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# ESP-NETIF Custom I/O Driver

[中文]

This section outlines implementing a new I/O driver with ESP-NETIF connection capabilities.

By convention, the I/O driver has to register itself as an ESP-NETIF driver, and thus holds a dependency on ESP-NETIF component and is responsible for providing data path functions, post-attach callback and in most cases, also default event handlers to define network interface actions based on driver's lifecycle transitions.

## Packet Input/Output

According to the diagram shown in the ESP-NETIF Architecture part, the following three API functions for the packet data path must be defined for connecting with ESP-NETIF:

- `esp_netif_transmit()`

- `esp_netif_free_rx_buffer()`

- `esp_netif_receive()`

The first two functions for transmitting and freeing the rx buffer are provided as callbacks, i.e., they get called from ESP-NETIF (and its underlying TCP/IP stack) and I/O driver provides their implementation.

The receiving function on the other hand gets called from the I/O driver, so that the driver's code simply calls `esp_netif_receive()` on a new data received event.

## Post Attach Callback

A final part of the network interface initialization consists of attaching the ESP-NETIF instance to the I/O driver, by means of calling the following API:

    esp_err_t esp_netif_attach(esp_netif_t *esp_netif, esp_netif_iodriver_handle driver_handle);

It is assumed that the `esp_netif_iodriver_handle` is a pointer to driver's object, a struct derived from `struct esp_netif_driver_base_s`, so that the first member of I/O driver structure must be this base structure with pointers to:

- post-attach function callback

- related ESP-NETIF instance

As a result, the I/O driver has to create an instance of the struct per below:

    typedef struct my_netif_driver_s {
            esp_netif_driver_base_t base;           /*!< base structure reserved as esp-netif driver */
            driver_impl             *h;             /*!< handle of driver implementation */
        } my_netif_driver_t;

with actual values of `my_netif_driver_t::base.post_attach` and the actual drivers handle `my_netif_driver_t::h`.

So when the `esp_netif_attach()` gets called from the initialization code, the post-attach callback from I/O driver's code gets executed to mutually register callbacks between ESP-NETIF and I/O driver instances. Typically the driver is started as well in the post-attach callback. An example of a simple post-attach callback is outlined below:

    static esp_err_t my_post_attach_start(esp_netif_t * esp_netif, void * args)
    {
        my_netif_driver_t *driver = args;
        const esp_netif_driver_ifconfig_t driver_ifconfig = {
                .driver_free_rx_buffer = my_free_rx_buf,
                .transmit = my_transmit,
                .handle = driver->driver_impl
        };
        driver->base.netif = esp_netif;
        ESP_ERROR_CHECK(esp_netif_set_driver_config(esp_netif, &driver_ifconfig));
        my_driver_start(driver->driver_impl);
        return ESP_OK;
    }

## Default Handlers

I/O drivers also typically provide default definitions of lifecycle behavior of related network interfaces based on state transitions of I/O drivers. For example driver start `->` network start, etc.

An example of such a default handler is provided below:

    esp_err_t my_driver_netif_set_default_handlers(my_netif_driver_t *driver, esp_netif_t * esp_netif)
    {
        driver_set_event_handler(driver->driver_impl, esp_netif_action_start, MY_DRV_EVENT_START, esp_netif);
        driver_set_event_handler(driver->driver_impl, esp_netif_action_stop, MY_DRV_EVENT_STOP, esp_netif);
        return ESP_OK;
    }

### Network Stack Connection

The packet data path functions for transmitting and freeing the rx buffer (defined in the I/O driver) are called from the ESP-NETIF, specifically from its TCP/IP stack connecting layer.

Note that ESP-IDF provides several network stack configurations for the most common network interfaces, such as for the Wi-Fi station or Ethernet. These configurations are defined in esp_netif/include/esp_netif_defaults.h and should be sufficient for most network drivers. In rare cases, expert users might want to define custom lwIP based interface layers; it is possible, but an explicit dependency to lwIP needs to be set.

The following API reference outlines these network stack interaction with the ESP-NETIF:

## Header File

- components/esp_netif/include/esp_netif_net_stack.h

- This header file can be included with:

  >     #include "esp_netif_net_stack.h"

- This header file is a part of the API provided by the `esp_netif` component. To declare that your component depends on `esp_netif`, add the following to your CMakeLists.txt:

  >     REQUIRES esp_netif

  > or

  >     PRIV_REQUIRES esp_netif

## Functions

esp_netif_t *esp_netif_get_handle_from_netif_impl(void *dev)

  Returns esp-netif handle.

  Parameters:

    dev -- [in] opaque ptr to network interface of specific TCP/IP stack

  Returns:

    handle to related esp-netif instance

void *esp_netif_get_netif_impl(esp_netif_t *esp_netif)

  Returns network stack specific implementation handle.

  Parameters:

    esp_netif -- [in] Handle to esp-netif instance

  Returns:

    handle to related network stack netif handle

esp_err_t esp_netif_set_link_speed(esp_netif_t *esp_netif, uint32_t speed)

  Set link-speed for the specified network interface.

  Parameters:

    - esp_netif -- [in] Handle to esp-netif instance

    - speed -- [in] Link speed in bit/s

  Returns:

    ESP_OK on success

esp_err_t esp_netif_transmit(esp_netif_t *esp_netif, void *data, size_t len)

  Outputs packets from the TCP/IP stack to the media to be transmitted.

  This function gets called from network stack to output packets to IO driver.

  Parameters:

    - esp_netif -- [in] Handle to esp-netif instance

    - data -- [in] Data to be transmitted

    - len -- [in] Length of the data frame

  Returns:

    ESP_OK on success, an error passed from the I/O driver otherwise

esp_err_t esp_netif_transmit_wrap(esp_netif_t *esp_netif, void *data, size_t len, void *netstack_buf)

  Outputs packets from the TCP/IP stack to the media to be transmitted.

  This function gets called from network stack to output packets to IO driver.

  Parameters:

    - esp_netif -- [in] Handle to esp-netif instance

    - data -- [in] Data to be transmitted

    - len -- [in] Length of the data frame

    - netstack_buf -- [in] net stack buffer

  Returns:

    ESP_OK on success, an error passed from the I/O driver otherwise

void esp_netif_free_rx_buffer(void *esp_netif, void *buffer)

  Free the rx buffer allocated by the media driver.

  This function gets called from network stack when the rx buffer to be freed in IO driver context, i.e. to deallocate a buffer owned by io driver (when data packets were passed to higher levels to avoid copying)

  Parameters:

    - esp_netif -- [in] Handle to esp-netif instance

    - buffer -- [in] Rx buffer pointer

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