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

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

[Lorenzo Delgado]

`ResourceSize` was a bare type alias for `resource_size_t`, so it
inherited every integer operation and `as` cast. That silently permits
operations that are meaningless for the size of a hardware resource:
mixing it with unrelated integers and truncating casts.

Wrap it in a `#[repr(transparent)]` newtype so each conversion at a
boundary becomes explicit and reviewable. The representation is
unchanged, so this is ABI-identical; only the spelling at the FFI
boundary changes. Provide the minimal conversion surface the call sites
need: `from_raw`/`into_raw`, `From` in both directions, and a fallible
`TryFrom<ResourceSize> for usize` for the page-count site that can
truncate on 32-bit. Update the two producers, `Resource::size` and
`SGEntry::dma_len`, and the `request_region` and nova-core consumers so
the tree builds green. Add doctest examples for the new type.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1203
Signed-off-by: Lorenzo Delgado <lnsdev@proton.me>
---
An earlier attempt at this conversion was posted by Moritz Zielke [1]. This
is a fresh, independent implementation that reuses none of that code; it
additionally converts the nova-core consumer and adds doctest examples.

[1] https://lore.kernel.org/all/20251203-res-size-newtype-v1-1-22ed0b8a7a18@gmail.com/

 drivers/gpu/nova-core/firmware/gsp.rs |  2 +-
 rust/kernel/io.rs                     | 73 +++++++++++++++++++++++++--
 rust/kernel/io/resource.rs            |  6 +--
 rust/kernel/scatterlist.rs            |  2 +-
 4 files changed, 74 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/nova-core/firmware/gsp.rs b/drivers/gpu/nova-core/firmware/gsp.rs
index 99a302bae567..d7593888e65b 100644
--- a/drivers/gpu/nova-core/firmware/gsp.rs
+++ b/drivers/gpu/nova-core/firmware/gsp.rs
@@ -175,7 +175,7 @@ pub(crate) fn radix3_dma_handle(&self) -> DmaAddress {
 fn map_into_lvl(sg_table: &SGTable<Owned<VVec<u8>>>, mut dst: VVec<u8>) -> Result<VVec<u8>> {
     for sg_entry in sg_table.iter() {
         // Number of pages we need to map.
-        let num_pages = usize::from_safe_cast(sg_entry.dma_len()).div_ceil(GSP_PAGE_SIZE);
+        let num_pages = usize::try_from(sg_entry.dma_len())?.div_ceil(GSP_PAGE_SIZE);
 
         for i in 0..num_pages {
             let entry = sg_entry.dma_address()
diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
index fcc7678fd9e3..ebf00b4536d6 100644
--- a/rust/kernel/io.rs
+++ b/rust/kernel/io.rs
@@ -6,9 +6,12 @@
 
 use crate::{
     bindings,
+    fmt,
     prelude::*, //
 };
 
+use core::num::TryFromIntError;
+
 pub mod mem;
 pub mod poll;
 pub mod register;
@@ -25,11 +28,73 @@
 /// `CONFIG_PHYS_ADDR_T_64BIT`, and it can be a u64 even on 32-bit architectures.
 pub type PhysAddr = bindings::phys_addr_t;
 
