Measure OS timer precision to refine calibration

Do not trust the OS at all and measure timer precision before
calibrating the delay loop and use that measurement to get reasonable
precision for our own delay code.

Print a measurement for a delay of 4x the OS timer resolution.

Be precise about how bad the deviation was if we had to recalculate.

Tested on Windows XP, 32 bit, built using MinGW by Uwe. Tested on
FreeDOS v1.0 Final, 32bit, built using DJGPP 4.3.2 by Idwer.

Corresponding to flashrom svn r1028.
Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Uwe Hermann <uwe@hermann-uwe.de>
Acked-by: Idwer Vollering <vidwer+lists.flashrom@gmail.com>

udelay.c

@@ -37,6 +37,30 @@ __attribute__ ((noinline)) void myusec_delay(int usecs)
 	}
 }
 
+unsigned long measure_os_delay_resolution(void)
+{
+	unsigned long timeusec;
+	struct timeval start, end;
+	unsigned long counter = 0;
+	
+	gettimeofday(&start, 0);
+	timeusec = 0;
+	
+	while (!timeusec && (++counter < 1000000000)) {
+		gettimeofday(&end, 0);
+		timeusec = 1000000 * (end.tv_sec - start.tv_sec) +
+			   (end.tv_usec - start.tv_usec);
+		/* Protect against time going forward too much. */
+		if ((end.tv_sec > start.tv_sec) &&
+		    ((end.tv_sec - start.tv_sec) >= LONG_MAX / 1000000 - 1))
+			timeusec = 0;
+		/* Protect against time going backwards during leap seconds. */
+		if ((end.tv_sec < start.tv_sec) || (timeusec > LONG_MAX))
+			timeusec = 0;
+	}
+	return timeusec;
+}
+
 unsigned long measure_delay(int usecs)
 {
 	unsigned long timeusec;
@@ -61,10 +85,16 @@ unsigned long measure_delay(int usecs)
 void myusec_calibrate_delay(void)
 {
 	unsigned long count = 1000;
-	unsigned long timeusec;
+	unsigned long timeusec, resolution;
 	int i, tries = 0;
 
 	msg_pinfo("Calibrating delay loop... ");
+	resolution = measure_os_delay_resolution();
+	if (resolution) {
+		msg_pdbg("OS timer resolution is %u usecs, ", resolution);
+	} else {
+		msg_pinfo("OS timer resolution is unusable. ");
+	}
 
 recalibrate:
 	count = 1000;
@@ -94,9 +124,27 @@ recalibrate:
 		 * a scheduler delay or something similar.
 		 */
 		for (i = 0; i < 4; i++) {
-			if (measure_delay(100) < 90) {
-				msg_pdbg("delay more than 10%% too short, "
-					 "recalculating... ");
+			if (resolution && (resolution < 10)) {
+				timeusec = measure_delay(100);
+			} else if (resolution && 
+				   (resolution < ULONG_MAX / 200)) {
+				timeusec = measure_delay(resolution * 10) *
+					   100 / (resolution * 10);
+			} else {
+				/* This workaround should be active for broken
+				 * OS and maybe libpayload. The criterion
+				 * here is horrible or non-measurable OS timer
+				 * resolution which will result in
+				 * measure_delay(100)=0 whereas a longer delay
+				 * (1000 ms) may be sufficient
+				 * to get a nonzero time measurement.
+				 */
+				timeusec = measure_delay(1000000) / 10000;
+			}
+			if (timeusec < 90) {
+				msg_pdbg("delay more than 10%% too short (got "
+					 "%lu%% of expected delay), "
+					 "recalculating... ", timeusec);
 				goto recalibrate;
 			}
 		}
@@ -113,6 +161,8 @@ recalibrate:
 	msg_pdbg("1000 myus = %ld us, ", timeusec);
 	timeusec = measure_delay(10000);
 	msg_pdbg("10000 myus = %ld us, ", timeusec);
+	timeusec = measure_delay(resolution * 4);
+	msg_pdbg("%ld myus = %ld us, ", resolution * 4, timeusec);
 
 	msg_pinfo("OK.\n");
 }