netdev - Re: What ixgbe devices support HWTSTAMP_FILTER

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Date:	Sat, 14 May 2016 19:11:50 -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 1:26 PM, Richard Cochran <richardcochran@...il.com> wrote:

> On Sat, May 14, 2016 at 11:47:22AM -0700, Guy Harris wrote:
>> 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.
> 
> You are going to have to implement #1 in any case, if you want your
> program to work on all kernels.

What libpcap currently implements is a combination of #2 and #3, where:

	if it's compiled with headers that define ETHTOOL_GET_TS_INFO, it tries to do ETHTOOL_GET_TS_INFO and, if that fails with EOPNOTSUPP or EINVAL, it offers all possibilities;

	if it's compiled with headers that don't define it, it just offers all possibilities.

It could do a combination of #2 and #1, where "offers all possibilities" is replaced by "opens the adapter, tries each of the possibilities, and offers the ones that don't fail" - but, other than the current bugs with ETHTOOL_GET_TS_INFO, I don't see any advantage to doing only #1, rather than trying #2, perhaps with some special-casing to work around the bugs in question, and only falling back on actually trying to set the options if we can't ask about them.