Re: [PATCH] samples: rust: fix endianness issue in rust_driver_pci

["Danilo Krummrich" <dakr@kernel.org>]

On Thu Nov 13, 2025 at 7:22 PM AEDT, Danilo Krummrich wrote:
> On Wed Nov 12, 2025 at 8:56 PM AEDT, Miguel Ojeda wrote:
>> I guess we could potentially consider something like returning a
>> wrapper to force us to explicitly pick either LE or BE to prevent
>> things like this.
>
> In general, the I/O backend (e.g. MMIO, I2C, etc.) is supposed to take care
> of endianness.
>
> In the case of MMIO we (currently) always assume that the device is
> little-endian; this is also what the C functions used to implement the MMIO
> backend (e.g. readl()) assume, i.e. they always convert from little-endian to
> CPU endianness.

I.e. if we'd support a big-endian architecture in Rust

	u32::from_le(bar.read32())

would be a bug.

> I don't think we should do anything else; the cases where we need to deal with
> big-endian devices for MMIO should be extremely rare -- rare enough that there
> aren't even corresponding *_be() implementations for all architectures.
>
> Given that, I think the proper fix is to drop the existing u32::from_le() call
> (that ended up in the sample driver by accident) and properly document the
> little-endian assumption of the MMIO backend.
>
> (Currently, this simply is the Io structure, but there is a patch series [1]
> from Zhi splitting things up, so we can start supporting arbitrary I/O
> backends.)
>
> @Marko: Two separate patches for this would be very welcome. :)
>
> [1] https://lore.kernel.org/all/20251110204119.18351-1-zhiw@nvidia.com/