linux-kernel - Re: gettimeofday() in 2.6.24

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Date:	Fri, 18 Apr 2008 08:35:52 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Jack Harvard <jack.harvard@...glemail.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: gettimeofday() in 2.6.24

Jack Harvard wrote:
> On Thu, Apr 17, 2008 at 7:13 PM, Chris Friesen <cfriesen@...tel.com>
> wrote:

>> 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.

Yes, I think xtime is updated by the tick timer.

system_timer->offset() may be updated by different mechanisms, depending 
on the specific arm subarch.

> Do you mean the code here 
> "http://lxr.linux.no/linux+v2.6.24/kernel/time/timekeeping.c#L45"

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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