Subsequent calls to SpiBus::write corrupt previous writes without flushing

[markus-k]

Subsequent calls to SpiBus::write seem to corrupt previous (unfinished) writes without calling SpiBus::flash() inbetween.

While this is somewhat documented, it does not follow the requirements by embedded-hal:

When calling another method when a previous operation is still in progress, implementations can either wait for the previous operation to finish, or enqueue the new one, but they must not return a “busy” error. Users must be able to do multiple method calls in a row and have them executed “as if” they were done sequentially, without having to check for “busy” errors.

This gets problematic with embedded-hal-bus transactions, where operations are just called one after eachother and only .flush()ed in the end: https://github.com/rust-embedded/embedded-hal/blob/5e21d566e25049a85a74010e0c92708f732b2a7f/embedded-hal-bus/src/spi/shared.rs#L23-L36.

Therefore something like the following example results in corrupted data on the bus:

    let mut spi_led0 = RefCellDevice::new(&spi_bus, spi_cs0, delay.clone());
    let mut txs = [
        embedded_hal::spi::Operation::Write(&[0x80, 0x20]),
        // this works if we insert a delay op here, which internally calls flush():
        // embedded_hal::spi::Operation::DelayNs(1),
        embedded_hal::spi::Operation::Write(&[0xff, 0xff, 0xff, 0xff]),
    ];
    spi_led0.transaction(&mut txs).unwrap();

The simplest solutions I would come up with is to call self.flush() before doing anything in Instance::write_bytes, but I don't know the inner workings of the SPI driver well enough to be sure this doesn't have any unwanted side-affects.

[bjoernQ]

The simplest solutions I would come up with is to call self.flush() before doing anything in Instance::write_bytes, but I don't know the inner workings of the SPI driver well enough to be sure this doesn't have any unwanted side-affects.

I guess that should be the way to go. It probably shouldn't cause problems

[markus-k]

Fixed with #1381