Re: [PATCH v6 07/20] rust: io: implement `Mmio` as view type

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Infallible 64-bit I/O accessors (`read64`, `write64`) and typed 64-bit registers cannot be used on MMIO regions, and custom view types are unsupported by standard offsets.
--

commit 09225b49c257719774237bd1b25407607dd9df87
Author: Gary Guo <gary@garyguo.net>

rust: io: implement Mmio as view type

Implement Mmio as view type and convert RelaxedMmio to view type as
well. I/O implementations of MmioOwned are changed to delegate to the
Mmio view type.

> diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
> index f93be7f780694..8110b49aa4307 100644
> --- a/rust/kernel/io.rs
> +++ b/rust/kernel/io.rs
[ ... ]
> @@ -847,17 +915,12 @@ unsafe fn io_write(self, value: $ty, address: usize) {
>  }
>  
>  // MMIO regions support 8, 16, and 32-bit accesses.
> -impl_mmio_io_capable!(MmioOwned, u8, readb, writeb);
> -impl_mmio_io_capable!(MmioOwned, u16, readw, writew);
> -impl_mmio_io_capable!(MmioOwned, u32, readl, writel);
> +impl_mmio_io_capable!(Mmio, u8, readb, writeb);
> +impl_mmio_io_capable!(Mmio, u16, readw, writew);
> +impl_mmio_io_capable!(Mmio, u32, readl, writel);
>  // MMIO regions on 64-bit systems also support 64-bit accesses.
> -impl_mmio_io_capable!(
> -    MmioOwned,
> -    #[cfg(CONFIG_64BIT)]
> -    u64,
> -    readq,
> -    writeq
> -);
> +#[cfg(CONFIG_64BIT)]
> +impl_mmio_io_capable!(Mmio, u64, readq, writeq);

[Severity: High]
Does this actually allow infallible 64-bit I/O accesses when the target is
Region<SIZE>?

Region<SIZE> has a MIN_ALIGN of 4, so infallible 64-bit accessors like
read64() and write64() will fail the compile-time assertion in
io_view_assert():

    const_assert!(
        Alignment::of::<U>().as_usize() <= IO::Target::MIN_ALIGN.as_usize()
    );

For U = u64, the alignment is 8 on 64-bit architectures, which fails the
assertion since 8 <= 4 is false.

Additionally, while this patch introduces Mmio as a generic view type, can
users create a custom target type (e.g. with align(8)) to work around this?

Looking at impl_usize_ioloc in rust/kernel/io.rs and the IoLoc implementation
in rust/kernel/io/register.rs, both usize offsets and typed registers are
strictly implemented only for IoLoc<Region<SIZE>, T>.

Because standard offsets and registers are restricted to Region<SIZE>, a custom
view type cannot be used with them. Does this leave 64-bit infallible accesses
and 64-bit typed registers unusable on MMIO regions?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260706-io_projection-v6-0-72cd5d055d54@garyguo.net?part=7