linux-kernel - Re: gettimeofday() in 2.6.24

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Message-ID: <a72f6a3c0804180154x28d0b528gc1cbd3d11279c4a1@mail.gmail.com>
Date:	Fri, 18 Apr 2008 09:54:45 +0100
From:	"Jack Harvard" <jack.harvard@...glemail.com>
To:	"Chris Friesen" <cfriesen@...tel.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: gettimeofday() in 2.6.24

On Thu, Apr 17, 2008 at 7:13 PM, Chris Friesen <cfriesen@...tel.com> wrote:
> Jack Harvard wrote:
>
>
> > 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));
> >
>
>
>
> > but I haven't quite figured out how gettimeofday() actually gets time
> > from this added timer, also how xtime is updated?
> >
>
>
>  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.

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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