LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
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, 15 Aug 2019 16:16:09 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1908151458560.1923@nanos.tec.linutronix.de> (raw)
In-Reply-To: <87y2zvt1hk.fsf@gmail.com>

Felipe,

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;
 
> >> +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?

> >> +{
> >> +	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.

> >> +	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?

Well, the function name says clearly: get_art_ns_now(). But you are not
using ART, you use the TSC and derive the ART value from it (incorrectly).

Thanks,

	tglx


  reply	other threads:[~2019-08-15 14:16 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 [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=alpine.DEB.2.21.1908151458560.1923@nanos.tec.linutronix.de \
    --to=tglx@linutronix.de \
    --cc=bp@alien8.de \
    --cc=christopher.s.hall@intel.com \
    --cc=felipe.balbi@linux.intel.com \
    --cc=hpa@zytor.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=netdev@vger.kernel.org \
    --cc=richardcochran@gmail.com \
    --cc=x86@kernel.org \
    --subject='Re: [RFC PATCH 1/5] x86: tsc: add tsc to art helpers' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).