ESP-NETIF Custom I/O Driver
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
As shown in the diagram, the following three API functions for the packet data path must be defined for connecting with esp-netif:
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 consequence 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 behaviour 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 IDF provides several network stack configurations for the most common network interfaces, such as for the WiFi 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
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 (if supported)
Note that it is not supported to acquire PPP netif impl pointer and this function will return NULL for esp_netif instances configured to PPP mode
- 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