Espressif ESP32-S2 User Manual

Chapter 2. API Reference
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:
• 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 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:
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Espressif ESP32-S2 Specifications

General

Brand	Espressif
Model	ESP32-S2
Category	Single board computers
Language	English