[PATCH v4 2/2] rust: io: convert ResourceSize into a transparent newtype

[Lorenzo Delgado]

`ResourceSize` is a bare type alias for `resource_size_t`, so it
inherits every integer operation and `as` cast. That allows operations
that make no sense for the size of a hardware resource, such as mixing
it with unrelated integers or truncating it with a cast.

Wrap it in a `#[repr(transparent)]` newtype so each conversion at a
boundary is explicit. The representation is unchanged, so this is
ABI-identical; only the spelling at the FFI boundary changes. Provide
`from_raw`/`into_raw`, `From` in both directions, and a fallible
`TryFrom<ResourceSize> for usize`, since `resource_size_t` can be wider
than `usize` on 32-bit.

Update the producer, `Resource::size`, and its consumers in `Region`
and `request_region`.

Suggested-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Link: https://github.com/Rust-for-Linux/linux/issues/1203
Signed-off-by: Lorenzo Delgado <lnsdev@xxxxxxxxx>
---
 rust/kernel/io.rs          | 74 ++++++++++++++++++++++++++++++++++++++++++----
 rust/kernel/io/resource.rs |  6 ++--
 2 files changed, 72 insertions(+), 8 deletions(-)

diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
index 5ce9fd129068..09b0586acf1a 100644
--- a/rust/kernel/io.rs
+++ b/rust/kernel/io.rs
@@ -6,11 +6,13 @@
 
 use core::{
     marker::PhantomData,
-    mem::MaybeUninit, //
+    mem::MaybeUninit,
+    num::TryFromIntError, //
 };
 
 use crate::{
     bindings,
+    fmt,
     prelude::*,
     ptr::{
         Alignment,
@@ -35,11 +37,73 @@
 /// `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())
+    }
+}
 
 /// Untyped I/O 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.

-- 
2.55.0