Zephyr Driver

Posted By admin On 31/01/22
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  3. Zephyr Express Delivery Driver

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Driver

The L2 stack is designed to hide the whole networking link-layer partand the related device drivers from the higher IP stack. This is madethrough a unique object known as the “network interface object”:structnet_if declared in include/net/net_if.h.

The IP layer is unaware of implementation details beyond the net_ifobject and the generic API provided by the L2 layer ininclude/net/net_l2.h as structnet_l2.

Only the L2 layer can talk to the device driver, linked to the net_ifobject. The L2 layer dictates the API provided by the device driver,specific for that device, and optimized for working together.

Currently, there are L2 layers for Ethernet, IEEE 802.15.4 Soft-MAC,Bluetooth IPSP, and a dummy one, which is a generic layer example thatcan be used as a template for writing a new one.

L2 layer API¶

In order to create an L2 layer, or even a driver for a specific L2layer, one needs to understand how the IP layer interacts with it andhow the L2 layer is supposed to behave. The generic L2 API has 3functions:

  • recv: All device drivers, once they receive a packet which they putinto a structnet_pkt, will push this buffer to the IPcore stack via net_recv_data(). At this point, the IP corestack does not know what to do with it. Instead, it passes thebuffer along to the L2 stack’s recv() function for handling. The L2stack does what it needs to do with the packet, for example, parsingthe link layer header, or handling link-layer only packets. Therecv() function will return NET_DROP in case of an erroneous packet,NET_OK if the packet was fully consumed by the L2, or NET_CONTINUEif the IP stack should then handle it as an IP packet.
  • reserve: Prior to creating any network buffer content, the Zephyrcore stack needs to know how much dedicated buffer space is neededfor the L2 layer (for example, space for the link layer header). Thisreserve function returns the number of bytes needed.
  • send: Similar to recv, the IP core stack will call this function toactually send a packet. All relevant link-layer content will begenerated and added by this function. As for recv, send returns averdict and can decide to drop the packet via NET_DROP if somethingwrong happened, or will return NET_OK.

Network Device drivers¶

Network device drivers fully follows Zephyr device driver model as abasis. Please refer to Device Drivers and Device Model.

There are, however, two differences:

  • the driver_api pointer must point to a valid structnet_if_api pointer.
  • The network device driver must use NET_DEVICE_INIT_INSTANCE(). Thismacro will call the DEVICE_AND_API_INIT() macro, and alsoinstantiate a unique structnet_if related to the createddevice driver instance.

Implementing a network device driver depends on the L2 stack itbelongs to: Ethernet, IEEE 802.15.4, etc. In the next section, we willdescribe how a device driver should behave when receiving or sending apacket. The rest is really hardware dependent and thus does not needto be detailed here.

Ethernet device driver¶

On reception, it is up to the device driver to fill-in the buffer withas many data fragments as required. The buffer itself is astructnet_pkt and should be allocated throughnet_pkt_get_reserve_rx(0)(). Then all fragments will beallocated through net_pkt_get_reserve_data(0)(). Of coursethe amount of required fragments depends on the size of the receivedpacket and on the size of a fragment, which is given byCONFIG_NET_BUF_DATA_SIZE.

Note that it is not up to the device driver to decide on thelink-layer space to be reserved in the buffer. Hence the 0 given asparameter here. The Ethernet L2 layer will update such informationonce the packet’s Ethernet header has been successfully parsed.

Zephyr

In case net_recv_data() call fails, it will be up to thedevice driver to unreference the buffer vianet_pkt_unref().

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On sending, it is up to the device driver to send the buffer all atonce, with all the fragments.

In case of a fully successful packet transmission only, the devicedriver must unreference the buffer via net_pkt_unref().

Each Ethernet device driver will need, in the end, to callNET_DEVICE_INIT_INSTANCE() like this:

IEEE 802.15.4 device driver¶

Device drivers for IEEE 802.15.4 L2 work basically the same as forEthernet. What has been described above, especially for recv, applieshere as well. There are two specific differences however:

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  • It requires a dedicated device driver API: structieee802154_radio_api, which overloads structnet_if_api. This is because 802.15.4 L2 needs more from the devicedriver than just send and recv functions. This dedicated API isdeclared in include/net/ieee802154_radio.h. Each and every IEEE802.15.4 device driver must provide a valid pointer on suchrelevantly filled-in API structure.
  • Sending a packet is slightly particular. IEEE 802.15.4 sendsrelatively small frames, 127 bytes all inclusive: frame header,payload and frame checksum. Buffer fragments are meant to fit suchframe size limitation. But a buffer containing an IPv6/UDP packetmight have more than one fragment. In the Ethernet device driver, itis up to the driver to handle all fragments. IEEE 802.15.4 drivershandle only one fragment at a time. This is why the structieee802154_radio_api requires a tx function pointer which differsfrom the structnet_if_api send function pointer.Instead, the IEEE 802.15.4 L2, provides a genericieee802154_radio_send() meant to be given asstructnet_if send function. It turn, the implementationof ieee802154_radio_send() will ensure the same behavior:sending one fragment at a time through structieee802154_radio_api tx function, and unreferencing the bufferonly when all the transmission were successful.

Zephyr Express Delivery Driver

Each IEEE 802.15.4 device driver, in the end, will need to callNET_DEVICE_INIT_INSTANCE() that way: