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
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