netdev - RE: [RFC PATCH 4/4] ixgbe: add support for extended PHC gettime

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Date:   Mon, 29 Oct 2018 16:02:57 +0000
From:   "Keller, Jacob E" <jacob.e.keller@...el.com>
To:     Miroslav Lichvar <mlichvar@...hat.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        Richard Cochran <richardcochran@...il.com>
Subject: RE: [RFC PATCH 4/4] ixgbe: add support for extended PHC gettime

> -----Original Message-----
> From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.kernel.org] On
> Behalf Of Miroslav Lichvar
> Sent: Monday, October 29, 2018 6:31 AM
> To: Keller, Jacob E <jacob.e.keller@...el.com>
> Cc: netdev@...r.kernel.org; intel-wired-lan@...ts.osuosl.org; Richard Cochran
> <richardcochran@...il.com>
> Subject: Re: [RFC PATCH 4/4] ixgbe: add support for extended PHC gettime
> 
> On Fri, Oct 26, 2018 at 04:54:57PM +0000, Keller, Jacob E wrote:
> > > -----Original Message-----
> > > From: Miroslav Lichvar [mailto:mlichvar@...hat.com]
> > > Sent: Friday, October 26, 2018 9:28 AM
> > > To: netdev@...r.kernel.org
> > > Cc: intel-wired-lan@...ts.osuosl.org; Richard Cochran
> <richardcochran@...il.com>;
> > > Keller, Jacob E <jacob.e.keller@...el.com>; Miroslav Lichvar
> <mlichvar@...hat.com>
> > > Subject: [RFC PATCH 4/4] ixgbe: add support for extended PHC gettime
> > >
> > > Cc: Richard Cochran <richardcochran@...il.com>
> > > Cc: Jacob Keller <jacob.e.keller@...el.com>
> > > Signed-off-by: Miroslav Lichvar <mlichvar@...hat.com>
> 
> > What about replacing gettime64 with:
> >
> > static int ixgbe_ptp_gettimex(struct ptp_clock_info *ptp, struct timespec64 *ts)
> > {
> >     struct ptp_system_timestamp sts
> >
> >     ixgbe_ptp_gettimex(ptp, &tst);
> >     *ts = sts.phc_ts
> > }
> 
> That will work, but it will be slower. With HPET as a clocksource
> there would be few microseconds of an extra (symmetric) delay and the
> applications would have to assume a larger maximum error.
> 

Right. My intention for this would be that we'd switch from gettime64 to gettime64x going forward, and provide this as a way to avoid having to duplicate logic in drivers while we're transitioning? Thus, new applications should be using the new call if it's available in the driver.

Hmm.
 
> I think there could be a flag in ptp_system_timestamp, or a parameter
> of gettimex64(), which would enable/disable reading of the system
> clock.
> 
> > Actually, could that even just be provided by the PTP core if gettime64 isn't
> implemented? This way new drivers only have to implement the new interface, and
> userspace will just get the old behavior if they use the old call?
> 
> Good idea.

Ideally we can find a way that minimizes the overhead for the old call.

Thanks,
Jake

> 
> Thanks,
> 
> --
> Miroslav Lichvar