Re: [dpdk-dev] [PATCH v2] doc: announce changes to ethdev rxconf structure

[Slava Ovsiienko <viacheslavo@mellanox.com>]

> -----Original Message-----
> From: Ferruh Yigit <ferruh.yigit@intel.com>
> Sent: Thursday, August 6, 2020 19:32
> To: Slava Ovsiienko <viacheslavo@mellanox.com>; dev@dpdk.org
> Cc: Matan Azrad <matan@mellanox.com>; Raslan Darawsheh
> <rasland@mellanox.com>; Thomas Monjalon <thomas@monjalon.net>;
> jerinjacobk@gmail.com; stephen@networkplumber.org;
> arybchenko@solarflare.com; ajit.khaparde@broadcom.com;
> maxime.coquelin@redhat.com; olivier.matz@6wind.com;
> david.marchand@redhat.com
> Subject: Re: [PATCH v2] doc: announce changes to ethdev rxconf structure
> 
> On 8/5/2020 9:49 AM, Viacheslav Ovsiienko wrote:
> > The DPDK datapath in the transmit direction is very flexible.
> > The applications can build multi-segment 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. The packet being received can only
> > be pushed to the chain of the mbufs of the same data buffer size and
> > allocated from the same pool. In order to extend the receiving
> > datapath buffer description 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 lengths */
> >     struct rte_mempool **mp; /* array of segment memory pools */
> >     ...
> > };
> 
> What is the way to say first 14 bytes will go first mempool and rest will go
> second one?
> Or do you have to define fixed sizes for all segments?
Yes - "rx_split_len" array defines the sizes of segments.

> What if that 'rest' part larger than given buffer size for that mempool?
Error. The supposed size for appropriate segment must conform the pool requirements.
So - the segment size must be less or equal to the mbuf buffer size from this pool.
And, must be supported by HW and PMD, if there are some other limitations - these
ones should be checked in rx_queue_setup and error returned if needed.

> 
> Intel NICs also has header split support, similar to what Jerin described,
> header and data goes to different buffers, which doesn't require fixed sizes
> and need only two mempools, not sure if it should be integrated to this
> feature but we can discuss later.
HEADER_SPLIT is port-wide feature, not per queue.
BTW, where the second pool (for the payload) is specified? Do I miss the second pool?

> 
> Also there are some valid concerns Andrew highlighted, like how application
> will know if PMD supports this feature etc.. and more.
> But since these are design/implementation related concerns, not a blocker
> for deprecation notice I think, overall no objection to config structure
> change, hence:
> 
> Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>

Thanks a lot for the review, discussion and Deprecation Note ack.

> 
> >
> > The non-zero value of rx_split_num field configures the receiving
> > queue to split ingress packets into multiple segments to the mbufs
> > allocated from various memory pools according to the specified
> > lengths. The zero value of rx_split_num field provides the backward
> > compatibility and queue should be configured in a regular way (with
> > single/multiple mbufs of the same data buffer length allocated from
> > the single memory pool).
> >
> > The new approach would allow splitting the ingress packets into
> > multiple parts pushed to the memory with different attributes.
> > For example, the packet headers can be pushed to the embedded data
> > buffers within mbufs and the application data into the external
> > buffers attached to mbufs allocated from the different memory pools.
> > The memory attributes for the split parts may differ either - for
> > example the application data may be pushed into the external memory
> > located on the dedicated physical device, say GPU or NVMe. This would
> > improve the DPDK receiving datapath flexibility preserving
> > compatibility with existing API.
> >
> > The proposed extended description of receiving buffers might be
> > considered by other vendors to be involved into similar features
> > support, it is the subject for the further discussion.
> >
> > Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
> > Acked-by: Jerin Jacob <jerinjacobk@gmail.com>
> >
> > ---
> > v1->v2: commit message updated, proposed to consider the new
> >         fields for supporting similar features by multiple
> > 	vendors
> > ---
> >  doc/guides/rel_notes/deprecation.rst | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/doc/guides/rel_notes/deprecation.rst
> > b/doc/guides/rel_notes/deprecation.rst
> > index acf87d3..b6bdb83 100644
> > --- a/doc/guides/rel_notes/deprecation.rst
> > +++ b/doc/guides/rel_notes/deprecation.rst
> > @@ -99,6 +99,11 @@ Deprecation Notices
> >    In 19.11 PMDs will still update the field even when the offload is not
> >    enabled.
> >
> > +* ethdev: add new fields to ``rte_eth_rxconf`` to configure the
> > +receiving
> > +  queues to split ingress packets into multiple segments according to
> > +the
> > +  specified lengths into the buffers allocated from the specified
> > +  memory pools. The backward compatibility to existing API is preserved.
> > +
> >  * ethdev: ``rx_descriptor_done`` dev_ops and
> ``rte_eth_rx_descriptor_done``
> >    will be deprecated in 20.11 and will be removed in 21.11.
> >    Existing ``rte_eth_rx_descriptor_status`` and
> > ``rte_eth_tx_descriptor_status``
> >