ESP-IDF offers a good starting point:

int uart_write_bytes_with_break(uart_port_t uart_num, const void *src, size_t size, int brk_len)

But this only allows a BRK after the bytes. I am interested in using this for LIN bus - so a break before bytes is needed.

To keep it simple, I would love to see the function for this just be:

fn send_break(bit_length: u8) -> nb::Result<(), Error> { }

Eventually, async would be great as well.

For LIN - inverting the level of the TX line does a nice job. Accomplished this by using the PAC to set UART_TXD_INV to 1 for the break duration, then UART_TXD_INV back to 0.

We can add this after 1.0 in a non-breaking way. Looks like it should be a new API to send breaks, and @zpg6 has some draft PRs proving this.