spi: make SpiDevice transaction take an operation slice instead of a closure.

[Dirbaio]

As discussed in today's WG meeting.

• Fixes https://github.com/rust-embedded/linux-embedded-hal/issues/87
• Gets rid of the cursed borrow issues in async.
• Makes the trait more consistent with I2C.

[ryankurte]

hey seems reasonable to me! a couple of questions:

• how do we represent busses with actual hardware CS, pass through a NullCS of some sort?
• can we test this with a PR against linux-embedded-hal as well as another hal and a driver?

[Dirbaio]

how do we represent busses with actual hardware CS, pass through a NullCS of some sort?

implementations with hardware CS would directly implement SpiDevice, not SpiBus. You dont take a "SpiBus with hardware CS" and then wrap it on an ExclusiveDevice with NullCS.

[Dirbaio]

can we test this with a PR against linux-embedded-hal as well as another hal and a driver?

• esp-idf-hal updated here https://github.com/esp-rs/esp-idf-hal/pull/224 (thanks @Vollbrecht !)
• embassy-nrf updated here https://github.com/embassy-rs/embassy/pull/1306
• nrf70 driver updated here https://github.com/embassy-rs/nrf70/pull/1 (WIP because waiting for embedded-hal and embassy, the SPI stuff is already working). Note that it's much nicer especially with async, it gets rid of the unsafe.
• I'm not familiar with the linux SPI API, hopefully someone else can do linux-embedded-hal.

@ryankurte @eldruin @therealprof could you please take a look? I'd really like to get this unblocked :)

[Dirbaio]

@eldruin: @Vollbrecht made an interesting comment about read_transaction but it seems only a mild HAL-impl-side inconvenience:

Yes, it's somewhat annoying but I think it's better than taking a &[Operation] and then asserting at runtime that it has only reads (or only writes). And I can't think of any other solution...

The duplication also shows up in #444 . It won't affect most HALs though, as they'll likely implement only SpiBus and let the user use embedded-hal-bus to get an SpiDevice. The only reasons a HAL would implement SpiDevice is for hardware CS (which is very annoying and the perf benefits are questionable in MCUs), or when the kernel handles CS for you (like in esp-idf or Linux).

@eldruin: It seems to me like this should be fine to implement on linux-embedded-hal but we should check on it.

Yeah I think so too. It has to convert from either &[Operation] or &[&[u8]] to the linux ioctl struct anyway...

[bors[bot]]

Build succeeded:

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• test (1.59.0, thumbv7m-none-eabi)
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• test (nightly, thumbv6m-none-eabi)
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• test (stable, thumbv6m-none-eabi)
• test (stable, thumbv7m-none-eabi)
• test (stable, x86_64-unknown-linux-gnu)