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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
Date: Tue, 01 Oct 2019 13:24:31 +0300	[thread overview]
Message-ID: <87y2y4vk4g.fsf@gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1908151458560.1923@nanos.tec.linutronix.de>

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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.
>> 
>> 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;

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 = clocksource_tsc.read(NULL);
>> >> +	cycles = tsc_counterval->cycles;
>> >> +	tsc_counterval->cs = art_related_clocksource;
>
> So this does more than returning the TSC time converted to nanoseconds. The
> 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

-- 
balbi

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  reply	other threads:[~2019-10-01 10:24 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 [this message]
2019-10-17 11:15           ` Thomas Gleixner
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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