[PATCH v2] rust: io: Add a BigEndianMmio wrapper

[Link Mauve]

This allows the user to read from and write to big-endian MMIO devices,
such as those found on PowerPC systems.

The implementation is pretty much a copy-paste of the RelaxedMmio type,
with the business logic changed.

This has been tested with various WIP drivers on the Nintendo Wii, and I
thought it would be nicer to upstream it before those are completely
ready, to get some review.

Signed-off-by: Link Mauve <linkmauve@xxxxxxxxxxxx>
---
 rust/helpers/io.c | 34 ++++++++++++++++++
 rust/kernel/io.rs | 88 +++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 120 insertions(+), 2 deletions(-)

Changes since v1:
- Link to v1:
  https://lore.kernel.org/rust-for-linux/178598049138.1212746.2443151507074972963.b4-review@b4/T/
- Fix copy/paste mistake in backend docstring (thank you Ethan Plant!)
- Fix safety comment in macro documentation.

diff --git a/rust/helpers/io.c b/rust/helpers/io.c
index 7ed9a4f77f1b..3bbb86a00d67 100644
--- a/rust/helpers/io.c
+++ b/rust/helpers/io.c
@@ -120,6 +120,40 @@ __rust_helper void rust_helper_writeq_relaxed(u64 value, void __iomem *addr)
 }
 #endif
 
+__rust_helper u16 rust_helper_ioread16be(const void __iomem *addr)
+{
+	return ioread16be(addr);
+}
+
+__rust_helper u32 rust_helper_ioread32be(const void __iomem *addr)
+{
+	return ioread32be(addr);
+}
+
+#ifdef CONFIG_64BIT
+__rust_helper u64 rust_helper_ioread64be(const void __iomem *addr)
+{
+	return ioread64be(addr);
+}
+#endif
+
+__rust_helper void rust_helper_iowrite16be(u16 value, void __iomem *addr)
+{
+	iowrite16be(value, addr);
+}
+
+__rust_helper void rust_helper_iowrite32be(u32 value, void __iomem *addr)
+{
+	iowrite32be(value, addr);
+}
+
+#ifdef CONFIG_64BIT
+__rust_helper void rust_helper_iowrite64be(u64 value, void __iomem *addr)
+{
+	iowrite64be(value, addr);
+}
+#endif
+
 __rust_helper resource_size_t rust_helper_resource_size(struct resource *res)
 {
 	return resource_size(res);
diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
index a38c20ba3d23..ab8b7864298a 100644
--- a/rust/kernel/io.rs
+++ b/rust/kernel/io.rs
@@ -1194,14 +1194,14 @@ impl IoCapable<$ty> for $backend {
             #[inline]
             fn io_read(view: <$backend as IoBackend>::View<'_, $ty>) -> $ty {
                 // SAFETY: `$backend::as_ptr(view)` is a valid pointer for MMIO operations for both
-                // `MmioBackend` and `RelaxedMmioBackend`.
+                // `MmioBackend`, `RelaxedMmioBackend` and `BigEndianMmioBackend`.
                 unsafe { bindings::$read_fn($backend::as_ptr(view).cast_const().cast()) }
             }
 
             #[inline]
             fn io_write(view: <$backend as IoBackend>::View<'_, $ty>, value: $ty) {
                 // SAFETY: `$backend::as_ptr(view)` is a valid pointer for MMIO operations for both
-                // `MmioBackend` and `RelaxedMmioBackend`.
+                // `MmioBackend`, `RelaxedMmioBackend` and `BigEndianMmioBackend`.
                 unsafe { bindings::$write_fn(value, $backend::as_ptr(view).cast()) }
             }
         }
@@ -1751,3 +1751,87 @@ macro_rules! io_write {
 }
 #[doc(inline)]
 pub use crate::io_write;
+
+/// [`Mmio`] wrapper using big-endian accessors.
+///
+/// This type provides an implementation of [`Io`] that uses big-endian I/O MMIO operands instead of
+/// the regular little-endian ones.
+///
+/// See [`Mmio::big_endian`] for a usage example.
+#[repr(transparent)]
+pub struct BigEndianMmio<'a, T: ?Sized>(Mmio<'a, T>);
+
+impl<T: ?Sized> Copy for BigEndianMmio<'_, T> {}
+impl<T: ?Sized> Clone for BigEndianMmio<'_, T> {
+    #[inline]
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+/// I/O Backend for memory-mapped I/O using big-endian accessors.
+pub struct BigEndianMmioBackend;
+
+impl IoBackend for BigEndianMmioBackend {
+    type View<'a, T: ?Sized + KnownSize> = BigEndianMmio<'a, T>;
+
+    #[inline]
+    fn as_ptr<'a, T: ?Sized + KnownSize>(view: Self::View<'a, T>) -> *mut T {
+        MmioBackend::as_ptr(view.0)
+    }
+
+    #[inline]
+    unsafe fn project_view<'a, T: ?Sized + KnownSize, U: ?Sized + KnownSize>(
+        view: Self::View<'a, T>,
+        ptr: *mut U,
+    ) -> Self::View<'a, U> {
+        // SAFETY: Per safety requirement.
+        BigEndianMmio(unsafe { MmioBackend::project_view(view.0, ptr) })
+    }
+}
+
+impl<'a, T: ?Sized + KnownSize> IoBase<'a> for BigEndianMmio<'a, T> {
+    type Backend = BigEndianMmioBackend;
+    type Target = T;
+
+    #[inline]
+    fn as_view(self) -> BigEndianMmio<'a, T> {
+        self
+    }
+}
+
+impl<'a, T: ?Sized> Mmio<'a, T> {
+    /// Returns a [`BigEndianMmio`] reference that performs big-endian I/O operations.
+    ///
+    /// Big-endian makes no change to 8-bit accesses, but will invert the bytes of 16-, 32- and
+    /// 64-bit accesses, to ensure numbers will be read in their correct order.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use kernel::io::{
+    ///     Io,
+    ///     Mmio,
+    ///     Region,
+    ///     BigEndianMmio,
+    /// };
+    ///
+    /// fn do_io(io: Mmio<'_, Region<0x100>>) {
+    ///     // The access is performed using `ioread32be` instead of `readl`.
+    ///     let v = io.big_endian().read32(0x10);
+    /// }
+    ///
+    /// ```
+    #[inline]
+    pub fn big_endian(self) -> BigEndianMmio<'a, T> {
+        BigEndianMmio(self)
+    }
+}
+
+// MMIO regions support 8, 16, and 32-bit accesses.
+impl_mmio_io_capable!(BigEndianMmioBackend, u8, readb, writeb);
+impl_mmio_io_capable!(BigEndianMmioBackend, u16, ioread16be, iowrite16be);
+impl_mmio_io_capable!(BigEndianMmioBackend, u32, ioread32be, iowrite32be);
+// MMIO regions on 64-bit systems also support 64-bit accesses.
+#[cfg(CONFIG_64BIT)]
+impl_mmio_io_capable!(BigEndianMmioBackend, u64, ioread64be, iowrite64be);

base-commit: 1701fda2f58e345c050f4309971bdc07cd6146ba
-- 
2.55.0