diff options
| -rw-r--r-- | udelay.c | 61 |
1 files changed, 48 insertions, 13 deletions
@@ -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); |