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_.
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.
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
--
