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
next prev parent 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
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