stm32/usart: impl {eio, eio_async}::Read for Uart, UartRx

[tcbennun]

Hopefully self-explanatory. 🙂

[Dirbaio]

The raw Uart can't impl embedded_io::Read for a few reasons:

• It is is unbuffered, this means it can only receive while the task is actively waiting on a .read().await call. Data that's received between read calls is lost. The embedded_io::Read contract requires that data is not so easily lost, drivers using it can assume that.
• It must return early if less than buf.len() bytes are received. Drivers can call read() with a buffer bigger than the data they're expecting (for example, if they don't know the size of the data in advance) and get back a partially filled buffer with the data that arrived and process that. With this impl, they'd hang instead.

if you want embedded_io::Read there's BufferedUartRx (irq-driven) or RingBufferedUartRx (dma-driven), which do extra fancy things to fulill the embedded-io contract.

[tcbennun]

Gotcha. Worth a shot.

Might look at impling the ring-buffered driver for GPDMA then