Unify `embedded_hal::spi` traits?

[madsmtm]

I'm unfamiliar with SPI, so maybe I'm completely off here, but it seems like the two main traits in embedded_hal::spi could be somewhat unified?

As an example, consider the API below.

trait Spi: ErrorType {
    fn read(&mut self, words: &mut [Word]) -> Result<(), Self::Error>;
    fn write(&mut self, words: &[Word]) -> Result<(), Self::Error>;
    fn transfer(&mut self, read: &mut [Word], write: &[Word]) -> Result<(), Self::Error>;
    fn transfer_in_place(&mut self, words: &mut [Word]) -> Result<(), Self::Error>;
    fn flush(&mut self) -> Result<(), Self::Error>;
}

trait SpiBus: Spi {}

trait SpiDevice: Spi {
    fn transaction(&mut self, operations: &mut [Operation<'_, Word>]) -> Result<(), Self::Error>;
}

impl<T: ?Sized + SpiDevice> Spi for T {
    fn read(&mut self, words: &mut [Word]) -> Result<(), Self::Error> {
        self.transaction(&mut [Operation::Read(words)])
    }
    fn write(&mut self, words: &[Word]) -> Result<(), Self::Error> {
        self.transaction(&mut [Operation::Write(words)])
    }
    fn transfer(&mut self, read: &mut [Word], write: &[Word]) -> Result<(), Self::Error> {
        self.transaction(&mut [Operation::Transfer(read, write)])
    }
    fn transfer_in_place(&mut self, words: &mut [Word]) -> Result<(), Self::Error> {
        self.transaction(&mut [Operation::TransferInPlace(words)])
    }
    fn flush(&mut self) -> Result<(), Self::Error> {
        // Noop for SPI devices, the flushing happens automatically inside the transaction
    }
}

Again, unsure if this is actually beneficial to driver / HAL authors or not?

[nyurik]

I just consolidated the Spi transaction (3 identical copies) - possibly related: #529

[Dirbaio]

I don't think this is a good idea. SpiBus and SpiDevice's write() methods have very different meaning, even if their signature is the same.

• SpiBus::write(): clock out the bytes, return possibly before it's finished
• SpiDevice::write(): assert CS, clock out the bytes, flush the bus to make sure it's indeed finished, deassert CS, return.

If they're in a shared trait Spi, a driver could do where T: Spi which would accept both SpiBus and SpiDevice objects. If the driver is expecting an SpiDevice (chip with CS), it'll break if the user passes a SpiBus, and vice versa. Also, there's no case where a driver would really want to be generic over SpiBus and SpiDevice: the chip either has CS or doesn't.

For the HALs it doesn't save code either, they don't implement SpiBus and SpiDevice on the same type at the same time for the same reason: it's either managing a CS pin or it's not.

[Tremoneck]

It would be useful to have the ability to use a bus as device. If the device has a CS pin, but it's permanent active by direct connection to GND or VCC. That would mean the driver implements device but it doesn't have a usable CS pin so for the user it would be a Bus.

So in the hal an explicit conversion from bus to device, that behaves like a device would be useful.

However here it was already discussed and the decision was against it. The argument was, that drivers depend on the CS toggling and some chips need the CS to actually toggle for correct functionality. However if this kind of error should be cought by the person designing the PCB not be the software developer.

[Dirbaio]

It would be useful to have the ability to use a bus as device. If the device has a CS pin, but it's permanent active by direct connection to GND or VCC.

You can still do it if you write a struct that wraps SpiBus and implements SpiDevice. Same as ExclusiveDevice in embedded-hal-bus, but without toggling the CS pin. This is an "incorrect" SpiDevice implementation because the SpiDevice contract says CS must toggle for each transaction, but you can still opt in to do it at your own risk.

I don't think we should allow silently using a SpiBus as SpiDevice. It doesn't enable new use cases (you can already do it with the current traits as I explained above), it just creates a footgun.