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

[Ferruh Yigit <ferruh.yigit@intel.com>]

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?
What if that 'rest' part larger than given buffer size for that mempool?

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.

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>

> 
> 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``
>