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authorDenys Vlasenko <vda.linux@googlemail.com>2010-04-14 16:19:20 (GMT)
committer Denys Vlasenko <vda.linux@googlemail.com>2010-04-14 16:19:20 (GMT)
commit60f659f9d998128d6db817888ab35a9de248be68 (patch)
tree3535a46dd228f1f8d7a079c84499cba9233c5161
parentd7b5289209131ae94b540b35f0f147d2302ec0c9 (diff)
downloadbusybox-60f659f9d998128d6db817888ab35a9de248be68.tar.gz
busybox-60f659f9d998128d6db817888ab35a9de248be68.tar.bz2
hwclock: improve, and then disable clever sync code: it's bloat
...and hardware is too stupid to benefit from it anyway function old new delta hwclock_main 439 319 -120 Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
-rw-r--r--util-linux/hwclock.c112
1 files changed, 82 insertions, 30 deletions
diff --git a/util-linux/hwclock.c b/util-linux/hwclock.c
index b830057..416271b 100644
--- a/util-linux/hwclock.c
+++ b/util-linux/hwclock.c
@@ -109,10 +109,53 @@ static void to_sys_clock(const char **pp_rtcname, int utc)
static void from_sys_clock(const char **pp_rtcname, int utc)
{
-#define TWEAK_USEC 200
- struct tm tm_time;
+#if 1
struct timeval tv;
+ struct tm tm_time;
+ int rtc;
+
+ rtc = rtc_xopen(pp_rtcname, O_WRONLY);
+ gettimeofday(&tv, NULL);
+ /* Prepare tm_time */
+ if (sizeof(time_t) == sizeof(tv.tv_sec)) {
+ if (utc)
+ gmtime_r((time_t*)&tv.tv_sec, &tm_time);
+ else
+ localtime_r((time_t*)&tv.tv_sec, &tm_time);
+ } else {
+ time_t t = tv.tv_sec;
+ if (utc)
+ gmtime_r(&t, &tm_time);
+ else
+ localtime_r(&t, &tm_time);
+ }
+#else
+/* Bloated code which tries to set hw clock with better precision.
+ * On x86, even though code does set hw clock within <1ms of exact
+ * whole seconds, apparently hw clock (at least on some machines)
+ * doesn't reset internal fractional seconds to 0,
+ * making all this a pointless excercise.
+ */
+ /* If we see that we are N usec away from whole second,
+ * we'll sleep for N-ADJ usecs. ADJ corrects for the fact
+ * that CPU is not infinitely fast.
+ * On infinitely fast CPU, next wakeup would be
+ * on (exactly_next_whole_second - ADJ). On real CPUs,
+ * this difference between current time and whole second
+ * is less than ADJ (assuming system isn't heavily loaded).
+ */
+ /* Small value of 256us gives very precise sync for 2+ GHz CPUs.
+ * Slower CPUs will fail to sync and will go to bigger
+ * ADJ values. qemu-emulated armv4tl with ~100 MHz
+ * performance ends up using ADJ ~= 4*1024 and it takes
+ * 2+ secs (2 tries with successively larger ADJ)
+ * to sync. Even straced one on the same qemu (very slow)
+ * takes only 4 tries.
+ */
+#define TWEAK_USEC 256
unsigned adj = TWEAK_USEC;
+ struct tm tm_time;
+ struct timeval tv;
int rtc = rtc_xopen(pp_rtcname, O_WRONLY);
/* Try to catch the moment when whole second is close */
@@ -124,55 +167,64 @@ static void from_sys_clock(const char **pp_rtcname, int utc)
t = tv.tv_sec;
rem_usec = 1000000 - tv.tv_usec;
- if (rem_usec < 1024) {
- /* Less than 1ms to next second. Good enough */
+ if (rem_usec < adj) {
+ /* Close enough */
small_rem:
t++;
}
- /* Prepare tm */
+ /* Prepare tm_time from t */
if (utc)
gmtime_r(&t, &tm_time); /* may read /etc/xxx (it takes time) */
else
localtime_r(&t, &tm_time); /* same */
- tm_time.tm_isdst = 0;
+
+ if (adj >= 32*1024) {
+ break; /* 32 ms diff and still no luck?? give up trying to sync */
+ }
/* gmtime/localtime took some time, re-get cur time */
gettimeofday(&tv, NULL);
- if (tv.tv_sec < t /* may happen if rem_usec was < 1024 */
- || (tv.tv_sec == t && tv.tv_usec < 1024)
+ if (tv.tv_sec < t /* we are still in old second */
+ || (tv.tv_sec == t && tv.tv_usec < adj) /* not too far into next second */
) {
- /* We are not too far into next second. Good. */
- break;
- }
- adj += 32; /* 2^(10-5) = 2^5 = 32 iterations max */
- if (adj >= 1024) {
- /* Give up trying to sync */
- break;
+ break; /* good, we are in sync! */
}
- /* Try to sync up by sleeping */
rem_usec = 1000000 - tv.tv_usec;
- if (rem_usec < 1024) {
- goto small_rem; /* already close, don't sleep */
+ if (rem_usec < adj) {
+ t = tv.tv_sec;
+ goto small_rem; /* already close to next sec, don't sleep */
}
- /* Need to sleep.
- * Note that small adj on slow processors can make us
- * to always overshoot tv.tv_usec < 1024 check on next
- * iteration. That's why adj is increased on each iteration.
- * This also allows it to be reused as a loop limiter.
- */
- usleep(rem_usec - adj);
- }
- xioctl(rtc, RTC_SET_TIME, &tm_time);
+ /* Try to sync up by sleeping */
+ usleep(rem_usec - adj);
- /* Debug aid to find "good" TWEAK_USEC.
+ /* Jump to 1ms diff, then increase fast (x2): EVERY loop
+ * takes ~1 sec, people won't like slowly converging code here!
+ */
+ //bb_error_msg("adj:%d tv.tv_usec:%d", adj, (int)tv.tv_usec);
+ if (adj < 512)
+ adj = 512;
+ /* ... and if last "overshoot" does not look insanely big,
+ * just use it as adj increment. This makes convergence faster.
+ */
+ if (tv.tv_usec < adj * 8) {
+ adj += tv.tv_usec;
+ continue;
+ }
+ adj *= 2;
+ }
+ /* Debug aid to find "optimal" TWEAK_USEC with nearly exact sync.
* Look for a value which makes tv_usec close to 999999 or 0.
- * for 2.20GHz Intel Core 2: TWEAK_USEC ~= 200
+ * For 2.20GHz Intel Core 2: optimal TWEAK_USEC ~= 200
*/
- //bb_error_msg("tv.tv_usec:%d adj:%d", (int)tv.tv_usec, adj);
+ //bb_error_msg("tv.tv_usec:%d", (int)tv.tv_usec);
+#endif
+
+ tm_time.tm_isdst = 0;
+ xioctl(rtc, RTC_SET_TIME, &tm_time);
if (ENABLE_FEATURE_CLEAN_UP)
close(rtc);