From: "Hall, Christopher S" <christopher.s.hall@intel.com>
To: 'John Stultz' <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Richard Cochran <richardcochran@gmail.com>,
Ingo Molnar <mingo@redhat.com>,
"Kirsher, Jeffrey T" <jeffrey.t.kirsher@intel.com>,
"Ronciak, John" <john.ronciak@intel.com>,
"H. Peter Anvin" <hpa@zytor.com>,
"x86@kernel.org" <x86@kernel.org>,
lkml <linux-kernel@vger.kernel.org>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>
Subject: RE: [PATCH 1/5] Add functions producing system time given a backing counter value
Date: Wed, 29 Jul 2015 01:41:34 +0000 [thread overview]
Message-ID: <B79D786B7111A34A8CF09F833429C493A909CEE1@ORSMSX109.amr.corp.intel.com> (raw)
In-Reply-To: <CALAqxLWSakB1zdnYAE=2yQTPLdzkjO3wLA82Q+CJoJccGLWM8Q@mail.gmail.com>
> -----Original Message-----
> From: John Stultz [mailto:john.stultz@linaro.org]
> Sent: Monday, July 27, 2015 8:44 PM
> To: Hall, Christopher S
> Cc: Thomas Gleixner; Richard Cochran; Ingo Molnar; Kirsher, Jeffrey T;
> Ronciak, John; H. Peter Anvin; x86@kernel.org; lkml;
> netdev@vger.kernel.org
> Subject: Re: [PATCH 1/5] Add functions producing system time given a
> backing counter value
>
> On Mon, Jul 27, 2015 at 5:46 PM, Christopher Hall
> <christopher.s.hall@intel.com> wrote:
> > * counter_to_rawmono64
> > * counter_to_mono64
> > * counter_to_realtime64
> >
> > Enables drivers to translate a captured system clock counter to system
> > time. This is useful for network and audio devices that capture
> timestamps
> > in terms of both the system clock and device clock.
>
> Huh. So for counter_to_realtime64 & mono64, this seems to ignore the
> fact that the multiplier is constantly adjusted and corrected. So that
> calling the function twice with the same counter value may result in
> different returned values.
>
> I've not yet groked the whole patchset, but it seems like there needs
> to be some mechanism that ensures the counter value is captured and
> used in the same (or at least close) interval that the timekeeper data
> is valid for.
The ART (and derived TSC) values are always in the past. There's no
chance that we could exceed the interval. I don't think any similar
usage would be a problem either.
Are you suggesting that, for completeness, this be enforced by the
conversion function?
I do a check here to make sure that the current counter value isn't before
the beginning of the current interval:
timekeeping_get_delta()
...
if (cycle_now < tkr->cycle_last &&
tkr->cycle_last - cycle_now < ROLLOVER_THRESHOLD)
return -EAGAIN;
If tkr->cycle_last - cycle_now is large, the assumption is that
rollover occurred. Otherwise, the caller should re-read the counter
so that it falls within the current interval. In my "normal use"
testing, re-read never occurred.
Thanks for your input.
Chris
>
> thanks
> -john
next prev parent reply other threads:[~2015-07-29 1:41 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-28 0:46 [PATCH 0/5] Patchset enabling hardware based cross-timestamps for next gen Intel platforms Christopher Hall
2015-07-28 0:46 ` [PATCH 1/5] Add functions producing system time given a backing counter value Christopher Hall
2015-07-28 3:44 ` John Stultz
2015-07-28 4:05 ` John Stultz
2015-07-29 10:19 ` Thomas Gleixner
2015-07-29 10:23 ` Thomas Gleixner
2015-07-29 10:51 ` Peter Zijlstra
2015-07-29 12:30 ` Richard Cochran
2015-07-29 10:49 ` Peter Zijlstra
2015-07-29 11:48 ` Richard Cochran
2015-07-29 12:35 ` Peter Zijlstra
2015-07-29 12:52 ` Thomas Gleixner
2015-07-29 1:41 ` Hall, Christopher S [this message]
2015-07-29 14:05 ` Thomas Gleixner
2015-07-28 0:46 ` [PATCH 2/5] Added functions mapping TSC value to system time Christopher Hall
2015-07-28 0:46 ` [PATCH 3/5] Add calls to translate Always Running Timer (ART) " Christopher Hall
2015-07-28 1:32 ` Andy Lutomirski
2015-07-29 1:18 ` Hall, Christopher S
2015-07-29 1:47 ` Andy Lutomirski
2015-07-28 4:11 ` John Stultz
2015-07-29 2:05 ` Hall, Christopher S
2015-07-29 8:25 ` Paul Bolle
2015-07-28 0:46 ` [PATCH 4/5] Add support for driver cross-timestamp to PTP_SYS_OFFSET ioctl Christopher Hall
2015-07-28 0:46 ` [PATCH 5/5] Enables cross timestamping in the e1000e driver Christopher Hall
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=B79D786B7111A34A8CF09F833429C493A909CEE1@ORSMSX109.amr.corp.intel.com \
--to=christopher.s.hall@intel.com \
--cc=hpa@zytor.com \
--cc=jeffrey.t.kirsher@intel.com \
--cc=john.ronciak@intel.com \
--cc=john.stultz@linaro.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=netdev@vger.kernel.org \
--cc=richardcochran@gmail.com \
--cc=tglx@linutronix.de \
--cc=x86@kernel.org \
/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
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
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).