[PATCH v4 04/20] rust: add new `num` module with useful integer operations

[Alexandre Courbot]

Introduce the `num` module, featuring the `NumExt` extension trait
that expands unsigned integers with useful operations for the kernel.

These are to be used by the nova-core driver, but they are so ubiquitous
that other drivers should be able to take advantage of them as well.

The currently implemented operations are:

- align_down()
- align_up()
- fls()

But this trait is expected to be expanded further.

`NumExt` is on unsigned types using a macro. An approach using another
trait constrained by the operator traits that we need (`Add`, `Sub`,
etc) was also considered, but had to be dropped as we need to use
wrapping operations, which are not provided by any trait.

Co-developed-by: Joel Fernandes <joelagnelf at nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf at nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot at nvidia.com>
---
 rust/kernel/lib.rs |  1 +
 rust/kernel/num.rs | 82 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 83 insertions(+)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index ab0286857061d2de1be0279cbd2cd3490e5a48c3..be75b196aa7a29cf3eed7c902ed8fb98689bbb50 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -67,6 +67,7 @@
 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..05d45b59313d830876c1a7b452827689a6dd5400
--- /dev/null
+++ b/rust/kernel/num.rs
@@ -0,0 +1,82 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Numerical and binary utilities for primitive types.
+
+/// Extension trait providing useful methods for the kernel on integers.
+pub trait NumExt {
+    /// Align `self` down to `alignment`.
+    ///
+    /// `alignment` must be a power of 2 for accurate results.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::num::NumExt;
+    ///
+    /// assert_eq!(0x4fffu32.align_down(0x1000), 0x4000);
+    /// assert_eq!(0x4fffu32.align_down(0x0), 0x0);
+    /// ```
+    fn align_down(self, alignment: Self) -> Self;
+
+    /// Align `self` up to `alignment`.
+    ///
+    /// `alignment` must be a power of 2 for accurate results.
+    ///
+    /// Wraps around to `0` if the requested alignment pushes the result above the type's limits.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::num::NumExt;
+    ///
+    /// assert_eq!(0x4fffu32.align_up(0x1000), 0x5000);
+    /// assert_eq!(0x4000u32.align_up(0x1000), 0x4000);
+    /// assert_eq!(0x0u32.align_up(0x1000), 0x0);
+    /// assert_eq!(0xffffu16.align_up(0x100), 0x0);
+    /// assert_eq!(0x4fffu32.align_up(0x0), 0x0);
+    /// ```
+    fn align_up(self, alignment: Self) -> Self;
+
+    /// Find Last Set Bit: return the 1-based index of the last (i.e. most significant) set bit in
+    /// `self`.
+    ///
+    /// Equivalent to the C `fls` function.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::num::NumExt;
+    ///
+    /// assert_eq!(0x0u32.fls(), 0);
+    /// assert_eq!(0x1u32.fls(), 1);
+    /// assert_eq!(0x10u32.fls(), 5);
+    /// assert_eq!(0xffffu32.fls(), 16);
+    /// assert_eq!(0x8000_0000u32.fls(), 32);
+    /// ```
+    fn fls(self) -> u32;
+}
+
+macro_rules! numext_impl {
+    ($($t:ty),+) => {
+        $(
+            impl NumExt for $t {
+                #[inline]
+                fn align_down(self, alignment: Self) -> Self {
+                    self & !alignment.wrapping_sub(1)
+                }
+
+                #[inline]
+                fn align_up(self, alignment: Self) -> Self {
+                    self.wrapping_add(alignment.wrapping_sub(1)).align_down(alignment)
+                }
+
+                #[inline]
+                fn fls(self) -> u32 {
+                    Self::BITS - self.leading_zeros()
+                }
+            }
+        )+
+    };
+}
+
+numext_impl!(usize, u8, u16, u32, u64, u128);

-- 
2.49.0