Tweak guidelines (#2482)

* Tweak guidelines

* Restore builder rule

* Address review comments

documentation/API-GUIDELINES.md

@@ -15,9 +15,21 @@ The following paragraphs contain additional recommendations.
 
 ## Construction and Destruction of Drivers
 
-- Drivers take peripherals and pins via the `PeripheralRef` pattern - they don't consume peripherals/pins.
+- Drivers should take peripherals via the `PeripheralRef` pattern - they don't consume peripherals directly.
+- If a driver requires pins, those pins should be configured using `fn with_signal_name(self, pin: impl Peripheral<P = InputConnection> + 'd) -> Self)` or `fn with_signal_name(self, pin: impl Peripheral<P = OutputConnection> + 'd) -> Self)`
+- If a driver supports multiple peripheral instances (for example, I2C0 is one such instance):
+  - The peripheral instance type should be positioned as the last type parameter of the driver type.
+  - The peripheral instance type should default to a type that supports any of the peripheral instances.
+  - The author is encouraged to use `crate::any_peripheral` to define the "any" peripheral instance type.
+  - The driver should implement a `new` constructor that automatically converts the peripheral instance into the any type, and a `new_typed` that preserves the peripheral type.
+- If a driver only supports a single peripheral instance, no instance type parameter is necessary.
+- If a driver implements both blocking and async operations, or only implements blocking operations, but may support asynchronous ones in the future, the driver's type signature should include a `crate::Mode` type parameter.
+- By default, constructors should configure the driver for blocking mode. The driver should implement `into_async` (and a matching `into_blocking`) function that reconfigures the driver.
+  - `into_async` should configure the driver and/or the associated DMA channels. This most often means enabling an interrupt handler.
+  - `into_blocking` should undo the configuration done by `into_async`.
+- The asynchronous driver implemntation should also expose the blocking methods (except for interrupt related functions).
 - Consider adding a `Drop` implementation resetting the peripheral to idle state.
-- Consider using a builder-like pattern for configuration which must be done during initialization.
+- Consider using a builder-like pattern for driver construction.
 
 ## Interoperability
 
@@ -25,6 +37,7 @@ The following paragraphs contain additional recommendations.
     - see [this example](https://github.com/esp-rs/esp-hal/blob/df2b7bd8472cc1d18db0d9441156575570f59bb3/esp-hal/src/spi/mod.rs#L15)
     - e.g. `#[cfg_attr(feature = "defmt", derive(defmt::Format))]`
 - Don't use `log::XXX!` macros directly - use the wrappers in `fmt.rs` (e.g. just `info!` instead of `log::info!` or importing `log::*`)!
+- Consider implementing common ecosystem traits, like the ones in `embedded-hal` or `embassy-embedded-hal`.
 
 ## API Surface
 
@@ -45,6 +58,9 @@ The following paragraphs contain additional recommendations.
     - For example starting a timer is fine for `&self`, worst case a timer will be started twice if two parts of the program call it. You can see a real example of this [here](https://github.com/esp-rs/esp-hal/pull/1500#pullrequestreview-2015911974)
 - Maintain order consistency in the API, such as in the case of pairs like RX/TX.
 - If your driver provides a way to listen for interrupts, the interrupts should be listed in a `derive(EnumSetType)` enum as opposed to one function per interrupt flag.
+- If a driver only implements a subset of a peripheral's capabilities, it should be placed in the `peripheral::subcategory` module.
+  - For example, if a driver implements the slave-mode I2C driver, it should be placed into `i2c::slave`.
+  - This helps us reducing the need of introducing breaking changes if we implement additional functionalities.
 
 ## Maintainability
 
@@ -56,6 +72,15 @@ The following paragraphs contain additional recommendations.
 - All `Future` objects (public or private) must be marked with ``#[must_use = "futures do nothing unless you `.await` or poll them"]``.
 - Prefer `cfg_if!` over multiple exclusive `#[cfg]` attributes. `cfg_if!` visually divides the options, often results in simpler conditions and simplifies adding new branches in the future.
 
+## Driver implementation
+
+- If a common `Instance` trait is used for multiple peripherals, those traits should not have any logic implemented in them.
+- The `Instance` traits should only be used to access information about a peripheral instance.
+- The internal implementation of the driver should be non-generic over the peripheral instance. This helps the compiler produce smaller code.
+- The author is encouraged to return a static shared reference to an `Info` and a `State` structure from the `Instance` trait.
+  - The `Info` struct should describe the peripheral. Do not use any interior mutability.
+  - The `State` struct should contain counters, wakers and other, mutable state. As this is accessed via a shared reference, interior mutability and atomic variables are preferred.
+
 ## Modules Documentation
 
 Modules should have the following documentation format:
@@ -88,4 +113,5 @@ Modules should have the following documentation format:
     ```
     #![doc = concat!("[ESP-IDF documentation](https://docs.espressif.com/projects/esp-idf/en/latest/", crate::soc::chip!(), "/api-reference/peripherals/etm.html)")]
     ```
-- Documentation examples should be short and basic, for more complex scenarios, create an example.
+  - In case of referencing a TRM chapter, use the `crate::trm_markdown_link!()` macro. If you are referring to a particular chapter, you may use `crate::trm_markdown_link!("#chapter_anchor")`.
+- Documentation examples should be short and basic. For more complex scenarios, create an example.