netdev - Re: [PATCH] Add PTP cross-timestamp to the PTP driver interface

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Message-ID: <20150626065507.GB1511@localhost.localdomain>
Date:	Fri, 26 Jun 2015 08:55:07 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Christopher Hall <christopher.s.hall@...el.com>
Cc:	netdev@...r.kernel.org, jacob.e.keller@...el.com,
	john.ronciak@...el.com, kevin.b.stanton@...el.com
Subject: Re: [PATCH] Add PTP cross-timestamp to the PTP driver interface

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.

> 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.

Thanks,
Richard
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