netdev - Re: [PATCH v3 1/5] net-timestamp: COMPLETION timestamp on packet tx completion

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Message-ID: <aef858a717b1547d35fbda78c0e928e57394329d.camel@iki.fi>
Date: Sat, 15 Feb 2025 14:43:27 +0200
From: Pauli Virtanen <pav@....fi>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
	linux-bluetooth@...r.kernel.org
Cc: Luiz Augusto von Dentz <luiz.dentz@...il.com>, netdev@...r.kernel.org, 
	davem@...emloft.net, kuba@...nel.org
Subject: Re: [PATCH v3 1/5] net-timestamp: COMPLETION timestamp on packet tx
 completion

Hi,

su, 2025-02-09 kello 22:29 -0500, Willem de Bruijn kirjoitti:
> Pauli Virtanen wrote:
> 
> >  1.3.2 Timestamp Reporting
> >  ^^^^^^^^^^^^^^^^^^^^^^^^^
> > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > index bb2b751d274a..3707c9075ae9 100644
> > --- a/include/linux/skbuff.h
> > +++ b/include/linux/skbuff.h
> > @@ -489,10 +489,14 @@ enum {
> >  
> >  	/* generate software time stamp when entering packet scheduling */
> >  	SKBTX_SCHED_TSTAMP = 1 << 6,
> > +
> > +	/* generate software time stamp on packet tx completion */
> > +	SKBTX_COMPLETION_TSTAMP = 1 << 7,
> >  };
> >  
> >  #define SKBTX_ANY_SW_TSTAMP	(SKBTX_SW_TSTAMP    | \
> > -				 SKBTX_SCHED_TSTAMP)
> > +				 SKBTX_SCHED_TSTAMP | \
> > +				 SKBTX_COMPLETION_TSTAMP)
> 
> These fields are used in the skb_shared_info tx_flags field.
> Which is a very scarce resource. This takes the last available bit.
> That is my only possible concern: the opportunity cost.

One alternative here could be:

Make SOF_TIMESTAMPING_TX_COMPLETION available only as a socket option,
and make it emit COMPLETION tstamp exactly for those packets that also
have SOF_TIMESTAMPING_TX_SOFTWARE enabled. User apps can then still
timestamp only selected packets via CMSG, however the choice between
SND and SND+COMPLETION is socket-wide. The API is then less uniform,
but probably usable in practice, although some use cases may not be
interested in the extra SND tstamps.

IIUC, sizeof skb_shared_info cannot be easily changed and I'm not sure
if there is room left there. So if the option of putting it into
protocol-specific skb cb is also out, then I'm not sure what the plan
here should be.

-- 
Pauli Virtanen