of %d Hz by more than 1%%\n",
-			       freq, i8254_freq);
-	}
-
 	set_i8254_freq(i8254_freq, hz);
 	tc_init(&i8254_timecounter);