159 lines
6.6 KiB
ReStructuredText
159 lines
6.6 KiB
ReStructuredText
.. _net_l2_interface:
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L2 Layer Management
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###################
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.. contents::
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:local:
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:depth: 2
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Overview
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********
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The L2 stack is designed to hide the whole networking link-layer part
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and the related device drivers from the upper network stack. This is made
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through a :c:type:`struct net_if` declared in
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:zephyr_file:`include/net/net_if.h`.
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The upper layers are unaware of implementation details beyond the net_if
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object and the generic API provided by the L2 layer in
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:zephyr_file:`include/net/net_l2.h` as :c:type:`struct net_l2`.
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Only the L2 layer can talk to the device driver, linked to the net_if
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object. The L2 layer dictates the API provided by the device driver,
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specific for that device, and optimized for working together.
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Currently, there are L2 layers for :ref:`Ethernet <ethernet_interface>`,
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:ref:`IEEE 802.15.4 Soft-MAC <ieee802154_interface>`,
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:ref:`Bluetooth IPSP <bluetooth-ipsp-sample>`, :ref:`CANBUS <can_interface>`,
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:ref:`OpenThread <thread_protocol_interface>`, Wi-Fi, and a dummy layer
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example that can be used as a template for writing a new one.
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L2 layer API
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************
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In order to create an L2 layer, or a driver for a specific L2 layer,
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one needs to understand how the L3 layer interacts with it and
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how the L2 layer is supposed to behave.
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See also :ref:`network stack architecture <network_stack_architecture>` for
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more details. The generic L2 API has these functions:
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- ``recv()``: All device drivers, once they receive a packet which they put
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into a :c:type:`struct net_pkt`, will push this buffer to the network
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stack via :c:func:`net_recv_data()`. At this point, the network
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stack does not know what to do with it. Instead, it passes the
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buffer along to the L2 stack's ``recv()`` function for handling.
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The L2 stack does what it needs to do with the packet, for example, parsing
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the link layer header, or handling link-layer only packets. The ``recv()``
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function will return ``NET_DROP`` in case of an erroneous packet,
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``NET_OK`` if the packet was fully consumed by the L2, or ``NET_CONTINUE``
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if the network stack should then handle it.
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- ``send()``: Similar to receive function, the network stack will call this
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function to actually send a network packet. All relevant link-layer content
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will be generated and added by this function.
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The ``send()`` function returns the number of bytes sent, or a negative
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error code if there was a failure sending the network packet.
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- ``enable()``: This function is used to enable/disable traffic over a network
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interface. The function returns ``<0`` if error and ``>=0`` if no error.
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- ``get_flags()``: This function will return the capabilities of an L2 driver,
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for example whether the L2 supports multicast or promiscuous mode.
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Network Device drivers
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**********************
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Network device drivers fully follows Zephyr device driver model as a
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basis. Please refer to :ref:`device_drivers`.
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There are, however, two differences:
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- The driver_api pointer must point to a valid :c:type:`struct net_if_api`
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pointer.
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- The network device driver must use ``NET_DEVICE_INIT_INSTANCE()``
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or ``ETH_NET_DEVICE_INIT()`` for Ethernet devices. These
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macros will call the ``DEVICE_AND_API_INIT()`` macro, and also
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instantiate a unique :c:type:`struct net_if` related to the created
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device driver instance.
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Implementing a network device driver depends on the L2 stack it
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belongs to: :ref:`Ethernet <ethernet_interface>`,
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:ref:`IEEE 802.15.4 <ieee802154_interface>`, etc.
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In the next section, we will describe how a device driver should behave when
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receiving or sending a network packet. The rest is hardware dependent
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and is not detailed here.
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Ethernet device driver
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======================
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On reception, it is up to the device driver to fill-in the network packet with
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as many data buffers as required. The network packet itself is a
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:c:type:`struct net_pkt` and should be allocated through
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:c:func:`net_pkt_rx_alloc_with_buffer()`. Then all data buffers will be
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automatically allocated and filled by :c:func:`net_pkt_write()`.
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After all the network data has been received, the device driver needs to
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call :c:func:`net_recv_data()`. If that call fails, it will be up to the
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device driver to unreference the buffer via :c:func:`net_pkt_unref()`.
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On sending, the device driver send function will be called, and it is up to
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the device driver to send the network packet all at once, with all the buffers.
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Each Ethernet device driver will need, in the end, to call
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``ETH_NET_DEVICE_INIT()`` like this:
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.. code-block:: c
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ETH_NET_DEVICE_INIT(..., CONFIG_ETH_INIT_PRIORITY,
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&the_valid_net_if_api_instance, 1500);
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IEEE 802.15.4 device driver
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===========================
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Device drivers for IEEE 802.15.4 L2 work basically the same as for
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Ethernet. What has been described above, especially for ``recv()``, applies
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here as well. There are two specific differences however:
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- It requires a dedicated device driver API: :c:type:`struct
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ieee802154_radio_api`, which overloads :c:type:`struct
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net_if_api`. This is because 802.15.4 L2 needs more from the device
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driver than just ``send()`` and ``recv()`` functions. This dedicated API is
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declared in :zephyr_file:`include/net/ieee802154_radio.h`. Each and every
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IEEE 802.15.4 device driver must provide a valid pointer on such
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relevantly filled-in API structure.
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- Sending a packet is slightly different than in Ethernet. IEEE 802.15.4 sends
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relatively small frames, 127 bytes all inclusive: frame header,
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payload and frame checksum. Buffers are meant to fit such
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frame size limitation. But a buffer containing an IPv6/UDP packet
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might have more than one fragment. IEEE 802.15.4 drivers
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handle only one buffer at a time. This is why the :c:type:`struct
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ieee802154_radio_api` requires a tx function pointer which differs
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from the :c:type:`struct net_if_api` send function pointer.
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Instead, the IEEE 802.15.4 L2, provides a generic
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:c:func:`ieee802154_radio_send()` meant to be given as
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:c:type:`struct net_if` send function. It turn, the implementation
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of :c:func:`ieee802154_radio_send()` will ensure the same behavior:
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sending one buffer at a time through :c:type:`struct
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ieee802154_radio_api` tx function, and unreferencing the network packet
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only when all the transmission were successful.
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Each IEEE 802.15.4 device driver, in the end, will need to call
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``NET_DEVICE_INIT_INSTANCE()`` that way:
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.. code-block:: c
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NET_DEVICE_INIT_INSTANCE(...,
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the_device_init_prio,
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&the_valid_ieee802154_radio_api_instance,
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IEEE802154_L2,
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NET_L2_GET_CTX_TYPE(IEEE802154_L2), 125);
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API Reference
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*************
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.. doxygengroup:: net_l2
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:project: Zephyr
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