rust: io: add read_val and write_val functions on Io

Provide `read_val` and `write_val` that allow I/O views to be accessed when
they're narrowed down to just views of primitives.

This is used to provide `io_read!` and `io_write!` macros, which are
generalized version of current `dma_read!` and `dma_write!` macro that work
for all types that implement `Io`.

Note though `io_read!` and `io_write!` only works if backend implements
`IoCapable` for the type; which is typically only implemented for
atomically accessible primitives. `dma_read!` and `dma_write!` currently
supports them via `read_volatile` and `write_volatile`; this can be
undesirable for aggregates as LLVM may turn them to multiple instructions
to access parts and re-assemble, even if they could be combined to a single
instruction. Thus, `io_read!()` and `io_write!()` does not fully replace
`dma_read!()` and `dma_write!()` in this scenario. The ability to
read/write aggregates (when atomicity is of no concern) is better served
with copying primitives (e.g. memcpy_{from,to}io).

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://patch.msgid.link/20260706-io_projection-v6-16-72cd5d055d54@garyguo.net
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/io.rs

@@ -399,6 +399,50 @@ fn try_cast<U>(self) -> Result<<Self::Backend as IoBackend>::View<'a, U>>
         Ok(unsafe { Self::Backend::project_view(view, ptr.cast()) })
     }
 
+    /// Read a value from I/O.
+    ///
+    /// This only works for primitives supported by the I/O backend.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// # use kernel::io::*;
+    /// # fn test_read_val(mmio: Mmio<'_, u32>) {
+    /// // let mmio: Mmio<'_, u32>;
+    /// let val: u32 = mmio.read_val();
+    /// # }
+    /// ```
+    #[inline]
+    fn read_val(self) -> Self::Target
+    where
+        Self::Backend: IoCapable<Self::Target>,
+        Self::Target: Sized,
+    {
+        Self::Backend::io_read(self.as_view())
+    }
+
+    /// Write a value to I/O.
+    ///
+    /// This only works for primitives supported by the I/O backend.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// # use kernel::io::*;
+    /// # fn test_write_val(mmio: Mmio<'_, u32>) {
+    /// // let mmio: Mmio<'_, u32>;
+    /// mmio.write_val(1u32);
+    /// # }
+    /// ```
+    #[inline]
+    fn write_val(self, value: Self::Target)
+    where
+        Self::Backend: IoCapable<Self::Target>,
+        Self::Target: Sized,
+    {
+        Self::Backend::io_write(self.as_view(), value)
+    }
+
     /// Fallible 8-bit read with runtime bounds check.
     #[inline(always)]
     fn try_read8(self, offset: usize) -> Result<u8>
@@ -1232,3 +1276,65 @@ macro_rules! io_project {
 }
 #[doc(inline)]
 pub use crate::io_project;
+
+/// Read from I/O memory.
+///
+/// The syntax is of form `io_read!(io, proj)` where `io` is an expression to a type that
+/// implements [`Io`] and `proj` is a [projection specification](kernel::ptr::project!).
+///
+/// # Examples
+///
+/// ```
+/// #[repr(C)]
+/// struct MyStruct { field: u32, }
+///
+/// # fn test(mmio: kernel::io::Mmio<'_, [MyStruct]>) -> Result {
+/// // let mmio: Mmio<'_, [MyStruct]>;
+/// let field: u32 = kernel::io::io_read!(mmio, [try: 2].field);
+/// # Ok::<(), Error>(()) }
+/// ```
+#[macro_export]
+#[doc(hidden)]
+macro_rules! io_read {
+    ($io:expr, $($proj:tt)*) => {
+        $crate::io::Io::read_val($crate::io_project!($io, $($proj)*))
+    };
+}
+#[doc(inline)]
+pub use crate::io_read;
+
+/// Writes to I/O memory.
+///
+/// The syntax is of form `io_write!(io, proj, val)` where `io` is an expression to a type that
+/// implements [`Io`] and `proj` is a [projection specification](kernel::ptr::project!),
+/// and `val` is the value to be written to the projected location.
+///
+/// # Examples
+///
+/// ```
+/// #[repr(C)]
+/// struct MyStruct { field: u32, }
+///
+/// # fn test(mmio: kernel::io::Mmio<'_, [MyStruct]>) -> Result {
+/// // let mmio: Mmio<'_, [MyStruct]>;
+/// kernel::io::io_write!(mmio, [try: 2].field, 10);
+/// # Ok::<(), Error>(()) }
+/// ```
+#[macro_export]
+#[doc(hidden)]
+macro_rules! io_write {
+    (@parse [$io:expr] [$($proj:tt)*] [, $val:expr]) => {
+        $crate::io::Io::write_val($crate::io_project!($io, $($proj)*), $val)
+    };
+    (@parse [$io:expr] [$($proj:tt)*] [.$field:tt $($rest:tt)*]) => {
+        $crate::io_write!(@parse [$io] [$($proj)* .$field] [$($rest)*])
+    };
+    (@parse [$io:expr] [$($proj:tt)*] [[$flavor:ident: $index:expr] $($rest:tt)*]) => {
+        $crate::io_write!(@parse [$io] [$($proj)* [$flavor: $index]] [$($rest)*])
+    };
+    ($io:expr, $($rest:tt)*) => {
+        $crate::io_write!(@parse [$io] [] [$($rest)*])
+    };
+}
+#[doc(inline)]
+pub use crate::io_write;