imxrt-iomuxc

i.MX RT pad definitions and pin configuration.

Goals

• Lay the foundation for split i.MX RT hardware abstraction layers (HALs). The approach de-couples one of the most chip-specific implementations -- the chip's pads -- from common driver code.
• Provide a better interface for pad configuration, as described in this issue.
• Define a common interface for pad configuration that can be used by HAL drivers, and also by advanced drivers maintained outside of the HAL.

Structure

• imxrt-iomuxc defines a set of traits that are implemented on pads. The traits specify that a pad may be used for a certain function, such as a UART transfer pin or an I2C clock pin. The root crate also provides common functions to configure pads. imxrt-iomuxc is used in crates that need to treat pads as resources. Without any feature flags, the imxrt-iomuxc provides the general pin configuration interface. There are no processor-specific APIs in the default build.
• imxrt-iomuxc feature flags, like imxrt106x, enable processor-specific pad definitions and pin implementations.
• imxrt-iomuxc-build provides build-time support for defining pads. It's used to simply generate all of the pads. It also implements simple, common functionality across pads, like GPIO pin traits.

Users

i.MX RT HAL designers, or advanced driver designers, want to treat pads as resources. They want to create strongly-typed, infallible interfaces that ensure a pad supports a capability. These users depend on the traits defined by imxrt-iomuxc to create their driver interfaces. These drivers will accept pads across all i.MX RT chip variants. These designers do not enable any imxrt-iomuxc feature flags.

When users are ready to run their code on hardware, they enable a feature in the imxrt-iomuxc crate that describes their i.MX RT variant. Users who want to generalize their code for different i.MX RT variants enable more feature flags.

Comparison to Variant-Specific IOMUXC Crates

A previous approach separated the imxrt-iomuxc interface and implementations across crates. Rather than having an imxrt106x feature in a single imxrt-iomuxc crate, we had separate crates, named like imxrt106x-iomuxc, that implemented the imxrt-iomuxc interfaces. We decided to use feature flags after realizing it was not only easier to maintain, but also equivalent to the multi-crate approach. This section compares the maintenance and equivalence of the two approaches.

Easier to maintain

A single crate with feature flags is easier to maintain than an interface crate with separate implementation crates:

• The interface crate does not need to support "public but internal" APIs. We signaled these APIs behind #[doc(hidden)] attributes on public types. When using features, these APIs are truly private. The approach reduces the documentation burden, since we do not need to identify which APIs are truly public, and which APIs are internal.
• It's easier to release and version a single crate than multiple crates. We don't need to plan an approach for releasing separate interface and implementation crates. Users do not need to be concerned with our versioning and release strategy.
• It's easier to document and study. We may generate documentation for the interface and all implementations simply using cargo doc --all-features. It's easier to link documentation across the implementations and the interface.

Equivalence

A single crate with feature flags is equivalent to an interface crate and separate implementation crates. Consider a user who wants to use the IOMUXC pin configuration interfaces. That user would depend on imxrt-iomuxc, regardless of the approach.

Now, consider a user who wants to use their code on an i.MX RT 106x processor variant. Under the old approach, that user would include the imxrt106x-iomuxc crate, which includes the imxrt-iomuxc crate:

```toml [dependencies] imxrt106x-iomuxc = "0.1"

imxrt-iomuxc = "0.1" - implicit dependency

```

When using feature flags, the user enables the imxrt106x feature:

toml [dependencies] imxrt-iomuxc = { version = "0.1", features = ["imxrt106x"] }

Both approaches result in the same changes to the dependency graph: the graph now includes code for i.MX RT 106x processor pads.

Since features are additive, users who want to support more processors enable more feature flags. This would have translated to the user explicitly including more crates:

```toml [dependencies] imxrt-iomuxc = { version = "0.1", features = ["imxrt102x", "imxrt106x"] }

Equivalent:

[dependencies] imxrt102x-iomuxc = "0.1" imxrt106x-iomuxc = "0.1"

imxrt-iomuxc = "0.1" - implicit dependency

```

Depending on the release strategy, the user would need to maintain the version for all implementation crates. The feature-flag approach requires a single version, which may make it easier for the user.

Discussion

Since the approaches are equivalent, the change has no effect on a split i.MX RT HAL. An imxrt-hal[-common] crate would depend on the imxrt-iomuxc crate without feature flags. Then, a processor-specific HAL would depend on imxrt-iomuxc with the appropriate feature flag. We achive the goals of a split HAL, as users are unconcerned with feature flags.

We realize that this approach perpetuates the need for feature flags. However, the imxrt-iomuxc crate is a much lower-level interface than the HAL crates; it's equivalent to a RAL crate or PAC. The imxrt-iomuxc crate is intended for HAL developers, not HAL users. HAL developers cannot escape feature flags, since using RALs and PACs already necessitates feature flags. By adopting a single imxrt-iomuxc crate with feature flags, HAL developers continue to use feature flags to support the i.MX RT variants.

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.