Re: [PATCH] rust: io: convert ResourceSize into a transparent newtype

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

On Sun Jul 19, 2026 at 12:40 PM CEST, Lorenzo Delgado wrote:
> I tried that, but it doesn't build. ResourceSize wraps resource_size_t,
> which is u64 on 64-bit (CONFIG_PHYS_ADDR_T_64BIT), so the impl the patch
> already has,
>
>     impl From<ResourceSize> for bindings::resource_size_t
>
> is already From<ResourceSize> for u64 there, and a second one conflicts:
>
>     error[E0119]: conflicting implementations of trait
>                   `From<ResourceSize>` for type `u64`

Sure, but in the case a cfg-gate is perfectly fine, as the impl will always be
availble.  Either through impl for u64 directly or through impl for
bindings::resource_size_t.

> There's a simpler way that stays infallible and adds nothing to io.rs.
> ResourceSize already has into_raw() (io/resource.rs uses it at the C
> boundaries), so nova-core can do:
>
>     let num_pages =
>         usize::from_safe_cast(sg_entry.dma_len().into_raw()).div_ceil(GSP_PAGE_SIZE);
>
> into_raw() gives back resource_size_t, and from_safe_cast handles that
> as u32 or u64 depending on the config, so it stays infallible. I'll use
> that in v2 unless you'd prefer something else.

I don't want to grow it arbitrarily in nova-core - which is also why I opened
the question of moving FromSafeCast to generic infrastructure again - but in
this case we should rather add the FromSafeCast<ResourceSize> for usize impl in
drivers/gpu/nova-core/num.rs for now and move it subsequently.