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File busybox-1.16.1-hwclock.patch of Package busybox (Revision 27)
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diff -urpN busybox-1.16.1/util-linux/hwclock.c busybox-1.16.1-hwclock/util-linux/hwclock.c --- busybox-1.16.1/util-linux/hwclock.c 2010-03-20 03:58:07.000000000 +0100 +++ busybox-1.16.1-hwclock/util-linux/hwclock.c 2010-04-14 18:29:37.000000000 +0200 @@ -109,10 +109,53 @@ static void to_sys_clock(const char **pp 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 ** 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);
