RE: [PATCH] x86: Export tsc related information in sysfs

[Dan Magenheimer]

> From: john stultz [mailto:johnstul@xxxxxxxxxx]
> 
> Yea, the relative instability of the tsc calibration at boot is an
> issue for folks who want very very precise timekeeping immediately
> after a reboot.
> 
> I proposed a solution to this awhile back via a boot option users
> could use to specify the tsc_khz freq, so it would be consistent from
> boot to boot. See: https://patchwork.kernel.org/patch/22492/
> 
> It didn't really go anywhere due to a lack of public interest.
> However, if you're interested in playing with it, I can try to revive
> the patch.

Another possibility: Optionally trust the stamped rate for the part?

I understand that on Nehalem this value is available in
MSR_PLATFORM_INFO[15:8] (google for MSR_PLATFORM_INFO 15 8),
but I don't know if this MSR is available on older (or AMD)
processors.

Just wondering:  If one were to put an ultra-precise scope on
a processor, how far off would the calibrated value be?  I'd
imagine the process of calibrating one unknown crystal against
a second crystal which has a known-but-not-highly-precise
frequency, though good enough for most purposes, is not particularly
accurate. In other words, maybe the stamped rate is more accurate
than the calibrated rate anyway?
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