-/// Resource Size type.
+/// Resource size type.
 ///
-/// This is a type alias to either `u32` or `u64` depending on the config option
-/// `CONFIG_PHYS_ADDR_T_64BIT`, and it can be a u64 even on 32-bit architectures.
-pub type ResourceSize = bindings::resource_size_t;
+/// This wraps either `u32` or `u64` depending on the config option
+/// `CONFIG_PHYS_ADDR_T_64BIT`, and it can be a `u64` even on 32-bit architectures.
+///
+/// # Examples
+///
+/// ```
+/// use kernel::io::ResourceSize;
+///
+/// let size = ResourceSize::from_raw(0x1000);
+/// assert_eq!(size.into_raw(), 0x1000);
+///
+/// // Round-trips through the raw C type.
+/// let raw: kernel::bindings::resource_size_t = size.into();
+/// assert_eq!(ResourceSize::from(raw), size);
+///
+/// // Fallible conversion to `usize` (can truncate on 32-bit).
+/// assert_eq!(usize::try_from(size)?, 0x1000);
+/// # Ok::<(), core::num::TryFromIntError>(())
+/// ```
+#[repr(transparent)]
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct ResourceSize(bindings::resource_size_t);
+
+impl ResourceSize {
+    /// Creates a resource size from the raw C type.
+    #[inline]
+    pub const fn from_raw(value: bindings::resource_size_t) -> Self {
+        Self(value)
+    }
+
+    /// Turns this resource size into the raw C type.
+    #[inline]
+    pub const fn into_raw(self) -> bindings::resource_size_t {
+        self.0
+    }
+}
+
+impl fmt::Debug for ResourceSize {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:#x}", self.0)
+    }
+}
+
+impl From<bindings::resource_size_t> for ResourceSize {
+    #[inline]
+    fn from(value: bindings::resource_size_t) -> Self {
+        Self::from_raw(value)
+    }
+}
+
+impl From<ResourceSize> for bindings::resource_size_t {
+    #[inline]
+    fn from(value: ResourceSize) -> Self {
+        value.into_raw()
+    }
+}
+
+impl TryFrom<ResourceSize> for usize {
+    type Error = TryFromIntError;
+
+    #[inline]
+    fn try_from(value: ResourceSize) -> Result<Self, Self::Error> {
+        Self::try_from(value.into_raw())
+    }
+}
 
 /// Raw representation of an MMIO region.
 ///
diff --git a/rust/kernel/io/resource.rs b/rust/kernel/io/resource.rs
index 17b0c174cfc5..a33a416b289a 100644
--- a/rust/kernel/io/resource.rs
+++ b/rust/kernel/io/resource.rs
@@ -58,7 +58,7 @@ fn drop(&mut self) {
         };
 
         // SAFETY: Safe as per the invariant of `Region`.
-        unsafe { release_fn(start, size) };
+        unsafe { release_fn(start, size.into_raw()) };
     }
 }
 
@@ -114,7 +114,7 @@ pub fn request_region(
             bindings::__request_region(
                 self.0.get(),
                 start,
-                size,
+                size.into_raw(),
                 name.as_char_ptr(),
                 flags.0 as c_int,
             )
@@ -130,7 +130,7 @@ pub fn request_region(
     pub fn size(&self) -> ResourceSize {
         let inner = self.0.get();
         // SAFETY: Safe as per the invariants of `Resource`.
-        unsafe { bindings::resource_size(inner) }
+        ResourceSize::from_raw(unsafe { bindings::resource_size(inner) })
     }
 
     /// Returns the start address of the resource.
diff --git a/rust/kernel/scatterlist.rs b/rust/kernel/scatterlist.rs
index b83c468b5c63..5d67242befd8 100644
--- a/rust/kernel/scatterlist.rs
+++ b/rust/kernel/scatterlist.rs
@@ -93,7 +93,7 @@ pub fn dma_address(&self) -> dma::DmaAddress {
     pub fn dma_len(&self) -> ResourceSize {
         #[allow(clippy::useless_conversion)]
         // SAFETY: `self.as_raw()` is a valid pointer to a `struct scatterlist`.
-        unsafe { bindings::sg_dma_len(self.as_raw()) }.into()
+        ResourceSize::from_raw(unsafe { bindings::sg_dma_len(self.as_raw()) }.into())
     }
 }
 

base-commit: 8cdeaa50eae8dad34885515f62559ee83e7e8dda
-- 
2.54.0

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

[Danilo Krummrich]

