[dpdk-dev] [RFC] ethdev: introduce Rx buffer split

[Thomas Monjalon]

12/10/2020 17:56, Ananyev, Konstantin:
> > From: Ananyev, Konstantin <konstantin.ananyev at intel.com>
> > > > 12/10/2020 11:56, Slava Ovsiienko:
> > > > > We have two approaches how to specify multiple segments to split Rx
> > > packets:
> > > > > 1. update queue configuration structure 2. introduce new
> > > > > rx_queue_setup_ex() routine with extra parameters.
> > > > >
> > > > > For [1] my only actual dislike is that we would have multiple places
> > > > > to specify the pool - in rx_queue_setup() and in the config
> > > > > structure. So, we should implement some checking (if we have offload
> > > > > flag set we should check whether mp parameter is NULL and segment
> > > > > descriptions array pointer/size is provided, if no offload flag set - we must
> > > check the description array is empty).
> > > > >
> > > > > > @Thomas, @Ferruh: I'd like to hear what other ethdev maintainers
> > > > > > think about it.
> > > > >
> > > > > Yes, it would be very nice to hear extra opinions. Do we think the
> > > > > providing of extra API function is worse than extending existing
> > > > > structure, introducing some conditional ambiguity and complicating
> > > > > the parameter compliance check?
> > > >
> > > > Let's try listing pros and cons of each approach, so we can conclude.
> > > >
> > > > 1/ update queue config struct
> > > >
> > > > 	1.1 pro: keep same queue setup function
> > > > 	1.2 con: two mempool pointers (struct or function)
> > > > 	1.3 con: variable size of segment description array
> > > >
> > > > 2/ new queue setup function
> > > >
> > > > 	2.1 con: two functions for queue setup
> > > > 	2.2 pro: mempool pointer is not redundant
> > > > 	2.3 pro: segment description array size defined by the caller
> > > >
> > > > What else I'm missing?
> > > >
> > >
> > > My 2 cents: can we make new (_ex) function to work for both original config
> > > (1 mp for all sizes, no split) and for new config (multiple mp, split allowed)?
> > > Then in future (21.11?) we can either get rid of original one, or even make it
> > > a wrapper around all one?
> > > Konstantin
> > 
> > Yes, actually the mlx5 PMD implementation follows this approach -
> > specifying the segment description array with the only element
> > and zero size/offset provides exactly the same configuration as existing
> > rte_eth_rx_queue_setup().
> > 
> > Currently I'm detailing the description  (how HEAD_ROOM is handled, what happens
> > if array is shorter the the buffer chain for segment of maximal size, the zero segment
> > size means follow the value deduced from the pool and so on).
> > 
> > So, may we consider this point as one more "pro" to setup_ex approach ? 😊
> 
> From my perspective, yes.
> It is sort of more gradual approach.
> I expect it would be experimental function for some time,
> so we'll have time to try it, adjust, fix, etc without breaking original one.

I like the wrapper idea.
Is it possible to call rte_eth_rx_queue_setup_ex()
from rte_eth_rx_queue_setup() using a rte_eth_rxseg object on the stack?