Subject: Re: Extending socket timestamping API for NTP

From: Miroslav Lichvar <[email-masked]>
Date: Mon, 27 Feb 2017 16:23:39 +0100

On Tue, Feb 07, 2017 at 02:32:04PM -0800, Willem de Bruijn wrote:
> >> 4) allow sockets to use both SW and HW TX timestamping at the same time
> >>
> >>    When using a socket which is not bound to a specific interface, it
> >>    would be nice to get transmit SW timestamps when HW timestamps are
> >>    missing. I suspect it's difficult to predict if a HW timestamp will
> >>    be available. Maybe it would be acceptable to get from the error
> >>    queue two messages per transmission if the interface supports both
> >>    SW and HW timestamping?
> >
> >
> > This seems useful,
> 
> Agreed, as long as it is optional so that it does not change the
> behavior for existing applications.

Do you think it is safe to assume that no application enabled both SW
and HW TX timestamping? Do we need a new option for this?

> > but not sure how best to implement it.
> 
> It might be sufficient to just remove the second line in sw_tx_timestamp
> 
> static inline void sw_tx_timestamp(struct sk_buff *skb)
> {
>         if (skb_shinfo(skb)->tx_flags & SKBTX_SW_TSTAMP &&
>             !(skb_shinfo(skb)->tx_flags & SKBTX_IN_PROGRESS))
>                 skb_tstamp_tx(skb, NULL);
> }

With this change I'm getting two error messages per transmission, but
it looks like it may need some additional changes.

If the first error message is received after the HW timestamp was
captured, it contains both timestamps as the HW timestamp is in the
shared info of the skb. Is it possible it could contain a partially
updated HW timestamp? I'm not sure how locking works here. Is
scm_timestamping actually allowed to contain more than one timestamp?
The timestamping.txt document says "Only one field is non-zero at any
time.", but that wasn't true even before if both SW and HW RX
timestamping was enabled.

If SO_TIMESTAMP{,NS} is enabled, ts[0] in the second error message
will contain a bogus SW timestamp added by __sock_recv_timestamp() for
a "Race occurred between timestamp enabling and packet receiving". Is
there a guarantee applications will get a timestamp for all messages
after enabling SO_TIMESTAMP? The original code is older than the git
repo, so I'm not sure what was the reason for this. To me it would
make more sense to not add any SCM_TIMESTAMP (and SW timestamp in
SCM_TIMESTAMPING) when the the timestamp is missing. If that's not
always acceptable, maybe it could be restricted to sockets that have
HW timestamping enabled?

Some drivers don't call skb_tx_timestamp() when HW timestamp was
requested. From a cursory look it is e1000e, xgbe, sxgbe, and stmmac.
This should hopefully be an easy fix.

Thoughts?

-- 
Miroslav Lichvar