On Sun Jul 12, 2026 at 1:36 PM CEST, Lorenzo Delgado wrote:
> diff --git a/drivers/gpu/nova-core/firmware/gsp.rs b/drivers/gpu/nova-core/firmware/gsp.rs
> index 99a302bae567..d7593888e65b 100644
> --- a/drivers/gpu/nova-core/firmware/gsp.rs
> +++ b/drivers/gpu/nova-core/firmware/gsp.rs
> @@ -175,7 +175,7 @@ pub(crate) fn radix3_dma_handle(&self) -> DmaAddress {
>  fn map_into_lvl(sg_table: &SGTable<Owned<VVec<u8>>>, mut dst: VVec<u8>) -> Result<VVec<u8>> {
>      for sg_entry in sg_table.iter() {
>          // Number of pages we need to map.
> -        let num_pages = usize::from_safe_cast(sg_entry.dma_len()).div_ceil(GSP_PAGE_SIZE);
> +        let num_pages = usize::try_from(sg_entry.dma_len())?.div_ceil(GSP_PAGE_SIZE);

I think this is worse, as the conversion becomes fallible.

You could implement From<ResourceSize> for u64 and then keep using
usize::from_safe_cast() in nova-core.

Alternatively, we could also consider moving the FromSafeCast trait to
rust/kernel/num.rs and add FromSafeCast<ResourceSize> impls for usize.

I originally proposed the approach [1], since in nova-core we had a lot of
fallible conversion, while our Kconfig mandates that they can never actually
fail.

However, by making it commonly availble I do see a risk with the cfg-gated impls
silently breaking the build.

Now, technically that's the feature and 0-day etc. should usually catch it, but
on the other hand we also usually try to avoid cfg-gates in the kernel.

Miguel, I think you were in favor of this as well, what do you think?

[1] https://lore.kernel.org/rust-for-linux/DDK4KADWJHMG.1FUPL3SDR26XF@kernel.org/

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

[Lorenzo Delgado]

On Thu Jul 16, 2026 at 11:45 PM CEST, Danilo Krummrich wrote:
> On Sun Jul 12, 2026 at 1:36 PM CEST, Lorenzo Delgado wrote:
> > -        let num_pages = usize::from_safe_cast(sg_entry.dma_len()).div_ceil(GSP_PAGE_SIZE);
> > +        let num_pages = usize::try_from(sg_entry.dma_len())?.div_ceil(GSP_PAGE_SIZE);
>
> I think this is worse, as the conversion becomes fallible.

Agreed, that's a regression for nova-core, where the Kconfig guarantees
the value fits and the conversion should stay infallible. I'll drop the
try_from() there.

> You could implement From<ResourceSize> for u64 and then keep using
> usize::from_safe_cast() in nova-core.

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`

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.

> Alternatively, we could also consider moving the FromSafeCast trait to
> rust/kernel/num.rs and add FromSafeCast<ResourceSize> impls for usize.
> [...]
> However, by making it commonly availble I do see a risk with the
> cfg-gated impls silently breaking the build.

Agreed on the risk. A FromSafeCast<ResourceSize> for usize impl would
have to be cfg-gated like the u64 one, which is the same
silent-breakage-under-randconfig case you mention.

Since into_raw() keeps this patch self-contained, moving FromSafeCast
into the kernel crate is a separate change from the newtype conversion.
It seems worth doing on its own, and I'm happy to send it as its own
series so it gets reviewed as a new core API, but it doesn't need to
block this patch. I'll post v2 with the into_raw() change.

Thanks for the review.

Lorenzo

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

[Danilo Krummrich]

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.

[PATCH v2] rust: io: convert ResourceSize into a transparent newtype

[Lorenzo Delgado]

`ResourceSize` was a bare type alias for `resource_size_t`, so it
inherited every integer operation and `as` cast. That silently permits
operations that are meaningless for the size of a hardware resource:
mixing it with unrelated integers and truncating casts.

