[PATCH RFC 1/3] rust: add useful ops for u64

[Alexandre Courbot <acourbot@nvidia.com>]

It is common to build a u64 from its high and low parts obtained from
two 32-bit registers. Conversely, it is also common to split a u64 into
two u32s to write them into registers. Add an extension trait for u64
that implement these methods in a new `num` module.

It is expected that this trait will be extended with other useful
operations, and similar extension traits implemented for other types.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/lib.rs |  1 +
 rust/kernel/num.rs | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 496ed32b0911a9fdbce5d26738b9cf7ef910b269..8c0c7c20a16aa96e3d3e444be3e03878650ddf77 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -59,6 +59,7 @@
 pub mod miscdevice;
 #[cfg(CONFIG_NET)]
 pub mod net;
+pub mod num;
 pub mod of;
 pub mod page;
 #[cfg(CONFIG_PCI)]
diff --git a/rust/kernel/num.rs b/rust/kernel/num.rs
new file mode 100644
index 0000000000000000000000000000000000000000..5e714cbda4575b8d74f50660580dc4c5683f8c2b
--- /dev/null
+++ b/rust/kernel/num.rs
@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Numerical and binary utilities for primitive types.
+
+/// Useful operations for `u64`.
+pub trait U64Ext {
+    /// Build a `u64` by combining its `high` and `low` parts.
+    ///
+    /// ```
+    /// use kernel::num::U64Ext;
+    /// assert_eq!(u64::from_u32s(0x01234567, 0x89abcdef), 0x01234567_89abcdef);
+    /// ```
+    fn from_u32s(high: u32, low: u32) -> Self;
+
+    /// Returns the `(high, low)` u32s that constitute `self`.
+    ///
+    /// ```
+    /// use kernel::num::U64Ext;
+    /// assert_eq!(u64::into_u32s(0x01234567_89abcdef), (0x1234567, 0x89abcdef));
+    /// ```
+    fn into_u32s(self) -> (u32, u32);
+}
+
+impl U64Ext for u64 {
+    fn from_u32s(high: u32, low: u32) -> Self {
+        ((high as u64) << u32::BITS) | low as u64
+    }
+
+    fn into_u32s(self) -> (u32, u32) {
+        ((self >> u32::BITS) as u32, self as u32)
+    }
+}

-- 
2.48.1