netdev - Re: [PATCH v1 net-next 5/6] net: allow simultaneous SW and HW transmit timestamping

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Date:   Tue, 2 May 2017 11:56:53 +0200
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Richard Cochran <richardcochran@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        "Keller, Jacob E" <jacob.e.keller@...el.com>,
        Denny Page <dennypage@...com>, Jiri Benc <jbenc@...hat.com>
Subject: Re: [PATCH v1 net-next 5/6] net: allow simultaneous SW and HW
 transmit timestamping

On Fri, Apr 28, 2017 at 04:07:29PM -0400, Willem de Bruijn wrote:
> On Fri, Apr 28, 2017 at 12:23 PM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> > On Fri, Apr 28, 2017 at 11:50:28AM -0400, Willem de Bruijn wrote:
> >> A more elegant solution would be to not set SKBTX_IN_PROGRESS
> >> at all if SOF_TIMESTAMPING_OPT_TX_SWHW is set on the socket.
> >> But the patch to do so is not elegant, having to update callsites in many
> >> device drivers.
> >
> > Also, it would change the meaning of the flag as it seems some drivers
> > actually use the SKBTX_IN_PROGRESS flag to check if they expect a
> > timestamp.
> >
> > How about allocating the last bit of tx_flags for SKBTX_SWHW_TSTAMP?
> 
> That is such a scarce resource that I really would prefer to avoid using
> that if we can.

Ok. I think it won't really matter. We should keep in mind that the
reason for adding the OPT_TX_SWHW option was to not break old
applications which enabled both SW and HW TX timestamping, even though
they could get only one timestamp. I think most applications in future
will either enable only SW or HW TX timestamping, or enable both
together with the OPT_TX_SWHW option in order to get a SW timestamp
when HW timestamp was requested but missing.

> >> Otherwise you may indeed have to call skb_tstamp_tx for every packet
> >> that has SKBTX_SW_TSTAMP set, as you do. We can at least move
> >> the skb->sk != NULL check into skb_tx_timestamp in skbuff.h.

There are other callers of skb_tx_timestamp() and it's not obvious to
me they are all safe (i.e. cannot pass skb will sk==NULL), so I think
this should rather be a separate patch if necessary.

I'll resend the series with the other changes you have suggested.

-- 
Miroslav Lichvar