gettimeofday() in 2.6.24

[Jack Harvard]

Hi, I was trying to figure out how gettimeofday() measures time in
2.6.24-arm2, in which a free-running timer is added to improve the
resolution of gettimeofday() from 10ms to us, I can trace down to
do_gettimeofday() as follows:

http://lxr.linux.no/linux+v2.6.24/arch/arm/kernel/time.c#L240
 239#ifndef CONFIG_GENERIC_TIME
 240void do_gettimeofday(struct timeval *tv)
 241{
 242        unsigned long flags;
 243        unsigned long seq;
 244        unsigned long usec, sec;
 245
 246        do {
 247                seq = read_seqbegin_irqsave(&xtime_lock, flags);
 248                usec = system_timer->offset();
 249                sec = xtime.tv_sec;
 250                usec += xtime.tv_nsec / 1000;
 251        } while (read_seqretry_irqrestore(&xtime_lock, seq, flags));
 252
 253        /* usec may have gone up a lot: be safe */
 254        while (usec >= 1000000) {
 255                usec -= 1000000;
 256                sec++;
 257        }
 258
 259        tv->tv_sec = sec;
 260        tv->tv_usec = usec;
 261}
 262
 263EXPORT_SYMBOL(do_gettimeofday);

but I haven't quite figured out how gettimeofday() actually gets time
from this added timer, also how xtime is updated?

Thanks, Jack
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