[flashrom] [PATCH] Handle insufficient DOS timer precision

Carl-Daniel Hailfinger c-d.hailfinger.devel.2006 at gmx.net
Wed Jun 2 15:21:22 CEST 2010


Totally new patch.

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.

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006 at gmx.net>

Index: flashrom-measure_delay_precision_dos/udelay.c
===================================================================
--- flashrom-measure_delay_precision_dos/udelay.c	(Revision 1023)
+++ flashrom-measure_delay_precision_dos/udelay.c	(Arbeitskopie)
@@ -37,6 +37,30 @@
 	}
 }
 
+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 @@
 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;
@@ -94,7 +124,22 @@
 		 * a scheduler delay or something similar.
 		 */
 		for (i = 0; i < 4; i++) {
-			if (measure_delay(100) < 90) {
+			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, "
 					 "recalculating... ");
 				goto recalibrate;


-- 
http://www.hailfinger.org/





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