38. Event Ethernet Rx Adapter Library

The DPDK Eventdev API allows the application to use an event driven programming model for packet processing. In this model, the application polls an event device port for receiving events that reference packets instead of polling Rx queues of ethdev ports. Packet transfer between ethdev and the event device can be supported in hardware or require a software thread to receive packets from the ethdev port using ethdev poll mode APIs and enqueue these as events to the event device using the eventdev API. Both transfer mechanisms may be present on the same platform depending on the particular combination of the ethdev and the event device. For SW based packet transfer, if the mbuf does not have a timestamp set, the adapter adds a timestamp to the mbuf using rte_get_tsc_cycles(), this provides a more accurate timestamp as compared to if the application were to set the timestamp since it avoids event device schedule latency.

The Event Ethernet Rx Adapter library is intended for the application code to configure both transfer mechanisms using a common API. A capability API allows the eventdev PMD to advertise features supported for a given ethdev and allows the application to perform configuration as per supported features.

38.1. API Walk-through

This section will introduce the reader to the adapter API. The application has to first instantiate an adapter which is associated with a single eventdev, next the adapter instance is configured with Rx queues that are either polled by a SW thread or linked using hardware support. Finally the adapter is started.

For SW based packet transfers from ethdev to eventdev, the adapter uses a DPDK service function and the application is also required to assign a core to the service function.

38.1.1. Creating an Adapter Instance

An adapter instance is created using rte_event_eth_rx_adapter_create(). This function is passed the event device to be associated with the adapter and port configuration for the adapter to setup an event port if the adapter needs to use a service function.

int err;
uint8_t dev_id;
struct rte_event_dev_info dev_info;
struct rte_event_port_conf rx_p_conf;

err = rte_event_dev_info_get(id, &dev_info);

rx_p_conf.new_event_threshold = dev_info.max_num_events;
rx_p_conf.dequeue_depth = dev_info.max_event_port_dequeue_depth;
rx_p_conf.enqueue_depth = dev_info.max_event_port_enqueue_depth;
err = rte_event_eth_rx_adapter_create(id, dev_id, &rx_p_conf);

If the application desires to have finer control of eventdev port allocation and setup, it can use the rte_event_eth_rx_adapter_create_ext() function. The rte_event_eth_rx_adapter_create_ext() function is passed a callback function. The callback function is invoked if the adapter needs to use a service function and needs to create an event port for it. The callback is expected to fill the struct rte_event_eth_rx_adapter_conf structure passed to it.

38.1.2. Adding Rx Queues to the Adapter Instance

Ethdev Rx queues are added to the instance using the rte_event_eth_rx_adapter_queue_add() function. Configuration for the Rx queue is passed in using a struct rte_event_eth_rx_adapter_queue_conf parameter. Event information for packets from this Rx queue is encoded in the ev field of struct rte_event_eth_rx_adapter_queue_conf. The servicing_weight member of the struct rte_event_eth_rx_adapter_queue_conf is the relative polling frequency of the Rx queue and is applicable when the adapter uses a service core function.

ev.queue_id = 0;
ev.sched_type = RTE_SCHED_TYPE_ATOMIC;
ev.priority = 0;

queue_config.rx_queue_flags = 0;
queue_config.ev = ev;
queue_config.servicing_weight = 1;

err = rte_event_eth_rx_adapter_queue_add(id,
                                        eth_dev_id,
                                        0, &queue_config);

38.1.3. Querying Adapter Capabilities

The rte_event_eth_rx_adapter_caps_get() function allows the application to query the adapter capabilities for an eventdev and ethdev combination. For e.g, if the RTE_EVENT_ETH_RX_ADAPTER_CAP_OVERRIDE_FLOW_ID is set, the application can override the adapter generated flow ID in the event using rx_queue_flags field in struct rte_event_eth_rx_adapter_queue_conf which is passed as a parameter to the rte_event_eth_rx_adapter_queue_add() function.

err = rte_event_eth_rx_adapter_caps_get(dev_id, eth_dev_id, &cap);

queue_config.rx_queue_flags = 0;
if (cap & RTE_EVENT_ETH_RX_ADAPTER_CAP_OVERRIDE_FLOW_ID) {
        ev.flow_id = 1;
        queue_config.rx_queue_flags =
                RTE_EVENT_ETH_RX_ADAPTER_QUEUE_FLOW_ID_VALID;
}

38.1.4. Configuring the Service Function

If the adapter uses a service function, the application is required to assign a service core to the service function as show below.

uint32_t service_id;

if (rte_event_eth_rx_adapter_service_id_get(0, &service_id) == 0)
        rte_service_map_lcore_set(service_id, RX_CORE_ID);

38.1.5. Starting the Adapter Instance

The application calls rte_event_eth_rx_adapter_start() to start the adapter. This function calls the start callbacks of the eventdev PMDs for hardware based eventdev-ethdev connections and rte_service_run_state_set() to enable the service function if one exists.

Note

The eventdev to which the event_eth_rx_adapter is connected needs to be started before calling rte_event_eth_rx_adapter_start().

38.1.6. Getting Adapter Statistics

The rte_event_eth_rx_adapter_stats_get() function reports counters defined in struct rte_event_eth_rx_adapter_stats. The received packet and enqueued event counts are a sum of the counts from the eventdev PMD callbacks if the callback is supported, and the counts maintained by the service function, if one exists. The service function also maintains a count of cycles for which it was not able to enqueue to the event device.

38.1.7. Interrupt Based Rx Queues

The service core function is typically set up to poll ethernet Rx queues for packets. Certain queues may have low packet rates and it would be more efficient to enable the Rx queue interrupt and read packets after receiving the interrupt.

The servicing_weight member of struct rte_event_eth_rx_adapter_queue_conf is applicable when the adapter uses a service core function. The application has to enable Rx queue interrupts when configuring the ethernet device using the rte_eth_dev_configure() function and then use a servicing_weight of zero when adding the Rx queue to the adapter.

The adapter creates a thread blocked on the interrupt, on an interrupt this thread enqueues the port id and the queue id to a ring buffer. The adapter service function dequeues the port id and queue id from the ring buffer, invokes the rte_eth_rx_burst() to receive packets on the queue and converts the received packets to events in the same manner as packets received on a polled Rx queue. The interrupt thread is affinitized to the same CPUs as the lcores of the Rx adapter service function, if the Rx adapter service function has not been mapped to any lcores, the interrupt thread is mapped to the master lcore.

38.1.8. Rx Callback for SW Rx Adapter

For SW based packet transfers, i.e., when the RTE_EVENT_ETH_RX_ADAPTER_CAP_INTERNAL_PORT is not set in the adapter’s capabilities flags for a particular ethernet device, the service function temporarily enqueues mbufs to an event buffer before batch enqueuing these to the event device. If the buffer fills up, the service function stops dequeuing packets from the ethernet device. The application may want to monitor the buffer fill level and instruct the service function to selectively enqueue packets to the event device. The application may also use some other criteria to decide which packets should enter the event device even when the event buffer fill level is low. The rte_event_eth_rx_adapter_cb_register() function allow the application to register a callback that selects which packets to enqueue to the event device.