[dpdk-dev] [RFC 0/8] mbuf: structure reorganization

[Ananyev, Konstantin]

> 
> Hi,
> 
> On Tue, 28 Feb 2017 09:05:07 +0000, "Ananyev, Konstantin"
> <konstantin.ananyev at intel.com> wrote:
> > Hi everyone,
> >
> > > >
> > > > In my opinion, if we have the room in the first cache line, we
> > > > should put it there. The only argument I see against is "we may
> > > > find something more important in the future, and we won't have
> > > > room for it in the first cache line". I don't feel we should
> > > > penalize today's use cases for hypothetic future use cases.
> > > >
> > > >
> > > >
> > > >> 2. timestamp normalization point
> > > >>      inside PMD RX vs somewhere later as user needs it (extra
> > > >> function in dev_ops?).
> > > >
> > > > This point could be changed. My initial proposition tries to
> > > > provide a generic API for timestamp. Let me remind it here:
> > > >
> > > > a- the timestamp is in nanosecond
> > > > b- the reference is always the same for a given path: if the
> > > > timestamp is set in a PMD, all the packets for this PMD will have
> > > > the same reference, but for 2 different PMDs (or a sw lib), the
> > > > reference would not be the same.
> > > >
> > > > We may remove a-, and just have:
> > > >  - the reference and the unit are always the same for a given
> > > > path: if the timestamp is set in a PMD, all the packets for this
> > > > PMD will have the same reference and unit, but for 2 different
> > > > PMDs (or a sw lib), they would not be the same.
> > > >
> > > > In both cases, we would need a conversion code (maybe in a
> > > > library) if the application wants to work with timestamps from
> > > > several sources. The second solution removes the normalization
> > > > code in the PMD when not needed, it is probably better.
> > >
> > > I agree.
> >
> > One question - does that mean that application would need to
> > keep a track from what PMD each particular packet came to do the
> > normalization? Konstantin
> 
> I'd say yes. It does not look very difficult to do, since the mbuf
> contains the input port id.
> 

I understand that we can use mbuf->port here, but it means that we'll
introduce new implicit dependency between timestamp and port values.
>From my point that introduces new implications:
1. all PMDs that do set a timestamp would also have to set port value too.
    Probably not a big deal as most of PMDs do set port value anyway right now,
    but it means it would be hard to get rid/change mbuf->port in future.
2. Applications would not allowed to change mbuf->port value before normalization is done
    (from what I heard some apps do update mbuf->port to store routing decisions).  
    BTW, how the app would keep track which mbufs were already normalized, and which were not?
3. In theory with eth_dev_detach() - mbuf->port value might be not valid at the point when application
     would decide to do normalization. 

So to me all that approach with delayed normalization seems unnecessary overcomplicated.
Original one suggested by Olivier, when normalization is done in PMD at RX look
much cleaner and more manageable. 
Konstantin