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>
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Denys Vlasenko
parent
d7b5289209
commit
60f659f9d9
@ -109,10 +109,53 @@ static void to_sys_clock(const char **pp_rtcname, int utc)
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static void from_sys_clock(const char **pp_rtcname, int utc)
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{
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#define TWEAK_USEC 200
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#if 1
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struct timeval tv;
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struct tm tm_time;
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int rtc;
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rtc = rtc_xopen(pp_rtcname, O_WRONLY);
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gettimeofday(&tv, NULL);
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/* Prepare tm_time */
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if (sizeof(time_t) == sizeof(tv.tv_sec)) {
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if (utc)
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gmtime_r((time_t*)&tv.tv_sec, &tm_time);
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else
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localtime_r((time_t*)&tv.tv_sec, &tm_time);
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} else {
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time_t t = tv.tv_sec;
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if (utc)
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gmtime_r(&t, &tm_time);
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else
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localtime_r(&t, &tm_time);
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}
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#else
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/* Bloated code which tries to set hw clock with better precision.
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* On x86, even though code does set hw clock within <1ms of exact
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* whole seconds, apparently hw clock (at least on some machines)
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* doesn't reset internal fractional seconds to 0,
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* making all this a pointless excercise.
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*/
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/* If we see that we are N usec away from whole second,
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* we'll sleep for N-ADJ usecs. ADJ corrects for the fact
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* that CPU is not infinitely fast.
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* On infinitely fast CPU, next wakeup would be
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* on (exactly_next_whole_second - ADJ). On real CPUs,
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* this difference between current time and whole second
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* is less than ADJ (assuming system isn't heavily loaded).
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*/
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/* Small value of 256us gives very precise sync for 2+ GHz CPUs.
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* Slower CPUs will fail to sync and will go to bigger
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* ADJ values. qemu-emulated armv4tl with ~100 MHz
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* performance ends up using ADJ ~= 4*1024 and it takes
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* 2+ secs (2 tries with successively larger ADJ)
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* to sync. Even straced one on the same qemu (very slow)
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* takes only 4 tries.
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*/
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#define TWEAK_USEC 256
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unsigned adj = TWEAK_USEC;
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struct tm tm_time;
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struct timeval tv;
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unsigned adj = TWEAK_USEC;
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int rtc = rtc_xopen(pp_rtcname, O_WRONLY);
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/* Try to catch the moment when whole second is close */
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@ -124,55 +167,64 @@ static void from_sys_clock(const char **pp_rtcname, int utc)
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t = tv.tv_sec;
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rem_usec = 1000000 - tv.tv_usec;
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if (rem_usec < 1024) {
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/* Less than 1ms to next second. Good enough */
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if (rem_usec < adj) {
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/* Close enough */
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small_rem:
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t++;
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}
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/* Prepare tm */
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/* Prepare tm_time from t */
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if (utc)
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gmtime_r(&t, &tm_time); /* may read /etc/xxx (it takes time) */
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else
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localtime_r(&t, &tm_time); /* same */
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tm_time.tm_isdst = 0;
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if (adj >= 32*1024) {
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break; /* 32 ms diff and still no luck?? give up trying to sync */
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}
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/* gmtime/localtime took some time, re-get cur time */
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gettimeofday(&tv, NULL);
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if (tv.tv_sec < t /* may happen if rem_usec was < 1024 */
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|| (tv.tv_sec == t && tv.tv_usec < 1024)
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if (tv.tv_sec < t /* we are still in old second */
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|| (tv.tv_sec == t && tv.tv_usec < adj) /* not too far into next second */
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) {
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/* We are not too far into next second. Good. */
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break;
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break; /* good, we are in sync! */
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}
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adj += 32; /* 2^(10-5) = 2^5 = 32 iterations max */
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if (adj >= 1024) {
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/* Give up trying to sync */
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break;
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rem_usec = 1000000 - tv.tv_usec;
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if (rem_usec < adj) {
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t = tv.tv_sec;
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goto small_rem; /* already close to next sec, don't sleep */
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}
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/* Try to sync up by sleeping */
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rem_usec = 1000000 - tv.tv_usec;
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if (rem_usec < 1024) {
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goto small_rem; /* already close, don't sleep */
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}
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/* Need to sleep.
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* Note that small adj on slow processors can make us
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* to always overshoot tv.tv_usec < 1024 check on next
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* iteration. That's why adj is increased on each iteration.
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* This also allows it to be reused as a loop limiter.
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*/
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usleep(rem_usec - adj);
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/* Jump to 1ms diff, then increase fast (x2): EVERY loop
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* takes ~1 sec, people won't like slowly converging code here!
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*/
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//bb_error_msg("adj:%d tv.tv_usec:%d", adj, (int)tv.tv_usec);
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if (adj < 512)
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adj = 512;
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/* ... and if last "overshoot" does not look insanely big,
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* just use it as adj increment. This makes convergence faster.
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*/
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if (tv.tv_usec < adj * 8) {
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adj += tv.tv_usec;
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continue;
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}
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adj *= 2;
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}
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xioctl(rtc, RTC_SET_TIME, &tm_time);
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/* Debug aid to find "good" TWEAK_USEC.
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/* Debug aid to find "optimal" TWEAK_USEC with nearly exact sync.
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* Look for a value which makes tv_usec close to 999999 or 0.
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* for 2.20GHz Intel Core 2: TWEAK_USEC ~= 200
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* For 2.20GHz Intel Core 2: optimal TWEAK_USEC ~= 200
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*/
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//bb_error_msg("tv.tv_usec:%d adj:%d", (int)tv.tv_usec, adj);
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//bb_error_msg("tv.tv_usec:%d", (int)tv.tv_usec);
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#endif
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tm_time.tm_isdst = 0;
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xioctl(rtc, RTC_SET_TIME, &tm_time);
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if (ENABLE_FEATURE_CLEAN_UP)
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close(rtc);
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