Wrap it in a `#[repr(transparent)]` newtype so each conversion at a
boundary becomes explicit and reviewable. The representation is
unchanged, so this is ABI-identical; only the spelling at the FFI
boundary changes. Provide the conversion surface `from_raw`/`into_raw`,
`From` in both directions, and a fallible `TryFrom<ResourceSize> for
usize` for checked narrowing. Update the two producers, `Resource::size`
and `SGEntry::dma_len`, and the `request_region` consumer. In nova-core,
add an `impl FromSafeCast<ResourceSize> for usize` so its page-count
conversion keeps the infallible `usize::from_safe_cast(sg_entry.dma_len())`
unchanged.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1203
Signed-off-by: Lorenzo Delgado <lnsdev@proton.me>
---
v2: keep the nova-core page-count conversion infallible, per review
    (Danilo Krummrich). v1 changed it to a fallible
    usize::try_from(sg_entry.dma_len())?; instead of touching the call
    site, add an impl FromSafeCast<ResourceSize> for usize in nova-core's
    num.rs, so drivers/gpu/nova-core/firmware/gsp.rs keeps
    usize::from_safe_cast(sg_entry.dma_len()) unchanged. FromSafeCast is
    to move to the kernel crate subsequently. io.rs is unchanged from v1.

v1: https://lore.kernel.org/r/20260712113602.389060-1-lnsdev@proton.me

 drivers/gpu/nova-core/num.rs | 10 +++++
 rust/kernel/io.rs            | 73 ++++++++++++++++++++++++++++++++++--
 rust/kernel/io/resource.rs   |  6 +--
 rust/kernel/scatterlist.rs   |  2 +-
 4 files changed, 83 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/nova-core/num.rs b/drivers/gpu/nova-core/num.rs
index 6eb174d136ab..7c0e878bbfb2 100644
--- a/drivers/gpu/nova-core/num.rs
+++ b/drivers/gpu/nova-core/num.rs
@@ -6,6 +6,7 @@
 //! crate.
 
 use kernel::{
+    io::ResourceSize,
     macros::paste,
     prelude::*, //
 };
@@ -137,6 +138,15 @@ fn from_safe_cast(value: u64) -> Self {
     }
 }
 
+// A `resource_size_t` fits into a `usize` on 64-bit platforms, where a `usize` is a `u64` and a
+// `resource_size_t` is at most a `u64`. Delegates to the primitive impl for the underlying type.
+#[cfg(CONFIG_64BIT)]
+impl FromSafeCast<ResourceSize> for usize {
+    fn from_safe_cast(value: ResourceSize) -> Self {
+        Self::from_safe_cast(value.into_raw())
+    }
+}
+
 /// Counterpart to the [`FromSafeCast`] trait, i.e. this trait is to [`FromSafeCast`] what [`Into`]
 /// is to [`From`].
 ///
diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
index fcc7678fd9e3..ebf00b4536d6 100644
--- a/rust/kernel/io.rs
+++ b/rust/kernel/io.rs
@@ -6,9 +6,12 @@
 
 use crate::{
     bindings,
+    fmt,
     prelude::*, //
 };
 
+use core::num::TryFromIntError;
+
 pub mod mem;
 pub mod poll;
 pub mod register;
@@ -25,11 +28,73 @@
 /// `CONFIG_PHYS_ADDR_T_64BIT`, and it can be a u64 even on 32-bit architectures.
 pub type PhysAddr = bindings::phys_addr_t;
 
