Re: [dpdk-dev] [PATCH v6 1/6] ethdev: introduce Rx buffer split

[Jerin Jacob <jerinjacobk@gmail.com>]

On Wed, Oct 14, 2020 at 11:42 PM Viacheslav Ovsiienko
<viacheslavo@nvidia.com> wrote:
>
> The DPDK datapath in the transmit direction is very flexible.
> An application can build the multi-segment packet and manages
> almost all data aspects - the memory pools where segments
> are allocated from, the segment lengths, the memory attributes
> like external buffers, registered for DMA, etc.
>
> In the receiving direction, the datapath is much less flexible,
> an application can only specify the memory pool to configure the
> receiving queue and nothing more. In order to extend receiving
> datapath capabilities it is proposed to add the way to provide
> extended information how to split the packets being received.
>
> The following structure is introduced to specify the Rx packet
> segment:
>
> struct rte_eth_rxseg {
>     struct rte_mempool *mp; /* memory pools to allocate segment from */
>     uint16_t length; /* segment maximal data length,
>                         configures "split point" */
>     uint16_t offset; /* data offset from beginning
>                         of mbuf data buffer */
>     uint32_t reserved; /* reserved field */
> };
>
> The segment descriptions are added to the rte_eth_rxconf structure:
>    rx_seg - pointer the array of segment descriptions, each element
>              describes the memory pool, maximal data length, initial
>              data offset from the beginning of data buffer in mbuf.
>              This array allows to specify the different settings for
>              each segment in individual fashion.
>    rx_nseg - number of elements in the array
>
> If the extended segment descriptions is provided with these new
> fields the mp parameter of the rte_eth_rx_queue_setup must be
> specified as NULL to avoid ambiguity.
>
> There are two options to specifiy Rx buffer configuration:
> - mp is not NULL, rx_conf.rx_seg is NULL, rx_conf.rx_nseg is zero,
>   it is compatible configuraion, follows existing implementation,
>   provides single pool and no description for segment sizes
>   and offsets.
> - mp is NULL, rx_conf.rx_seg is not NULL, rx_conf.rx_nseg is not
>   zero, it provides the extended configuration, individually for
>   each segment.
>
> The new offload flag RTE_ETH_RX_OFFLOAD_BUFFER_SPLIT in device
> capabilities is introduced to present the way for PMD to report to
> application about supporting Rx packet split to configurable
> segments. Prior invoking the rte_eth_rx_queue_setup() routine
> application should check RTE_ETH_RX_OFFLOAD_BUFFER_SPLIT flag.
>
> If the Rx queue is configured with new settings the packets being
> received will be split into multiple segments pushed to the mbufs
> with specified attributes. The PMD will split the received packets
> into multiple segments according to the specification in the
> description array:
>
> - the first network buffer will be allocated from the memory pool,
>   specified in the first segment description element, the second
>   network buffer - from the pool in the second segment description
>   element and so on. If there is no enough elements to describe
>   the buffer for entire packet of maximal length the pool from the
>   last valid element will be used to allocate the buffers from for the
>   rest of segments
>
> - the offsets from the segment description elements will provide
>   the data offset from the buffer beginning except the first mbuf -
>   for this one the offset is added to the RTE_PKTMBUF_HEADROOM to get
>   actual offset from the buffer beginning. If there is no enough
>   elements to describe the buffer for entire packet of maximal length
>   the offsets for the rest of segment will be supposed to be zero.
>
> - the data length being received to each segment is limited  by the
>   length specified in the segment description element. The data
>   receiving starts with filling up the first mbuf data buffer, if the
>   specified maximal segment length is reached and there are data
>   remaining (packet is longer than buffer in the first mbuf) the
>   following data will be pushed to the next segment up to its own
>   maximal length. If the first two segments is not enough to store
>   all the packet remaining data  the next (third) segment will
>   be engaged and so on. If the length in the segment description
>   element is zero the actual buffer size will be deduced from
>   the appropriate memory pool properties. If there is no enough
>   elements to describe the buffer for entire packet of maximal
>   length the buffer size will be deduced from the pool of the last
>   valid element for the remaining segments.
>
> For example, let's suppose we configured the Rx queue with the
> following segments:
>     seg0 - pool0, len0=14B, off0=2
>     seg1 - pool1, len1=20B, off1=128B
>     seg2 - pool2, len2=20B, off2=0B
>     seg3 - pool3, len3=512B, off3=0B


Sorry for chime in late. This API lookout looks good to me.
But, I am wondering how the application can know the capability or "limits" of
struct rte_eth_rxseg structure for the specific PMD. The other
descriptor limit, it's being exposed with struct
rte_eth_dev_info::rx_desc_lim;
If PMD can support a specific pattern rather than returning the
blanket error, the application should know the limit.
IMO, it is better to add
struct rte_eth_rxseg *rxsegs;
unint16_t nb_max_rxsegs
in rte_eth_dev_info structure to express the capablity.
Where the en and offset can define the max offset.

Thoughts?