Re: 2.6.14-rt13

From: Vojtech Pavlik
Date: Sat Nov 19 2005 - 02:46:40 EST

On Fri, Nov 18, 2005 at 06:28:24PM -0800, George Anzinger wrote:

> >There are many mechanisms to keep time:
> >
> >1) RTC: 0.5 sec resolution, interrupts
> >2) PIT: takes ages to read, overflows at each timer interrupt
> >3) PMTMR: takes ages to read, overflows in approx 4 seconds, no interrupt
> The PMTMR can be read from user space (if you can find it). See the
> "iopl" man page. It is an I/O access and so is slow, but you can read
> it.

Yes, however this must be limited to a small number of privileged
applications - iopl() is only available to CAP_SYS_RAWIO IIRC,
and thus it's not suitable for general use.

> Finding it is another matter. It does not have a fixed address (i.e.
> it differs from machine to machine, but is constant on any given
> machine). The boot code roots it out of an info block put in memory
> by the BIOS. I suppose one could put a printk in the boot code to
> disclose it...

There is really no reason to do that, since the time to read it (~1200
ns) is much less than the time to enter the kernel (less than 200 ns),
so gettimeofday() is definitely easier to use and also doesn't overflow.

Vojtech Pavlik
SuSE Labs, SuSE CR
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