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-rw-r--r--udelay.c61
1 files changed, 48 insertions, 13 deletions
diff --git a/udelay.c b/udelay.c
index ac58017..ff6620a 100644
--- a/udelay.c
+++ b/udelay.c
@@ -24,13 +24,16 @@
#include <limits.h>
#include "flash.h"
-// count to a billion. Time it. If it's < 1 sec, count to 10B, etc.
+/* loops per microsecond */
unsigned long micro = 1;
-void myusec_delay(int usecs)
+__attribute__ ((noinline)) void myusec_delay(int usecs)
{
- volatile unsigned long i;
- for (i = 0; i < usecs * micro; i++) ;
+ unsigned long i;
+ for (i = 0; i < usecs * micro; i++) {
+ /* Make sure the compiler doesn't optimize the loop away. */
+ asm volatile ("" : : "rm" (i) );
+ }
}
unsigned long measure_delay(int usecs)
@@ -43,29 +46,61 @@ unsigned long measure_delay(int usecs)
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 = LONG_MAX;
+ /* Protect against time going backwards during leap seconds. */
+ if ((end.tv_sec < start.tv_sec) || (timeusec > LONG_MAX))
+ timeusec = 1;
return timeusec;
}
void myusec_calibrate_delay(void)
{
- int count = 1000;
+ unsigned long count = 1000;
unsigned long timeusec;
- int ok = 0;
+ int i, tries = 0;
printf("Calibrating delay loop... ");
- while (!ok) {
+recalibrate:
+ while (1) {
timeusec = measure_delay(count);
+ if (timeusec > 1000000 / 4)
+ break;
+ if (count >= ULONG_MAX / 2) {
+ msg_pinfo("timer loop overflow, reduced precision. ");
+ break;
+ }
count *= 2;
- if (timeusec < 1000000 / 4)
- continue;
- ok = 1;
}
+ tries ++;
+
+ /* Avoid division by zero, but in that case the loop is shot anyway. */
+ if (!timeusec)
+ timeusec = 1;
+
+ /* Compute rounded up number of loops per microsecond. */
+ micro = (count * micro) / timeusec + 1;
+ msg_pdbg("%luM loops per second, ", micro);
- // compute one microsecond. That will be count / time
- micro = count / timeusec;
- msg_pdbg("%ldM loops per second, ", micro);
+ /* Did we try to recalibrate less than 5 times? */
+ if (tries < 5) {
+ /* Recheck our timing to make sure we weren't just hitting
+ * 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... ");
+ goto recalibrate;
+ }
+ }
+ } else {
+ msg_perr("delay loop is unreliable, trying to continue ");
+ }
/* We're interested in the actual precision. */
timeusec = measure_delay(10);
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