Deepak Joshi wrote:
> Hi all,
>
> I read that in order to get the current time by using
> do_gettimeofday( struct timeval *tv) function we need
> to disable the inturrepts because we can't access both
> fields of timeval struct tv_sec and tv_usec. Why is it
> so ?
The function do_gettimeofday() makes sure that
the tv_sec and tv_usec values returned by it are
consistent, using an internal kernel lock. The
values returned in your tv structure are a snapshot
of the time, and will not be modified by ongoing
kernel timekeeping after the call.
There is no need to disable interrupts around a call
to do_gettimeofday().
FYI, on most platforms, do_gettimeofday() returns
values that have a resolution that is much better
than jiffies. That is, more than JUST the timer
interrupt is a factor in determining the value to
be returned. Usually it's a combination of
jiffies and some other high resolution clock
on the system.
> also i read code from Linux device drivers book
> that
> they are reading all the values in /proc interface. is
> it compulsary to do so ?
I don't understand this question. Who is "they"?
What is the purpose of reading the time, and in what
context (kernel or user-space) is the time value wanted?
> cli();
> do_gettimeofday(&tv);
> printk("\n\t The current time is %f",( time_t
> )tv.tv_sec);
> restore_flags( flags );
> sti();
I'm not sure I understand the purpose of the module.
Is it to provide a user-space manifestation of the current
time? If so, why not use a system call instead of a kernel
module?
With regard to the code itself, even if there was a
requirement to disable interrupts when using the value,
this particular code only uses one of the two parts of
the time value (tv_sec and not tv_usec), so it wouldn't
have a problem anyway.
=============================
Tim Bird
Architecture Group Chair, CE Linux Forum
Senior Staff Engineer, Sony Electronics
=============================
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