x86: quick TSC calibration

Introduce a fast TSC-calibration method on sane hardware.

It only uses 17920 PIT timer ticks to calibrate the TSC, plus 256 ticks on
each side to make sure the TSC values were very close to the tick, so the
whole calibration takes 15ms. Yet, despite only takign 15ms,
we can actually give pretty stringent guarantees of accuracy:

 - the code requires that we hit each 256-counter block at least 50 times,
   so the TSC error is basically at *MOST* just a few PIT cycles off in
   any direction. In practice, it's going to be about one microseconds
   off (which is how long it takes to read the counter)

 - so over 17920 PIT cycles, we can pretty much guarantee that the
   calibration error is less than one half of a percent.

My testing bears this out: on my machine, the quick-calibration reports
2934.085kHz, while the slow one reports 2933.415.

Yes, the slower calibration is still more precise. For me, the slow
calibration is stable to within about one hundreth of a percent, so it's
(at a guess) roughly an order-and-a-half of magnitude more precise. The
longer you wait, the more precise you can be.

However, the nice thing about the fast TSC PIT synchronization is that
it's pretty much _guaranteed_ to give that 0.5% precision, and fail
gracefully (and very quickly) if it doesn't get it. And it really is
fairly simple (even if there's a lot of _details_ there, and I didn't get
all of those right ont he first try or even the second ;)

The patch says "110 insertions", but 63 of those new lines are actually
comments.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 arch/x86/kernel/tsc.c |  111 ++++++++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 110 insertions(+), 1 deletions(-)