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From:   Felipe Balbi <felipe.balbi@linux.intel.com>
To:     Thomas Gleixner <tglx@linutronix.de>
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
In-Reply-To: <alpine.DEB.2.21.1908151458560.1923@nanos.tec.linutronix.de>
References: <20190716072038.8408-1-felipe.balbi@linux.intel.com> <20190716072038.8408-2-felipe.balbi@linux.intel.com> <alpine.DEB.2.21.1907160952040.1767@nanos.tec.linutronix.de> <87y2zvt1hk.fsf@gmail.com> <alpine.DEB.2.21.1908151458560.1923@nanos.tec.linutronix.de>
Date:   Tue, 01 Oct 2019 13:24:31 +0300
Message-ID: <87y2y4vk4g.fsf@gmail.com>
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Hi,

(sorry for the long delay, got caught up in other tasks)

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.
>>=20
>> 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 =3D art_to_tsc_offset + ART * scale;

From=20silicon folks I got the equation:

ART =3D ECX * EBX / EAX;

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();
}

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

>> >> +void get_tsc_ns(struct system_counterval_t *tsc_counterval, u64 *tsc=
_ns)
>
> Why is this not returning the result instead of having that pointer
> indirection?

That can be changed easily, no worries.

>> >> +{
>> >> +	u64 tmp, res, rem;
>> >> +	u64 cycles;
>> >> +
>> >> +	tsc_counterval->cycles =3D clocksource_tsc.read(NULL);
>> >> +	cycles =3D tsc_counterval->cycles;
>> >> +	tsc_counterval->cs =3D art_related_clocksource;
>
> So this does more than returning the TSC time converted to nanoseconds. T=
he
> function name should reflect this. Plus both functions want kernel-doc
> explaining what they do.

convert_tsc_to_art_ns()? That would be analogous to convert_art_to_tsc()
and convert_art_ns_to_tsc().

cheers

=2D-=20
balbi

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