Re: [PATCH v4 net-next 6/7] net: allow simultaneous SW and HW transmit timestamping

[Miroslav Lichvar <mlichvar@redhat.com>]

On Thu, May 18, 2017 at 04:16:26PM -0400, Willem de Bruijn wrote:
> On Thu, May 18, 2017 at 9:06 AM, Miroslav Lichvar <mlichvar@redhat.com> wrote:
> > +/* On transmit, software and hardware timestamps are returned independently.
> > + * As the two skb clones share the hardware timestamp, which may be updated
> > + * before the software timestamp is received, a hardware TX timestamp may be
> > + * returned only if there is no software TX timestamp. A false software
> > + * timestamp made for SOCK_RCVTSTAMP when a real timestamp is missing must
> > + * be ignored.
> 
> Please expand on why this case can be ignored. It is quite subtle. How about
> something like
> 
> *
> * A false software timestamp is one made inside the __sock_recv_timestamp
> * call itself. These are generated whenever SO_TIMESTAMP(NS) is enabled
> * on the socket, even when the timestamp reported is for another option, such
> * as hardware tx timestamp.
> *
> * Ignore these when deciding whether a timestamp source is hw or sw.
> */

That seems a bit too verbose to me. :) Would the following work?

/* On transmit, software and hardware timestamps are returned independently.
 * As the two skb clones share the hardware timestamp, which may be updated
 * before the software timestamp is received, a hardware TX timestamp may be
 * returned only if there is no software TX timestamp. Ignore false software
 * timestamps, which may be made in the __sock_recv_timestamp() call when the
 * option SO_TIMESTAMP(NS) is enabled on the socket, even when the skb has a
 * hardware timestamp.
 */

> > +static bool skb_is_swtx_tstamp(const struct sk_buff *skb,
> > +                              const struct sock *sk, int false_tstamp)
> > +{
> > +       if (false_tstamp && sk->sk_tsflags & SOF_TIMESTAMPING_OPT_TX_SWHW)
> 
> Also, why is it ignored only for the new mode?

Good point. That should not be there. The function can be now reduced
to a single line again. I originally tried a different approach,
disabling false timestamps in the new mode, but then I thought it's
better to not complicate it unnecessarily and keep it consistent.

-- 
Miroslav Lichvar