linux-kernel - Re: [PATCH v2] x86/tsc: Allow quick PIT calibration despite interruptions

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Date:   Fri, 5 Apr 2019 00:06:16 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jan H. Schönherr <jan@...nhrr.de>
cc:     Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        x86@...nel.org, Paul Menzel <pmenzel@...gen.mpg.de>,
        Thomas Lendacky <Thomas.Lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2] x86/tsc: Allow quick PIT calibration despite
 interruptions

On Thu, 14 Feb 2019, Thomas Gleixner wrote:
> On Thu, 14 Feb 2019, Jan H. Schönherr wrote:
> 
> Cc+: Linus (he wrote the original implementation and might have opinions)
> 
> > Some systems experience regular interruptions (60 Hz SMI?), that prevent
> > the quick PIT calibration from succeeding: individual interruptions can be
> > so long, that the PIT MSB is observed to decrement by 2 or 3 instead of 1.
> > The existing code cannot recover from this.
> > 
> > The system in question is an AMD Ryzen Threadripper 2950X, microcode
> > 0x800820b, on an ASRock Fatal1ty X399 Professional Gaming, BIOS P3.30.
> > 
> > Change the code to handle (almost) arbitrary interruptions, as long
> > as they happen only once in a while and they do not take too long.
> > Specifically, also cover an interruption during the very first reads.
> > 
> > Signed-off-by: Jan H. Schönherr <jan@...nhrr.de>
> > ---
> > 
> > v2:
> > - Dropped the other hacky patch for the time being.
> > - Fixed the early exit check.
> > - Hopefully fixed all inaccurate math in v1.
> > - Extended comments.
> 
> That looks halfways sane, but I'm way too tired to wrap my head around
> it right now.

Went through it again and did not find any obvious issue. Nice work!

> Vs. comments: The big comment above pit_verify_msb() needs a big overhaul
> as well.

Would you please fix that up and repost?

Thanks,

	tglx