Re: [dpdk-dev] [PATCH] doc: announce changes to ethdev rxconf structure

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 6 Aug 2020 16:58:22 +0100
Ferruh Yigit <ferruh.yigit@intel.com> wrote:

> On 8/4/2020 2:32 PM, Jerin Jacob wrote:
> > On Mon, Aug 3, 2020 at 6:36 PM Slava Ovsiienko <viacheslavo@mellanox.com> wrote:  
> >>
> >> Hi, Jerin,
> >>
> >> Thanks for the comment,  please, see below.
> >>  
> >>> -----Original Message-----
> >>> From: Jerin Jacob <jerinjacobk@gmail.com>
> >>> Sent: Monday, August 3, 2020 14:57
> >>> To: Slava Ovsiienko <viacheslavo@mellanox.com>
> >>> Cc: dpdk-dev <dev@dpdk.org>; Matan Azrad <matan@mellanox.com>;
> >>> Raslan Darawsheh <rasland@mellanox.com>; Thomas Monjalon
> >>> <thomas@monjalon.net>; Ferruh Yigit <ferruh.yigit@intel.com>; Stephen
> >>> Hemminger <stephen@networkplumber.org>; Andrew Rybchenko
> >>> <arybchenko@solarflare.com>; Ajit Khaparde
> >>> <ajit.khaparde@broadcom.com>; Maxime Coquelin
> >>> <maxime.coquelin@redhat.com>; Olivier Matz <olivier.matz@6wind.com>;
> >>> David Marchand <david.marchand@redhat.com>
> >>> Subject: Re: [PATCH] doc: announce changes to ethdev rxconf structure
> >>>
> >>> On Mon, Aug 3, 2020 at 4:28 PM Viacheslav Ovsiienko
> >>> <viacheslavo@mellanox.com> wrote:  
> >>>>
> >>>> The DPDK datapath in the transmit direction is very flexible.
> >>>> The applications can build multisegment packets and manages almost all
> >>>> data aspects - the memory pools where segments are allocated from, the
> >>>> segment lengths, the memory attributes like external, registered, etc.
> >>>>
> >>>> In the receiving direction, the datapath is much less flexible, the
> >>>> applications can only specify the memory pool to configure the
> >>>> receiving queue and nothing more. In order to extend the receiving
> >>>> datapath capabilities it is proposed to add the new fields into
> >>>> rte_eth_rxconf structure:
> >>>>
> >>>> struct rte_eth_rxconf {
> >>>>     ...
> >>>>     uint16_t rx_split_num; /* number of segments to split */
> >>>>     uint16_t *rx_split_len; /* array of segment lengthes */
> >>>>     struct rte_mempool **mp; /* array of segment memory pools */  
> >>>
> >>> The pool has the packet length it's been configured for.
> >>> So I think, rx_split_len can be removed.  
> >>
> >> Yes, it is one of the supposed options - if pointer to array of segment lengths
> >> is NULL , the queue_setup() could use the lengths from the pool's properties.
> >> But we are talking about packet split, in general, it should not depend
> >> on pool properties. What if application provides the single pool
> >> and just wants to have the tunnel header in the first dedicated mbuf?
> >>  
> >>>
> >>> This feature also available in Marvell HW. So it not specific to one vendor.
> >>> Maybe we could just the use case mention the use case in the depreciation
> >>> notice and the tentative change in rte_eth_rxconf and exact details can be
> >>> worked out at the time of implementation.
> >>>  
> >> So, if I understand correctly, the struct changes in the commit message
> >> should be marked as just possible implementation?  
> > 
> > Yes.
> > 
> > We may need to have a detailed discussion on the correct abstraction for various
> > HW is available with this feature.
> > 
> > On Marvell HW, We can configure TWO pools for given eth Rx queue.
> > One pool can be configured as a small packet pool and other one as
> > large packet pool.
> > And there is a threshold value to decide the pool between small and large.
> > For example:
> > - The small pool is configured 2k
> > - The large pool is configured with 10k
> > - And if the threshold value is configured as 2k.
> > Any packet size <=2K will land in small pool and others in a large pool.
> > The use case, we are targeting is to save the memory space for jumbo frames.  
> 
> Out of curiosity, do you provide two different buffer address in the descriptor
> and HW automatically uses one based on the size,
> or driver uses one of the pools based on the configuration and possible largest
> packet size?

I am all for allowing more configuration of buffer pool.
But don't want that to be exposed as a hardware specific requirement in the
API for applications. The worst case would be if your API changes required:

  if (strcmp(dev->driver_name, "marvell") == 0) {
     // make another mempool for this driver