test the PM timer for glitches (using the algorithm from FreeBSD)

and use a single read if the timer looks good,
patch from Juan RP, with minor simplification by me

sys/dev/acpi/acpi_timer.c

@@ -1,4 +1,4 @@
-/* $NetBSD: acpi_timer.c,v 1.1 2006/06/21 17:47:23 drochner Exp $ */
+/* $NetBSD: acpi_timer.c,v 1.2 2006/06/26 12:29:36 drochner Exp $ */
 
 #include <sys/types.h>
 
@@ -10,14 +10,17 @@
 #include <dev/acpi/acpica.h>
 #include <dev/acpi/acpi_timer.h>
 
-static u_int acpitimer_read(struct timecounter *);
+static int acpitimer_test(void);
+static uint32_t acpitimer_delta(uint32_t, uint32_t);
+static u_int acpitimer_read_safe(struct timecounter *);
+static u_int acpitimer_read_fast(struct timecounter *);
 
 static struct timecounter acpi_timecounter = {
-	acpitimer_read,
+	acpitimer_read_safe,
 	0,
 	0x00ffffff,
 	PM_TIMER_FREQUENCY,
-	"ACPI_PM_TMR",
+	"ACPI-Safe",
 	900
 };
 
@@ -25,6 +28,7 @@ int
 acpitimer_init()
 {
 	uint32_t bits;
+	int i, j;
 	ACPI_STATUS res;
 
 	res = AcpiGetTimerResolution(&bits);
@@ -33,21 +37,38 @@ acpitimer_init()
 
 	if (bits == 32)
 		acpi_timecounter.tc_counter_mask = 0xffffffff;
-	tc_init(&acpi_timecounter);
 
-	printf("acpitimer: %d bits\n", bits);
+	j = 0;
+	for (i = 0; i < 10; i++)
+		j += acpitimer_test();
+	aprint_verbose("acpitimer_test(): %d\n", j);
+	if (j >= 10) {
+		acpi_timecounter.tc_name = "ACPI-Fast";
+		acpi_timecounter.tc_get_timecount = acpitimer_read_fast;
+		acpi_timecounter.tc_quality = 1000;
+	}
+		
+	tc_init(&acpi_timecounter);
+	aprint_normal("%s %d-bit timer\n", acpi_timecounter.tc_name, bits);
 
 	return (0);
 }
 
+static u_int
+acpitimer_read_fast(struct timecounter *tc)
+{
+	uint32_t t;
+
+	AcpiGetTimer(&t);
+	return (t);
+}
+
 /*
  * Some chipsets (PIIX4 variants) do not latch correctly; there
  * is a chance that a transition is hit.
- * For now, just be conservative. We might detect the situation later
- * (by testing, or chipset quirks).
  */
 static u_int
-acpitimer_read(struct timecounter *tc)
+acpitimer_read_safe(struct timecounter *tc)
 {
 	uint32_t t1, t2, t3;
 
@@ -61,6 +82,53 @@ acpitimer_read(struct timecounter *tc)
 	return (t2);
 }
 
+static uint32_t
+acpitimer_delta(uint32_t end, uint32_t start)
+{
+	uint32_t delta;
+	u_int mask = acpi_timecounter.tc_counter_mask;
+
+	if (end >= start)
+		delta = end - start;
+	else
+		delta = ((mask - start) + end + 1) & mask;
+
+	return (delta);
+}
+
+#define N 2000
+static int
+acpitimer_test()
+{
+	uint32_t last, this, delta;
+	int minl, maxl, n;
+
+	minl = 10000000;
+	maxl = 0;
+
+	disable_intr();
+	AcpiGetTimer(&last);
+	for (n = 0; n < N; n++) {
+		AcpiGetTimer(&this);
+		delta = acpitimer_delta(this, last);
+		if (delta > maxl)
+			maxl = delta;
+		else if (delta < minl)
+			minl = delta;
+		last = this;
+	}
+	enable_intr();
+
+	if (maxl - minl > 2 )
+		n = 0;
+	else if (minl < 0 || maxl == 0)
+		n = 0;
+	else
+		n = 1;
+
+	return (n);
+}
+
 #else
 
 int