Re: 2.6.14-rt13

[Vojtech Pavlik]

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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