Re: [PATCH v5 02/20] rust: io: add missing safety requirement in `IoCapable` methods

[Alexandre Courbot]

On Fri Jul 3, 2026 at 10:49 PM JST, Gary Guo wrote:
> On Fri Jul 3, 2026 at 2:09 PM BST, Alexandre Courbot wrote:
>> On Fri Jul 3, 2026 at 9:35 PM JST, Gary Guo wrote:
>>> On Fri Jul 3, 2026 at 3:57 AM BST, Alexandre Courbot wrote:
>>>> On Fri Jun 26, 2026 at 11:45 PM JST, Gary Guo wrote:
>>>> <...>
>>>>> @@ -309,7 +312,11 @@ pub trait Io {
>>>>>      // Always inline to optimize out error path of `build_assert`.
>>>>>      #[inline(always)]
>>>>>      fn io_addr_assert<U>(&self, offset: usize) -> usize {
>>>>> -        build_assert!(offset_valid::<U>(offset, Self::Target::MIN_SIZE));
>>>>> +        // We cannot check alignment with `offset_valid` using `self.addr()`. So set 0 for it and
>>>>> +        // ensure alignment by checking that the alignment of `U` is smaller or equal to the
>>>>> +        // alignment of `Self::Target`.
>>>>> +        const_assert!(Alignment::of::<U>().as_usize() <= Self::Target::MIN_ALIGN.as_usize());
>>>>
>>>> With `Region::MIN_ALIGN` being `4`, my understanding is that this will
>>>> make `read64` and other infallible 64-bit accessors unusable on untyped
>>>> I/O regions?
>>>
>>> That's correct.
>>
>> Isn't that a limitation we may want to eventually address? The fallible
>> accessors are still usable, but it seems arbitrary that the non-fallible
>> ones stop at 32 bits...
>
> It'd probably be solved by having typed regions. 32-bit is the most common
> alignment for I/O memory, so that's chosen.
>
> None of the existing Rust drivers require 64-bit accessors yet.

Mmm I'd like to see whether we can't make it work for untyped regions as
well, but since we don't have many users (nor do we expect many to come
I guess) this can be a follow-up.

Reviewed-by: Alexandre Courbot <acourbot@xxxxxxxxxx>