[dpdk-dev] [PATCH v8 1/6] ethdev: introduce Rx buffer split

[Ferruh Yigit]

On 10/16/2020 8:48 AM, Viacheslav Ovsiienko 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 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.
> 
> The following structure is introduced to specify the Rx packet
> segment for RTE_ETH_RX_OFFLOAD_BUFFER_SPLIT offload:
> 
> struct rte_eth_rxseg_split {
> 
>      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 specify Rx buffer configuration:
> - mp is not NULL, rx_conf.rx_seg is NULL, rx_conf.rx_nseg is zero,
>    it is compatible configuration, 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.
> 
> f 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.
> 
> 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
> 
> The packet 46 bytes long will look like the following:
>      seg0 - 14B long @ RTE_PKTMBUF_HEADROOM + 2 in mbuf from pool0
>      seg1 - 20B long @ 128 in mbuf from pool1
>      seg2 - 12B long @ 0 in mbuf from pool2
> 
> The packet 1500 bytes long will look like the following:
>      seg0 - 14B @ RTE_PKTMBUF_HEADROOM + 2 in mbuf from pool0
>      seg1 - 20B @ 128 in mbuf from pool1
>      seg2 - 20B @ 0 in mbuf from pool2
>      seg3 - 512B @ 0 in mbuf from pool3
>      seg4 - 512B @ 0 in mbuf from pool3
>      seg5 - 422B @ 0 in mbuf from pool3
> 
> The offload RTE_ETH_RX_OFFLOAD_SCATTER must be present and
> configured to support new buffer split feature (if rx_nseg
> is greater than one).
> 
> The split limitations imposed by underlying PMD is reported
> in the new introduced rte_eth_dev_info->rx_seg_capa field.
> 
> 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 with preserving compatibility
> with existing API.
> 
> Signed-off-by: Viacheslav Ovsiienko <viacheslavo at nvidia.com>
> Acked-by: Ajit Khaparde <ajit.khaparde at broadcom.com>
> Acked-by: Jerin Jacob <jerinj at marvell.com>

<...>

> +/**
>    * A structure used to configure an RX ring of an Ethernet port.
>    */
>   struct rte_eth_rxconf {
> @@ -977,6 +998,46 @@ struct rte_eth_rxconf {
>   	uint16_t rx_free_thresh; /**< Drives the freeing of RX descriptors. */
>   	uint8_t rx_drop_en; /**< Drop packets if no descriptors are available. */
>   	uint8_t rx_deferred_start; /**< Do not start queue with rte_eth_dev_start(). */
> +	uint16_t rx_nseg; /**< Number of descriptions in rx_seg array. */
> +	/**
> +	 * Points to the array of segment descriptions. Each array element
> +	 * describes the properties for each segment in the receiving
> +	 * buffer according to feature descripting structure.
> +	 *
> +	 * The supported capabilities of receiving segmentation is reported
> +	 * in rte_eth_dev_info ->rx_seg_capa field.
> +	 *
> +	 * If RTE_ETH_RX_OFFLOAD_BUFFER_SPLIT flag is set in offloads field,
> +	 * 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 array element, the second buffer, from the
> +	 *   pool in the second element, and so on.
> +	 *
> +	 * - the offsets from the segment description elements specify
> +	 *   the data offset from the buffer beginning except the first mbuf.
> +	 *   For this one the offset is added with RTE_PKTMBUF_HEADROOM.
> +	 *
> +	 * - the lengths in the elements define the maximal data amount
> +	 *   being received to each segment. The receiving starts with filling
> +	 *   up the first mbuf data buffer up to specified length. If the
> +	 *   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 length, 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 not enough elements to describe the buffer for entire
> +	 *   packet of maximal length the following parameters will be used
> +	 *   for the all remaining segments:
> +	 *     - pool from the last valid element
> +	 *     - the buffer size from this pool
> +	 *     - zero offset
> +	 */
> +	struct rte_eth_rxseg *rx_seg;

"struct rte_eth_rxconf" is very commonly used, I think all applications does the 
'rte_eth_rx_queue_setup()', but "buffer split" is not a common usage,

I am against the "struct rte_eth_rxseg *rx_seg;" field creating this much noise 
in the "struct rte_eth_rxconf" documentation.
As mentioned before, can you please move the above detailed documentation to 
where "struct rte_eth_rxseg" defined, and in this struct put a single comment 
for "struct rte_eth_rxseg *rx_seg" ?