Re: [dpdk-dev] [PATCH] net/ring: advertise multi segment support.

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Sep 28, 2020 at 11:25:34AM +0100, Ferruh Yigit wrote:
> On 9/28/2020 8:31 AM, Dumitru Ceara wrote:
> > On 9/22/20 4:21 PM, Ferruh Yigit wrote:
> > > On 9/18/2020 11:36 AM, Dumitru Ceara wrote:
> > > > Even though ring interfaces don't support any other TX/RX offloads they
> > > > do support sending multi segment packets and this should be advertised
> > > > in order to not break applications that use ring interfaces.
> > > > 
> > > 
> > > Does ring PMD support sending multi segmented packets?
> > > 
> > 
> > Yes, sending multi segmented packets works fine with ring PMD.
> > 
> 
> Define "works fine" :)
> 
> All PMDs can put the first mbuf of the chained mbuf to the ring, in that
> case what is the difference between the ones supports
> 'DEV_TX_OFFLOAD_MULTI_SEGS' and the ones doesn't support?
> 
> If the traffic is only from ring PMD to ring PMD, you won't recognize the
> difference between segmented or not-segmented mbufs, and it will look like
> segmented packets works fine.
> But if there is other PMDs involved in the forwarding, or if need to process
> the packets, will it still work fine?
> 

What other PMDs do or don't do should be irrelevant here, I think. The fact
that multi-segment PMDs make it though the ring PMD in valid form should be
sufficient to mark it as supported.

> > > As far as I can see ring PMD doesn't know about the mbuf segments.
> > > 
> > 
> > Right, the PMD doesn't care about the mbuf segments but it implicitly
> > supports sending multi segmented packets. From what I see it's actually
> > the case for most of the PMDs, in the sense that most don't even check
> > the DEV_TX_OFFLOAD_MULTI_SEGS flag and if the application sends multi
> > segment packets they are just accepted.
> >
> 
> As far as I can see, if the segmented packets sent, the ring PMD will put
> the first mbuf into the ring without doing anything specific to the next
> segments.
> 
> If the 'DEV_TX_OFFLOAD_MULTI_SEGS' is supported I expect it should detect
> the segmented packets and put each chained mbuf into the separate field in
> the ring.
> 

Why, what would be the advantage of that? Right now if you send in a valid
packet chain to the Ring PMD, you get a valid packet chain out again the
other side, so I don't see what needs to change about that behaviour.

> > 
> > However, the fact that the ring PMD doesn't advertise this implicit
> > support forces applications that use ring PMD to have a special case for
> > handling ring interfaces. If the ring PMD would advertise
> > DEV_TX_OFFLOAD_MULTI_SEGS this would allow upper layers to be oblivious
> > to the type of underlying interface.
> > 
> 
> This is not handling the special case for the ring PMD, this is why he have
> the offload capability flag. Application should behave according capability
> flags, not per specific PMD.
> 
> Is there any specific usecase you are trying to cover?