Re: [PATCH v4 07/20] rust: io: implement `Mmio` as view type

[Gary Guo]

On Mon Jun 15, 2026 at 3:52 PM BST, Alexandre Courbot wrote:
> On Fri Jun 12, 2026 at 1:28 AM JST, Gary Guo wrote:
> <...>
>> +/// A view of memory-mapped I/O region.
>> +///
>> +/// # Invariant
>> +///
>> +/// `ptr` points to a valid and aligned memory-mapped I/O region for the duration lifetime `'a`.
>> +pub struct Mmio<'a, T: ?Sized> {
>> +    ptr: *mut T,
>> +    phantom: PhantomData<&'a ()>,
>> +}
>> +
>> +impl<T: ?Sized> Copy for Mmio<'_, T> {}
>> +impl<T: ?Sized> Clone for Mmio<'_, T> {
>> +    #[inline]
>> +    fn clone(&self) -> Self {
>> +        *self
>> +    }
>> +}
>> +
>> +impl<'a, T: ?Sized> Mmio<'a, T> {
>> +    /// Create a `Mmio`, providing the accessors to the MMIO mapping.
>> +    ///
>> +    /// # Safety
>> +    ///
>> +    /// `raw` represents an valid and aligned memory-mapped I/O region while `'a` is alive.
>
> typo: "a valid".
>
> <...>
>> -/// [`MmioOwned`] wrapper using relaxed accessors.
>> +/// [`Mmio`] but using relaxed accessors.
>>  ///
>>  /// This type provides an implementation of [`Io`] that uses relaxed I/O MMIO operands instead of
>>  /// the regular ones.
>>  ///
>> -/// See [`MmioOwned::relaxed`] for a usage example.
>> -#[repr(transparent)]
>> -pub struct RelaxedMmio<const SIZE: usize = 0>(MmioOwned<SIZE>);
>> +/// See [`Mmio::relaxed`] for a usage example.
>> +///
>> +/// # Invariant
>> +///
>> +/// `ptr` points to a valid and aligned memory-mapped I/O region for the duration lifetime `'a`.
>> +pub struct RelaxedMmio<'a, T: ?Sized> {
>> +    ptr: *mut T,
>> +    phantom: PhantomData<&'a ()>,
>> +}
>
> Is there a reason for not just declaring `RelaxedMmio` as
>
>     #[repr(transparent)]
>     pub struct RelaxedMmio<'a, T: ?Sized>(Mmio<'a, T>);
>
> similarly to what the original code did with `MmioOwned`?
>
> I tried locally and could build. This avoids declaring `Mmio` and
> `RelaxedMmio` as basically identical types, and lets us remove the
> explicit `Send` and `Sync` implementations. IIUC you can also reduce or
> even remove the invariant section as it is enforced by `Mmio`.

This is what I did originally, but this would cause duplication for
RelaxedMmioBackend, as now you have to do

     unsafe { bindings::$read_fn(view.0.ptr.cast_const().cast()) }

and the `.0` causes the macro not being shared with MmioBackend.

Best,
Gary