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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: Thu, 15 Aug 2019 08:57:11 +0300	[thread overview]
Message-ID: <87y2zvt1hk.fsf@gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1907160952040.1767@nanos.tec.linutronix.de>


Hi,


Thomas Gleixner <tglx@linutronix.de> writes:

> Felipe,
>
> On Tue, 16 Jul 2019, Felipe Balbi wrote:
>
> -ENOCHANGELOG
>
> As you said in the cover letter:
>
>>  (3) The change in arch/x86/kernel/tsc.c needs to be reviewed at length
>>      before going in.
>
> 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.

>> +void get_tsc_ns(struct system_counterval_t *tsc_counterval, u64 *tsc_ns)
>> +{
>> +	u64 tmp, res, rem;
>> +	u64 cycles;
>> +
>> +	tsc_counterval->cycles = clocksource_tsc.read(NULL);
>> +	cycles = tsc_counterval->cycles;
>> +	tsc_counterval->cs = art_related_clocksource;
>> +
>> +	rem = do_div(cycles, tsc_khz);
>> +
>> +	res = cycles * USEC_PER_SEC;
>> +	tmp = rem * USEC_PER_SEC;
>> +
>> +	do_div(tmp, tsc_khz);
>> +	res += tmp;
>> +
>> +	*tsc_ns = res;
>> +}
>> +EXPORT_SYMBOL(get_tsc_ns);
>> +
>> +u64 get_art_ns_now(void)
>> +{
>> +	struct system_counterval_t tsc_cycles;
>> +	u64 tsc_ns;
>> +
>> +	get_tsc_ns(&tsc_cycles, &tsc_ns);
>> +
>> +	return tsc_ns;
>> +}
>> +EXPORT_SYMBOL(get_art_ns_now);
>
> While the changes look innocuous I'm missing the big picture why this needs
> to emulate ART instead of simply using TSC directly.

i don't think we're emulating ART here (other than the name in the
function). We're just reading TSC and converting to nanoseconds, right?

Cheers

-- 
balbi

  reply	other threads:[~2019-08-15  5:57 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 [this message]
2019-08-15 14:16       ` Thomas Gleixner
2019-10-01 10:24         ` Felipe Balbi
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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