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

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 6 Aug 2020 17:03:31 +0000
Slava Ovsiienko <viacheslavo@mellanox.com> wrote:

> > -----Original Message-----
> > From: Stephen Hemminger <stephen@networkplumber.org>
> > Sent: Thursday, August 6, 2020 19:26
> > To: Ferruh Yigit <ferruh.yigit@intel.com>
> > Cc: Jerin Jacob <jerinjacobk@gmail.com>; Slava Ovsiienko
> > <viacheslavo@mellanox.com>; dpdk-dev <dev@dpdk.org>; Matan Azrad
> > <matan@mellanox.com>; Raslan Darawsheh <rasland@mellanox.com>;
> > Thomas Monjalon <thomas@monjalon.net>; 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 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
> >   
> I thought about adding some other segment attributes, vendor specific.
> We could describe the segments with some descriptor structure (size, pool)
> and add flags field to one. The proposals from other vendors are welcome.
> 

Please no snowflake API's "are driver is special"...

Think of how it can fit into a general model.
Also, just because your hardware has a special feature does not mean
the DPDK has to support it!