Re: [PATCH v14 20/25] x86/tsc: calibrate tsc only once

Peter Zijlstra
2018-07-19T10:33:40Z

On Tue, Jul 17, 2018 at 10:22:06PM -0400, Pavel Tatashin wrote:
> During boot tsc is calibrated twice: once in tsc_early_delay_calibrate(),
> and the second time in tsc_init().
>
> Rename tsc_early_delay_calibrate() to tsc_early_init(), and rework it so
> the calibration is done only early, and make tsc_init() to use the values
> already determined in tsc_early_init().
>
> Sometimes it is not possible to determine tsc early, as the subsystem that
> is required is not yet initialized, in such case try again later in
> tsc_init().

It might be nice to preserve some of the information tglx dug out during
review of all this. Like the various methods of calibrate_*() and their
dependencies.

And I note that this patch relies on the magic of native_calibrate_cpu()
working really early and not exploding in the quick calibration run.
This either wants fixing or documenting.

I think the initial idea was to only do the fast_calibrate (cpuid, msr
and possibly the quick_pit) things early and delay the HPET/PMTIMER
magic until later.