-/// Resource Size type.
+/// Resource size type.
 ///
-/// This is a type alias to either `u32` or `u64` depending on the config option
-/// `CONFIG_PHYS_ADDR_T_64BIT`, and it can be a u64 even on 32-bit architectures.
-pub type ResourceSize = bindings::resource_size_t;
+/// This wraps either `u32` or `u64` depending on the config option
+/// `CONFIG_PHYS_ADDR_T_64BIT`, and it can be a `u64` even on 32-bit architectures.
+///
+/// # Examples
+///
+/// ```
+/// use kernel::io::ResourceSize;
+///
+/// let size = ResourceSize::from_raw(0x1000);
+/// assert_eq!(size.into_raw(), 0x1000);
+///
+/// // Round-trips through the raw C type.
+/// let raw: kernel::bindings::resource_size_t = size.into();
+/// assert_eq!(ResourceSize::from(raw), size);
+///
+/// // Fallible conversion to `usize` (can truncate on 32-bit).
+/// assert_eq!(usize::try_from(size)?, 0x1000);
+/// # Ok::<(), core::num::TryFromIntError>(())
+/// ```
+#[repr(transparent)]
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct ResourceSize(bindings::resource_size_t);
+
+impl ResourceSize {
+    /// Creates a resource size from the raw C type.
+    #[inline]
+    pub const fn from_raw(value: bindings::resource_size_t) -> Self {
+        Self(value)
+    }
+
+    /// Turns this resource size into the raw C type.
+    #[inline]
+    pub const fn into_raw(self) -> bindings::resource_size_t {
+        self.0
+    }
+}
+
+impl fmt::Debug for ResourceSize {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:#x}", self.0)
+    }
+}
+
+impl From<bindings::resource_size_t> for ResourceSize {
+    #[inline]
+    fn from(value: bindings::resource_size_t) -> Self {
+        Self::from_raw(value)
+    }
+}
+
+impl From<ResourceSize> for bindings::resource_size_t {
+    #[inline]
+    fn from(value: ResourceSize) -> Self {
+        value.into_raw()
+    }
+}
+
+impl TryFrom<ResourceSize> for usize {
+    type Error = TryFromIntError;
+
+    #[inline]
+    fn try_from(value: ResourceSize) -> Result<Self, Self::Error> {
+        Self::try_from(value.into_raw())
+    }
+}
 
 /// Raw representation of an MMIO region.
 ///
diff --git a/rust/kernel/io/resource.rs b/rust/kernel/io/resource.rs
index 17b0c174cfc5..a33a416b289a 100644
--- a/rust/kernel/io/resource.rs
+++ b/rust/kernel/io/resource.rs
@@ -58,7 +58,7 @@ fn drop(&mut self) {
         };
 
         // SAFETY: Safe as per the invariant of `Region`.
-        unsafe { release_fn(start, size) };
+        unsafe { release_fn(start, size.into_raw()) };
     }
 }
 
@@ -114,7 +114,7 @@ pub fn request_region(
             bindings::__request_region(
                 self.0.get(),
                 start,
-                size,
+                size.into_raw(),
                 name.as_char_ptr(),
                 flags.0 as c_int,
             )
@@ -130,7 +130,7 @@ pub fn request_region(
     pub fn size(&self) -> ResourceSize {
         let inner = self.0.get();
         // SAFETY: Safe as per the invariants of `Resource`.
-        unsafe { bindings::resource_size(inner) }
+        ResourceSize::from_raw(unsafe { bindings::resource_size(inner) })
     }
 
     /// Returns the start address of the resource.
diff --git a/rust/kernel/scatterlist.rs b/rust/kernel/scatterlist.rs
index b83c468b5c63..5d67242befd8 100644
--- a/rust/kernel/scatterlist.rs
+++ b/rust/kernel/scatterlist.rs
@@ -93,7 +93,7 @@ pub fn dma_address(&self) -> dma::DmaAddress {
     pub fn dma_len(&self) -> ResourceSize {
         #[allow(clippy::useless_conversion)]
         // SAFETY: `self.as_raw()` is a valid pointer to a `struct scatterlist`.
-        unsafe { bindings::sg_dma_len(self.as_raw()) }.into()
+        ResourceSize::from_raw(unsafe { bindings::sg_dma_len(self.as_raw()) }.into())
     }
 }
 

base-commit: 7059bdf4f04a3e14f4fafb3ac35fdca913e3e21a
-- 
2.54.0

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

[Gary Guo]

On Sun Jul 19, 2026 at 3:53 PM BST, Danilo Krummrich wrote:
> 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.

Given that `resource_size_t` is either u32 or u64 depending on cfg, essentially
`From<ResourceSize> for bindings::resource_size_t` has cfg-gating. So basically
we always unconditionally have u64 impl, and for `!CONFIG_PHYS_ADDR_T_64BIT` we
have a condition u32 impl.

Best,
Gary