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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system_timer->offset() uses the added timer to return the number of

usecs since the last timer tick.  It's potentially different for each

specific type of arm blade, and the function often has "gettimeoffset"

in the name.
"xtime" is updated in the core kernel code.
Chris

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is "xtime" updated by the time tick clock timer, i.e., the timer which

generates interrupts every 1/HZ second to the kernel. Put it in

another way,

does gettimeofday get time in two parts 1) seconds from xtime.tv_sec,

updated by timer0, 2) microseconds from xtime.tv_nsec +

system_timer->offset(), updated by timer0 and timer3.
Do you mean the code here

"http://lxr.linux.no/linux+v2.6.24/kernel/time/timekeeping.c#L45"

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Yes, I think xtime is updated by the tick timer.
system_timer->offset() may be updated by different mechanisms, depending 
No.  As I said, the function usually has "gettimeoffset" in the name and

it's specific to arm so it would be under arch/arm.  For instance, on

Xscale boards it's ixp2000_gettimeoffset() in arch/arm/mach-ixp2000/core.c.
For more information you might want to look at the code in arch/arm or

else talk to the arm developers on the linux-arm-kernel mailing list.
Chris

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