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Date: Fri, 26 Jun 2015 15:24:25 +0000 From: "Keller, Jacob E" <jacob.e.keller@...el.com> To: "Hall, Christopher S" <christopher.s.hall@...el.com>, "richardcochran@...il.com" <richardcochran@...il.com> CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "Stanton, Kevin B" <kevin.b.stanton@...el.com>, "Ronciak, John" <john.ronciak@...el.com> Subject: Re: [PATCH] Add PTP cross-timestamp to the PTP driver interface On Fri, 2015-06-26 at 08:55 +0200, Richard Cochran wrote: > Chris, > > Basically this patch looks okay to me. Could you please add LKML, > John Stultz and tglx (the time guys) onto CC? I would like to get > their Acks or at least let them have a chance to review it. > > On Thu, Jun 25, 2015 at 04:42:56PM -0700, Christopher Hall wrote: > > This patch allows system and device time ("cross-timestamp") to be > > performed > > by the driver. Currently, the timestamping is performed in the > > PTP_SYS_OFFSET > > ioctl. It reads gettimeofday() and the gettime64() callback > > provided by the > > driver. The cross-timestamp is best effort ignoring the latency > > between the > > capture of system time (getnstimeofday()) and the device time > > (driver callback). > > You can make the motivation more clear by mentioning how the newer > PCIe spec foresees "perfect" timestamps. If I didn't already know > the > background, I would wonder who would ever want "best effort" single > cross timestamps. > FYI, I read "best effort" as a comment on how it's implemented today, not what the implementation could provide. > > Additionally, the callback, getsynctime64(), will only be called > > when > > n_samples == 1 because the driver returns only 1 cross-timestamp > > where > > multiple samples cannot be chained together. > > There should be a way for user space to find out whether a particular > device offers the cross timestamp capability. There are reserved > fields in 'struct ptp_clock_caps' that could be used. > Yes, I agree. Otherwise we'd have to use only one sample in ptp4l in order to benefit, but we'd be unable to tell it was worth it. Regards, Jake > Thanks, > Richard > -- > To unsubscribe from this list: send the line "unsubscribe netdev" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html
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