Crate embassy_net_driver_channel
source ·Expand description
§embassy-net-driver-channel
This crate provides a toolkit for implementing embassy-net
drivers in a
higher level way than implementing the embassy-net-driver
trait directly.
The embassy-net-driver
trait is polling-based. To implement it, you must write the packet receive/transmit state machines by
hand, and hook up the Waker
s provided by embassy-net
to the right interrupt handlers so that embassy-net
knows when to poll your driver again to make more progress.
With embassy-net-driver-channel
you get a “channel-like” interface instead, where you can send/receive packets
to/from embassy-net. The intended usage is to spawn a “driver task” in the background that does this, passing
packets between the hardware and the channel.
§A note about deadlocks
When implementing a driver using this crate, it might be tempting to write it in the most straightforward way:
loop {
// Wait for either..
match select(
// ... the chip signaling an interrupt, indicating a packet is available to receive, or
irq_pin.wait_for_low(),
// ... a TX buffer becoming available, i.e. embassy-net wants to send a packet
tx_chan.tx_buf(),
).await {
Either::First(_) => {
// a packet is ready to be received!
let buf = rx_chan.rx_buf().await; // allocate a rx buf from the packet queue
let n = receive_packet_over_spi(buf).await;
rx_chan.rx_done(n);
}
Either::Second(buf) => {
// a packet is ready to be sent!
send_packet_over_spi(buf).await;
tx_chan.tx_done();
}
}
}
However, this code has a latent deadlock bug. The symptom is it can hang at rx_chan.rx_buf().await
under load.
The reason is that, under load, both the TX and RX queues can get full at the same time. When this happens, the embassy-net
task stalls trying to send because the TX queue is full, therefore it stops processing packets in the RX queue. Your driver task also stalls because the RX queue is full, therefore it stops processing packets in the TX queue.
The fix is to make sure to always service the TX queue while you’re waiting for space to become available in the RX queue. For example, select on either “tx_chan.tx_buf() available” or “INT is low AND rx_chan.rx_buf() available”:
loop {
// Wait for either..
match select(
async {
// ... the chip signaling an interrupt, indicating a packet is available to receive
irq_pin.wait_for_low().await;
// *AND* the buffer is ready...
rx_chan.rx_buf().await
},
// ... or a TX buffer becoming available, i.e. embassy-net wants to send a packet
tx_chan.tx_buf(),
).await {
Either::First(buf) => {
// a packet is ready to be received!
let n = receive_packet_over_spi(buf).await;
rx_chan.rx_done(n);
}
Either::Second(buf) => {
// a packet is ready to be sent!
send_packet_over_spi(buf).await;
tx_chan.tx_done();
}
}
}
§Examples
These embassy-net
drivers are implemented using this crate. You can look at them for inspiration.
cyw43
for WiFi on CYW43xx chips, used in the Raspberry Pi Pico Wembassy-usb
for Ethernet-over-USB (CDC NCM) support.embassy-net-wiznet
for Wiznet SPI Ethernet MAC+PHY chips.embassy-net-esp-hosted
for using ESP32 chips with theesp-hosted
firmware as WiFi adapters for another non-ESP32 MCU.
§Interoperability
This crate can run on any executor.
Re-exports§
pub use embassy_net_driver as driver;
Structs§
- Channel device.
- Represents a packet of size MTU.
- Channel runner.
- RX runner.
- A rx token.
- Channel state.
- State runner.
- TX runner.
- A tx token.
Functions§
- Create a channel.