Re: [PATCH] Fix TSC calibration issues

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, 3 Sep 2008, Arjan van de Ven wrote:
>
> On Wed, 3 Sep 2008 20:59:05 -0700 (PDT)
> Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> > The only frequency we can trust on 99% of all machines is the PIT,
> 
> pmtimer is also quite ok, with the exception of some K6 based boxes.
> (I'm surprised the K6 boxes even have enough modern stuff to have
> pmtimer; I'd think they would fall under the date cutoff)

Quite frankly, pmtimer isn't all that much better than PIT. It has a 
slightly bigger range, but it has a much more limited format, and it 
doesn't have a reliable frequency. It was designed for something else.

At least HPET is clearly better than PIT as a _timer_.

All the big HPET problems are with its idiotic interface. 

Of course, in any _sane_ situation, the timer really would have been in 
the local APIC instead, with a fixed and architected frequency, and it 
should run in all power states. But noo, that obviously won't ever work, 
because that would have been _sensible_.

> one of the options we have is to start with an initial
> rough-but-conservative estimate, and refine it over time as the system
> is running.... sort of like ntp but for the calibration.

I do agree that we could aim for something like that. But even to get the 
rough estimate, we'd probably have to do the 5ms thing.

> another option for calibrating the tsc rate is to read it from the
> msr's/cpuid/aperf of what the hardware says it should be, and then all
> we need is to verify it is that; that we could do over timer or quickly.
> (of course that only works for systems with constant tsc)

I don't think it's reliable even for systems with a constant TSC. Because 
the msr/cpuid thing isn't going to actualyl give the right frequency. It 
might be the frequency the thing is _rated_ at, but it will be off when 
people over- or under-clock the front-side bus etc.

This is why it's so important that the clock input be a _known_ frequency. 
The thing that makes the PIT still so useful is not that it's a good 
timer, but that we *know* the frequency it runs at.

			Linus