On Tue, Mar 21, 2023 at 4:02 AM Nathan Hartman wrote: > On Mon, Mar 20, 2023 at 10:36 PM Gregory Nutt wrote: > > > NuttX offers a function clock_synchronize() that synchronizes system > > clock > > > with clock from RTC (internal or external). The synchronization is done > > > during board initialization and I suppose this can also be called from an > > > application level if required. > > I think clock_synchronize() is okay for startup initialization, but > > would be inappropriate at run time. That is because it could cause the > > current time to apparently jump backward. That could be fatal under > > certain circumstances as described for adjtime() in > > https://github.com/apache/nuttx/issues/8858 > > > > I remember once reading about how FreeBSD handles this (I think it was > FreeBSD; this was a long time ago): > > If there is a difference between the system clock and the reference clock, > the system clock is _not_ assigned the reference clock's time, exactly > because of the "clock running backwards" problem. Rather, when the clock is > advanced, the advancement is adjusted by a small amount, so that the system > clock always runs forwards, never backwards, and over time the system clock > gradually comes into synchronization with the reference clock. > > We probably need something like that to avoid the same problem.
Here are some references :-) https://github.com/freebsd/freebsd-src/blob/master/contrib/ntp/libntp/adjtime.c https://github.com/freebsd/freebsd-src/blob/master/contrib/ntp/include/ntp_syscall.h https://github.com/freebsd/freebsd-src/blob/main/sys/kern/kern_time.c And the man page :-) https://man.freebsd.org/cgi/man.cgi?query=adjtime&sektion=2&apropos=0&manpath=FreeBSD+13.1-RELEASE+and+Ports The adjtime() system call makes small adjustments to the system time, as returned by gettimeofday(2), advancing or retarding it by the time speci- fied by the timeval delta. If delta is negative, the clock is slowed down by incrementing it more slowly than normal until the correction is complete. If delta is positive, a larger increment than normal is used. The skew used to perform the correction is generally a fraction of one percent. Thus, the time is always a monotonically increasing function. A time correction from an earlier call to adjtime() may not be finished when adjtime() is called again. If olddelta is not a null pointer, the structure pointed to will contain, upon return, the number of microsec- onds still to be corrected from the earlier call. This call may be used by time servers that synchronize the clocks of com- puters in a local area network. Such time servers would slow down the clocks of some machines and speed up the clocks of others to bring them to the average network time. The adjtime() system call is restricted to the super-user. Hope that helps :-) -- CeDeROM, SQ7MHZ, http://www.tomek.cedro.info
