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RX/TX Callbacks Sample Application
==================================

The RX/TX Callbacks sample application is a packet forwarding application that
demonstrates the use of user defined callbacks on received and transmitted
packets. The application performs a simple latency check, using callbacks, to
determine the time packets spend within the application.

In the sample application a user defined callback is applied to all received
packets to add a timestamp. A separate callback is applied to all packets
prior to transmission to calculate the elapsed time, in CPU cycles.


Compiling the Application
-------------------------

To compile the sample application see :doc:`compiling`.

The application is located in the ``rxtx_callbacks`` sub-directory.

The callbacks feature requires that the ``CONFIG_RTE_ETHDEV_RXTX_CALLBACKS``
setting is on in the ``config/common_`` config file that applies to the
target. This is generally on by default:

.. code-block:: console

    CONFIG_RTE_ETHDEV_RXTX_CALLBACKS=y

Running the Application
-----------------------

To run the example in a ``linuxapp`` environment:

.. code-block:: console

    ./build/rxtx_callbacks -l 1 -n 4

Refer to *DPDK Getting Started Guide* for general information on running
applications and the Environment Abstraction Layer (EAL) options.



Explanation
-----------

The ``rxtx_callbacks`` application is mainly a simple forwarding application
based on the :doc:`skeleton`. See that section of the documentation for more
details of the forwarding part of the application.

The sections below explain the additional RX/TX callback code.


The Main Function
~~~~~~~~~~~~~~~~~

The ``main()`` function performs the application initialization and calls the
execution threads for each lcore. This function is effectively identical to
the ``main()`` function explained in :doc:`skeleton`.

The ``lcore_main()`` function is also identical.

The main difference is in the user defined ``port_init()`` function where the
callbacks are added. This is explained in the next section:


The Port Initialization  Function
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The main functional part of the port initialization is shown below with
comments:

.. code-block:: c

    static inline int
    port_init(uint16_t port, struct rte_mempool *mbuf_pool)
    {
        struct rte_eth_conf port_conf = port_conf_default;
        const uint16_t rx_rings = 1, tx_rings = 1;
        struct ether_addr addr;
        int retval;
        uint16_t q;

        if (port >= rte_eth_dev_count())
            return -1;

        /* Configure the Ethernet device. */
        retval = rte_eth_dev_configure(port, rx_rings, tx_rings, &port_conf);
        if (retval != 0)
            return retval;

        /* Allocate and set up 1 RX queue per Ethernet port. */
        for (q = 0; q < rx_rings; q++) {
            retval = rte_eth_rx_queue_setup(port, q, RX_RING_SIZE,
                    rte_eth_dev_socket_id(port), NULL, mbuf_pool);
            if (retval < 0)
                return retval;
        }

        /* Allocate and set up 1 TX queue per Ethernet port. */
        for (q = 0; q < tx_rings; q++) {
            retval = rte_eth_tx_queue_setup(port, q, TX_RING_SIZE,
                    rte_eth_dev_socket_id(port), NULL);
            if (retval < 0)
                return retval;
        }

        /* Start the Ethernet port. */
        retval = rte_eth_dev_start(port);
        if (retval < 0)
            return retval;

        /* Enable RX in promiscuous mode for the Ethernet device. */
        rte_eth_promiscuous_enable(port);


        /* Add the callbacks for RX and TX.*/
        rte_eth_add_rx_callback(port, 0, add_timestamps, NULL);
        rte_eth_add_tx_callback(port, 0, calc_latency, NULL);

        return 0;
    }


The RX and TX callbacks are added to the ports/queues as function pointers:

.. code-block:: c

        rte_eth_add_rx_callback(port, 0, add_timestamps, NULL);
        rte_eth_add_tx_callback(port, 0, calc_latency,   NULL);

More than one callback can be added and additional information can be passed
to callback function pointers as a ``void*``. In the examples above ``NULL``
is used.

The ``add_timestamps()`` and ``calc_latency()`` functions are explained below.


The add_timestamps() Callback
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``add_timestamps()`` callback is added to the RX port and is applied to
all packets received:

.. code-block:: c

    static uint16_t
    add_timestamps(uint16_t port __rte_unused, uint16_t qidx __rte_unused,
            struct rte_mbuf **pkts, uint16_t nb_pkts, void *_ __rte_unused)
    {
        unsigned i;
        uint64_t now = rte_rdtsc();

        for (i = 0; i < nb_pkts; i++)
            pkts[i]->udata64 = now;

        return nb_pkts;
    }

The DPDK function ``rte_rdtsc()`` is used to add a cycle count timestamp to
each packet (see the *cycles* section of the *DPDK API Documentation* for
details).


The calc_latency() Callback
~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``calc_latency()`` callback is added to the TX port and is applied to all
packets prior to transmission:

.. code-block:: c

    static uint16_t
    calc_latency(uint16_t port __rte_unused, uint16_t qidx __rte_unused,
            struct rte_mbuf **pkts, uint16_t nb_pkts, void *_ __rte_unused)
    {
        uint64_t cycles = 0;
        uint64_t now = rte_rdtsc();
        unsigned i;

        for (i = 0; i < nb_pkts; i++)
            cycles += now - pkts[i]->udata64;

        latency_numbers.total_cycles += cycles;
        latency_numbers.total_pkts   += nb_pkts;

        if (latency_numbers.total_pkts > (100 * 1000 * 1000ULL)) {
            printf("Latency = %"PRIu64" cycles\n",
                    latency_numbers.total_cycles / latency_numbers.total_pkts);

            latency_numbers.total_cycles = latency_numbers.total_pkts = 0;
        }

        return nb_pkts;
    }

The ``calc_latency()`` function accumulates the total number of packets and
the total number of cycles used. Once more than 100 million packets have been
transmitted the average cycle count per packet is printed out and the counters
are reset.
