Re: [PATCH v3 14/16] rust: io: register: remove relative registers

[Alexandre Courbot]

On Wed Aug 19, 2026 at 8:09 PM JST, Gary Guo wrote:
> Relative registers can be better served by projection to subregion instead
> of ad-hoc handling in register macro. Projection composes better (e.g. it
> natively allows relative registers of relative registers without needing
> additional support).
>
> Remove relative register support, and update the documentation to
> demonstrate how projection and subregions can be used to achieve this
> instead.
>
> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>

Reviewed-by: Alexandre Courbot <acourbot@xxxxxxxxxx>

And good riddance!

<...>
> @@ -763,115 +511,83 @@ pub const fn element_alias_offset<Base: ?Sized, Alias: RegisterArray<Base = Base
>  /// # }
>  /// ```
>  ///
> -/// ## Relative arrays of registers
> +/// ## Relative registers
>  ///
> -/// Combining the two features described in the sections above, arrays of registers accessible from
> -/// a base can also be defined:
> +/// There are cases where a register region is subdivided into small subregions, and you may wish to
> +/// have your register definition be relative to these subregions. This may be needed, for example,
> +/// if these subregions are instantiated several times, or you just want it for encapsulation
> +/// purpose.
>  ///
> -/// ```ignore
> -/// register! {
> -///     ...
> -///     pub RELATIVE_REGISTER_ARRAY(u8)[10, stride = 4] @ Base + 0x100 {
> -///         ...
> -///     }
> -/// }
> +/// For instance, imagine the following I/O space:
> +///
> +/// ```text
> +///           +-----------------------------+
> +///           |             ...             |
> +///           |                             |
> +///  0x100--->+------------CPU0-------------+
> +///           |                             |
> +///  0x110--->+-----------------------------+
> +///           |           CPU_CTL           |
> +///           +-----------------------------+
> +///           |             ...             |
> +///           |                             |
> +///           |                             |
> +///  0x200--->+------------CPU1-------------+
> +///           |                             |
> +///  0x210--->+-----------------------------+
> +///           |           CPU_CTL           |
> +///           +-----------------------------+
> +///           |             ...             |
> +///           +-----------------------------+
>  /// ```
>  ///
> -/// Like relative registers, they implement the [`WithBase`] trait. However the return value of
> -/// [`WithBase::of`] cannot be used directly as a location and must be further specified using the
> -/// [`at`](RelativeRegisterLoc::at) method.
> +/// `CPU0` and `CPU1` both have a `CPU_CTL` register that starts at offset `0x10` of their I/O
> +/// space segment. Since both instances of `CPU_CTL` share the same layout, we don't want to define
> +/// them twice and would prefer a way to select which one to use from a single definition.
> +///
> +/// This can be done define a new type for the subregion, and then define registers that use the new

nit: "done by defining ... and then defining registers"