netdev - Re: [PATCH net-next 3/3] docs: networking: timestamping: add a set of frequently asked questions

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Date:   Mon, 20 Jul 2020 11:54:30 -0700
From:   Jacob Keller <jacob.e.keller@...el.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
        richardcochran@...il.com, sorganov@...il.com,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH net-next 3/3] docs: networking: timestamping: add a set of
 frequently asked questions

On 7/18/2020 4:35 AM, Vladimir Oltean wrote:
> On Fri, Jul 17, 2020 at 04:12:07PM -0700, Jacob Keller wrote:
>> On 7/17/2020 9:10 AM, Vladimir Oltean wrote:
>>> +When the interface they represent offers both ``SOF_TIMESTAMPING_TX_HARDWARE``
>>> +and ``SOF_TIMESTAMPING_TX_SOFTWARE``.
>>> +Originally, the network stack could deliver either a hardware or a software
>>> +time stamp, but not both. This flag prevents software timestamp delivery.
>>> +This restriction was eventually lifted via the ``SOF_TIMESTAMPING_OPT_TX_SWHW``
>>> +option, but still the original behavior is preserved as the default.
>>> +
>>
>> So, this implies that we set this only if both are supported? I thought
>> the intention was to set this flag whenever we start a HW timestamp.
>>
> 
> It's only _required_ when SOF_TIMESTAMPING_TX_SOFTWARE is used, it
> seems. I had also thought of setting 'SKBTX_IN_PROGRESS' as good
> practice, but there are many situations where it can do more harm than
> good.
> 

I guess I've only ever implemented a driver with software timestamping
enabled as an option. What sort of issues arise when you have this set?
I'm guessing that it's some configuration of stacked devices as in the
other cases? If the issue can't be fixed I'd at least like more
explanation here, since the prevailing convention is that we set this
flag, so understanding when and why it's problematic would be useful.

Thanks,
Jake