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From: Thomas Gleixner <tglx@linutronix.de>
To: Felipe Balbi <felipe.balbi@linux.intel.com>
Cc: Richard Cochran <richardcochran@gmail.com>,
	netdev@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>, "H . Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, linux-kernel@vger.kernel.org,
	"Christopher S . Hall" <christopher.s.hall@intel.com>
Subject: Re: [RFC PATCH 1/5] x86: tsc: add tsc to art helpers
Date: Thu, 17 Oct 2019 13:15:15 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1910171256580.1824@nanos.tec.linutronix.de> (raw)
In-Reply-To: <87y2y4vk4g.fsf@gmail.com>

Hi,

On Tue, 1 Oct 2019, Felipe Balbi wrote:
> (sorry for the long delay, got caught up in other tasks)

Delayed by vacation :)

> Thomas Gleixner <tglx@linutronix.de> writes:
> > On Thu, 15 Aug 2019, Felipe Balbi wrote:
> >> Thomas Gleixner <tglx@linutronix.de> writes:
> >> > On Tue, 16 Jul 2019, Felipe Balbi wrote:
> >> >
> >> > So some information what those interfaces are used for and why they are
> >> > needed would be really helpful.
> >> 
> >> Okay, I have some more details about this. The TGPIO device itself uses
> >> ART since TSC is not directly available to anything other than the
> >> CPU. The 'problem' here is that reading ART incurs extra latency which
> >> we would like to avoid. Therefore, we use TSC and scale it to
> >> nanoseconds which, would be the same as ART to ns.
> >
> > Fine. But that's not really correct:
> >
> >       TSC = art_to_tsc_offset + ART * scale;
> 
> From silicon folks I got the equation:
> 
> ART = ECX * EBX / EAX;

What is the content of ECX/EBX/EAX and where is it coming from?
 
> If I'm reading this correctly, that's basically what
> native_calibrate_tsc() does (together with some error checking the safe
> defaults). Couldn't we, instead, just have a single function like below?
> 
> u64 convert_tsc_to_art_ns()
> {
> 	return x86_platform.calibrate_tsc();
> }

Huch? How is that supposed to work? calibrate_tsc() returns the TSC
frequency.

> Another way would be extract the important parts from
> native_calibrate_tsc() into a separate helper. This would safe another
> call to cpuid(0x15,...);

What for?

The relation between TSC and ART is already established via detect_art()
which reads all relevant data out of CPUID(ART_CPUID_LEAF).

We use exactly that information for convert_art_to_tsc() so the obvious
solution for calculating ART from TSC is to do the reverse operation.

convert_art_to_tsc()
{
        rem = do_div(art, art_to_tsc_denominator);

        res = art * art_to_tsc_numerator;
        tmp = rem * art_to_tsc_numerator;

        do_div(tmp, art_to_tsc_denominator);
        res += tmp + art_to_tsc_offset;
}

which is translated into math:

      TSC = ART * SCALE + OFFSET

where

      SCALE = N / D

and

      N = CPUID(ART_CPUID_LEAF).EAX
      D = CPUID(ART_CPUID_LEAF).EBX

So the obvious reverse operation is:

     ART = (TSC - OFFSET) / SCALE;

Translating that into code should not be rocket science.

Thanks,

	tglx

  reply	other threads:[~2019-10-17 11:15 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-16  7:20 [RFC PATCH 0/5] PTP: add support for Intel's TGPIO controller Felipe Balbi
2019-07-16  7:20 ` [RFC PATCH 1/5] x86: tsc: add tsc to art helpers Felipe Balbi
2019-07-16  7:57   ` Thomas Gleixner
2019-08-15  5:57     ` Felipe Balbi
2019-08-15 14:16       ` Thomas Gleixner
2019-10-01 10:24         ` Felipe Balbi
2019-10-17 11:15           ` Thomas Gleixner [this message]
2019-10-17 12:01             ` Felipe Balbi
2019-07-16  7:20 ` [RFC PATCH 2/5] PTP: add a callback for counting timestamp events Felipe Balbi
2019-07-16  7:20 ` [RFC PATCH 3/5] PTP: implement PTP_EVENT_COUNT_TSTAMP ioctl Felipe Balbi
2019-07-16  7:20 ` [RFC PATCH 4/5] PTP: Add flag for non-periodic output Felipe Balbi
2019-07-16 16:39   ` Richard Cochran
2019-07-17  6:49     ` Felipe Balbi
2019-07-17 17:36       ` Richard Cochran
2019-07-18  8:59         ` Felipe Balbi
2019-07-18 16:41           ` Richard Cochran
2019-08-13  7:53             ` Felipe Balbi
2019-08-13 17:48               ` Richard Cochran
2019-08-13 18:06                 ` Richard Cochran
2019-08-14  7:05                   ` Felipe Balbi
2019-07-16  7:20 ` [RFC PATCH 5/5] PTP: Add support for Intel PMC Timed GPIO Controller Felipe Balbi
2019-07-16 19:14   ` Shannon Nelson
2019-07-17  6:51     ` Felipe Balbi
2019-07-16 16:41 ` [RFC PATCH 0/5] PTP: add support for Intel's TGPIO controller Richard Cochran
2019-07-17  6:52   ` Felipe Balbi
2019-07-17 17:39     ` Richard Cochran
2019-07-18  8:58       ` Felipe Balbi
2019-07-18 19:50 ` Andrew Lunn
2019-07-19  7:35   ` Felipe Balbi
2019-07-19 13:20     ` Andrew Lunn
2019-08-13  7:50       ` Felipe Balbi
2019-08-13 17:49         ` Richard Cochran

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