netdev - Re: What ixgbe devices support HWTSTAMP_FILTER

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Message-Id: <4AAA3CE1-B137-4AC1-AB85-B4690E8E3E12@alum.mit.edu>
Date:	Sat, 14 May 2016 11:47:22 -0700
From:	Guy Harris <guy@...m.mit.edu>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	Network Development <netdev@...r.kernel.org>
Subject: Re: What ixgbe devices support HWTSTAMP_FILTER_ALL for hardware time stamping?

On May 14, 2016, at 12:30 AM, Richard Cochran <richardcochran@...il.com> wrote:

> On Fri, May 13, 2016 at 04:12:52PM -0700, Guy Harris wrote:
>> The Linux implementation currently implements the inquiry by doing a
>> ETHTOOL_GET_TS_INFO SIOETHTOOL ioctl and looking at the
>> so_timestamping bits, if the linux/ethtool.h header defines
>> ETHTOOL_GET_TS_INFO and the ioctl succeeds on the device.
> 
> So far, so good. 
> 
>> This is inadequate - as libpcap requests hardware time stamping for
>> all packets, it should also check whether HWTSTAMP_FILTER_ALL is set
>> in rx_filters, and only offer hardware time stamping if it's set.
> 
> The SO_TIMESTAMPING and SIOCSHWTSTAMP interfaces predate
> ETHTOOL_GET_TS_INFO, and they work fine without it.  Applications
> should simply use SIOCSHWTSTAMP to request the mode that they need and
> check the result.

So if you have a GUI application for packet capture, with a combo box to select the type of time stamping, should it:

	1) regardless of whether ETHTOOL_GET_TS_INFO is available, open the adapter, try each of the time stamp types to see whether it works, and show a combo box based on that;

	2) use ETHTOOL_GET_TS_INFO if available;

	3) offer all possibilities regardless of whether they work with the adapter or not, and just report an error for possibilities that don't work?

My preference is 2) - which is the main reason why libpcap offers "what possibilities are available?" APIs, not just "request this possibility" APIs.