embedded-hal-bus no longer builds with target thumbv6m-none-eabi

[Tschrock]

Added embedded-hal-bus = "0.2.0" to my project and it failed to build.
Using target thumbv7m-none-eabi works, using target thumbv6m-none-eabi fails. Version 0.1.0 works.

Trying to build the latest master branch:

./embedded-hal-bus> cargo build --target thumbv6m-none-eabi
   Compiling embedded-hal v1.0.0 (/home/cyber/Documents/VSCode/embedded-hal/embedded-hal)
   Compiling portable-atomic v1.6.0
   Compiling embedded-io-adapters v0.6.1 (/home/cyber/Documents/VSCode/embedded-hal/embedded-io-adapters)
   Compiling embedded-io-async v0.6.1 (/home/cyber/Documents/VSCode/embedded-hal/embedded-io-async)
   Compiling embedded-hal-async v1.0.0 (/home/cyber/Documents/VSCode/embedded-hal/embedded-hal-async)
   Compiling nb v1.1.0
   Compiling embedded-io v0.6.1 (/home/cyber/Documents/VSCode/embedded-hal/embedded-io)
   Compiling embedded-hal-bus v0.2.0 (/home/cyber/Documents/VSCode/embedded-hal/embedded-hal-bus)
   Compiling critical-section v1.1.2
   Compiling embedded-can v0.4.1 (/home/cyber/Documents/VSCode/embedded-hal/embedded-can)
   Compiling embedded-hal-nb v1.0.0 (/home/cyber/Documents/VSCode/embedded-hal/embedded-hal-nb)
error[E0599]: no method named `compare_exchange` found for struct `portable_atomic::AtomicBool` in the current scope
   --> embedded-hal-bus/src/i2c/atomic.rs:122:14
    |
120 | /         self.bus
121 | |             .busy
122 | |             .compare_exchange(
    | |             -^^^^^^^^^^^^^^^^ method not found in `AtomicBool`
    | |_____________|
    | 

error[E0599]: no method named `compare_exchange` found for struct `portable_atomic::AtomicBool` in the current scope
   --> embedded-hal-bus/src/spi/atomic.rs:127:14
    |
125 | /         self.bus
126 | |             .busy
127 | |             .compare_exchange(
    | |             -^^^^^^^^^^^^^^^^ method not found in `AtomicBool`
    | |_____________|
    | 

For more information about this error, try `rustc --explain E0599`.
error: could not compile `embedded-hal-bus` (lib) due to 2 previous errors

Seems to be caused by #593

embedded-hal/embedded-hal-bus/src/spi/atomic.rs

Lines 125 to 133 in a0ccb65

	self.bus
	.busy
	.compare_exchange(
	false,
	true,
	core::sync::atomic::Ordering::SeqCst,
	core::sync::atomic::Ordering::SeqCst,
	)
	.map_err(|_| AtomicError::Busy)?;

embedded-hal/embedded-hal-bus/src/i2c/atomic.rs

Lines 120 to 128 in a0ccb65

	self.bus
	.busy
	.compare_exchange(
	false,
	true,
	core::sync::atomic::Ordering::SeqCst,
	core::sync::atomic::Ordering::SeqCst,
	)
	.map_err(|_| AtomicError::<T::Error>::Busy)?;

ARMv6-M does not have atomic CAS, so portable_atomic does not implement AtomicBool::compare_exchange on this target.

[Rahix]

I guess the most straight forward solution to fix this regression would be to add a feature-flag for AtomicDevice. We did the same thing back in shared-bus to cater for platforms without the necessary atomics support.

Another solution would be to allow using AtomicDevice on single-core microcontrollers by use of critical sections instead of real atomics. Not sure how best to expose this API-wise, though.

[Rahix]

Also, maybe a lesson learned from this regression could be that we need to set up CI infrastructure in embedded-hal to check builds on all relevant targets? There may be more subtle incompatibilities between platforms that could hit in this way, I think.

[dan-corneanu]

I have stumbled upon this issue right now. Is there any workaround for the time being?

[adamgreig]

Because AtomicDevice uses the portable-atomic crate, you can enable support for CAS on targets that don't normally have it with either the critical-section or unsafe-assume-single-core features: https://crates.io/crates/portable-atomic#optional-features-critical-section

Having to add an explicit dependency on portable-atomic and enable the feature just to get this crate to build isn't great, but it should work as a workaround now.

[adamgreig]

We discussed this in today's meeting and the plan is:

• Add a new atomic-device feature, default off
• Only enable AtomicDevice when either the atomic-device feature is enabled or cfg(target_has_atomic...) is set, so it's by default available on platforms with CAS
• Add new portable-atomic-critical-section and portable-atomic-unsafe-assume-single-core features which proxy to the corresponding portable-atomic features (so users don't have to depend on portable-atomic directly) and enable our atomic-device feature
• Document these new features
• Release as 0.3