Re: [PATCH 3/8] rust: io: use pointer types instead of address

["Gary Guo" <gary@garyguo.net>]

On Thu Mar 26, 2026 at 2:20 PM GMT, Andreas Hindborg wrote:
> "Gary Guo" <gary@kernel.org> writes:
>
>> From: Gary Guo <gary@garyguo.net>
>>
>> This carries the size information with the pointer type and metadata, makes
>> it possible to use I/O projections and paves the way for IO view types.
>>
>> With this change, minimum size information becomes available through types;
>> so `KnownSize::MIN_SIZE` can be used and `IoKnownSize` trait is no longer
>> necessary. The trait is kept for compatibility and can be removed when
>> users stop using it for bounds.
>>
>> PCI config space uses only offsets and not pointers like MMIO; for this
>> null pointers (with proper size metadata) is used. This is okay as I/O
>> trait impl and I/O projections can operate on invalid pointers, and for PCI
>> config space we will only use address info and ignore the provenance.
>>
>> The current safety comment on `io_read`/`io_write` does not cover the topic
>> about alignment, although this is guaranteed by checks in `Io`. Add it so
>> it can be relied on by implementor of `IoCapable`.
>>
>> Signed-off-by: Gary Guo <gary@garyguo.net>
>> ---
>
> <cut>
>
>> +impl<T: Io + ?Sized> IoKnownSize for T {}
>>
>>  /// Implements [`IoCapable`] on `$mmio` for `$ty` using `$read_fn` and `$write_fn`.
>>  macro_rules! impl_mmio_io_capable {
>>      ($mmio:ident, $(#[$attr:meta])* $ty:ty, $read_fn:ident, $write_fn:ident) => {
>>          $(#[$attr])*
>>          impl<T: ?Sized> IoCapable<$ty> for $mmio<T> {
>> -            unsafe fn io_read(&self, address: usize) -> $ty {
>> +            unsafe fn io_read(&self, address: *mut $ty) -> $ty {
>>                  // SAFETY: By the trait invariant `address` is a valid address for MMIO operations.
>>                  unsafe { bindings::$read_fn(address as *const c_void) }
>
> Should we change this to a `cast()`?

This is a `*mut T` -> `*const c_void` cast which needs
`cast_const().cast::<c_void>` which I think is less readable than just `as`.

It's also existing code that I haven't touched in this commit.

>
> Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
>
> Side question: Can you explain a situation where `IoLoc::offset` is not identity?

By identity, you mean the impl of `IoLoc for usize`? `IoLoc` is also implemented
for registers generated by `register!()` macro.

Best,
Gary