[PATCH v3 1/2] rust: clk: Add `ExclusiveClk` wrapper for `clk_rate_exclusive_get`

[Maurice Hieronymus]

Add Rust bindings for `clk_rate_exclusive_get()` and
`clk_rate_exclusive_put()`.

`Clk::rate_exclusive_get()` consumes the `Clk<Enabled>` and returns an
`ExclusiveClk`. The matching put is issued from its Drop impl.
`ExclusiveClk` derefs to `Clk<Enabled>` so existing APIs remain
available on the locked handle.

Reviewed-by: Alice Ryhl <aliceryhl at google.com>
Signed-off-by: Maurice Hieronymus <mhi at mailbox.org>
---
 rust/kernel/clk.rs | 67 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)

diff --git a/rust/kernel/clk.rs b/rust/kernel/clk.rs
index 1412a2f0aedf..89f662b67b95 100644
--- a/rust/kernel/clk.rs
+++ b/rust/kernel/clk.rs
@@ -95,6 +95,7 @@ mod common_clk {
     use core::{
         marker::PhantomData,
         mem::ManuallyDrop,
+        ops::Deref,
         ptr, //
     };
 
@@ -621,6 +622,23 @@ pub fn set_rate(&self, rate: Hertz) -> Result {
             // [`clk_set_rate`].
             to_result(unsafe { bindings::clk_set_rate(self.as_raw(), rate.as_hz()) })
         }
+
+        /// Acquire exclusive control over the clock's rate.
+        ///
+        /// Consumes the [`Clk<Enabled>`] and returns an [`ExclusiveClk`] that releases
+        /// the exclusivity when dropped. While held, no other consumer may change the clock's rate.
+        ///
+        /// Equivalent to the kernel's [`clk_rate_exclusive_get`] API. Must not be called from
+        /// atomic context.
+        ///
+        /// [`clk_rate_exclusive_get`]: https://docs.kernel.org/core-api/kernel-api.html#c.clk_rate_exclusive_get
+        #[inline]
+        pub fn rate_exclusive_get(self) -> Result<ExclusiveClk> {
+            // SAFETY: By the type invariants, self.as_raw() is a valid argument for
+            // clk_rate_exclusive_get.
+            to_result(unsafe { bindings::clk_rate_exclusive_get(self.as_raw()) })?;
+            Ok(ExclusiveClk(self))
+        }
     }
 
     impl<T: ClkState> Clk<T> {
@@ -650,6 +668,55 @@ fn drop(&mut self) {
             unsafe { bindings::clk_put(self.as_raw()) };
         }
     }
+
+    /// A [`Clk<Enabled>`] with exclusive control over its rate.
+    ///
+    /// While an [`ExclusiveClk`] exists, no other consumer of the same clock may change its rate.
+    /// Obtained by calling [`Clk::rate_exclusive_get`]; the exclusivity is released automatically
+    /// when the value is dropped, after which the inner [`Clk<Enabled>`] is dropped as usual.
+    ///
+    /// # Invariants
+    ///
+    /// An [`ExclusiveClk`] instance owns a [`Clk<Enabled>`] for which `clk_rate_exclusive_get` has
+    /// been called and the matching `clk_rate_exclusive_put` has not yet been called.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::clk::{
+    ///     Clk,
+    ///     ExclusiveClk,
+    ///     Enabled //
+    /// };
+    /// use kernel::device::{
+    ///     Bound,
+    ///     Device, //
+    /// };
+    /// use kernel::error::Result;
+    ///
+    /// fn lock_rate(dev: &Device<Bound>) -> Result<ExclusiveClk> {
+    ///     let clk = Clk::<Enabled>::get(dev, None)?;
+    ///     clk.rate_exclusive_get()
+    /// }
+    /// ```
+    pub struct ExclusiveClk(Clk<Enabled>);
+
+    impl Deref for ExclusiveClk {
+        type Target = Clk<Enabled>;
+
+        fn deref(&self) -> &Clk<Enabled> {
+            &self.0
+        }
+    }
+
+    impl Drop for ExclusiveClk {
+        fn drop(&mut self) {
+            // SAFETY: By the type invariants, self.as_raw() is a valid argument for
+            // clk_rate_exclusive_put and balances the clk_rate_exclusive_get call from
+            // Clk::rate_exclusive_get.
+            unsafe { bindings::clk_rate_exclusive_put(self.as_raw()) };
+        }
+    }
 }
 
 #[cfg(CONFIG_COMMON_CLK)]

-- 
2.51.2