rust: bits: add support for bits/genmask macros

In light of bindgen being unable to generate bindings for macros, and
owing to the widespread use of these macros in drivers, manually define
the bit and genmask C macros in Rust.

The *_checked version of the functions provide runtime checking while
the const version performs compile-time assertions on the arguments via
the build_assert!() macro.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250714-topics-tyr-genmask2-v9-1-9e6422cbadb6@collabora.com
[ `expect`ed Clippy warning in doctests, hid single `use`, grouped
  examples. Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: danielalmeida-collabora

rust/kernel/bits.rs

@@ -0,0 +1,203 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Bit manipulation macros.
+//!
+//! C header: [`include/linux/bits.h`](srctree/include/linux/bits.h)
+
+use crate::prelude::*;
+use core::ops::RangeInclusive;
+use macros::paste;
+
+macro_rules! impl_bit_fn {
+    (
+        $ty:ty
+    ) => {
+        paste! {
+            /// Computes `1 << n` if `n` is in bounds, i.e.: if `n` is smaller than
+            /// the maximum number of bits supported by the type.
+            ///
+            /// Returns [`None`] otherwise.
+            #[inline]
+            pub fn [<checked_bit_ $ty>](n: u32) -> Option<$ty> {
+                (1 as $ty).checked_shl(n)
+            }
+
+            /// Computes `1 << n` by performing a compile-time assertion that `n` is
+            /// in bounds.
+            ///
+            /// This version is the default and should be used if `n` is known at
+            /// compile time.
+            #[inline]
+            pub const fn [<bit_ $ty>](n: u32) -> $ty {
+                build_assert!(n < <$ty>::BITS);
+                (1 as $ty) << n
+            }
+        }
+    };
+}
+
+impl_bit_fn!(u64);
+impl_bit_fn!(u32);
+impl_bit_fn!(u16);
+impl_bit_fn!(u8);
+
+macro_rules! impl_genmask_fn {
+    (
+        $ty:ty,
+        $(#[$genmask_checked_ex:meta])*,
+        $(#[$genmask_ex:meta])*
+    ) => {
+        paste! {
+            /// Creates a contiguous bitmask for the given range by validating
+            /// the range at runtime.
+            ///
+            /// Returns [`None`] if the range is invalid, i.e.: if the start is
+            /// greater than the end or if the range is outside of the
+            /// representable range for the type.
+            $(#[$genmask_checked_ex])*
+            #[inline]
+            pub fn [<genmask_checked_ $ty>](range: RangeInclusive<u32>) -> Option<$ty> {
+                let start = *range.start();
+                let end = *range.end();
+
+                if start > end {
+                    return None;
+                }
+
+                let high = [<checked_bit_ $ty>](end)?;
+                let low = [<checked_bit_ $ty>](start)?;
+                Some((high | (high - 1)) & !(low - 1))
+            }
+
+            /// Creates a compile-time contiguous bitmask for the given range by
+            /// performing a compile-time assertion that the range is valid.
+            ///
+            /// This version is the default and should be used if the range is known
+            /// at compile time.
+            $(#[$genmask_ex])*
+            #[inline]
+            pub const fn [<genmask_ $ty>](range: RangeInclusive<u32>) -> $ty {
+                let start = *range.start();
+                let end = *range.end();
+
+                build_assert!(start <= end);
+
+                let high = [<bit_ $ty>](end);
+                let low = [<bit_ $ty>](start);
+                (high | (high - 1)) & !(low - 1)
+            }
+        }
+    };
+}
+
+impl_genmask_fn!(
+    u64,
+    /// # Examples
+    ///
+    /// ```
+    /// # #![expect(clippy::reversed_empty_ranges)]
+    /// # use kernel::bits::genmask_checked_u64;
+    /// assert_eq!(genmask_checked_u64(0..=0), Some(0b1));
+    /// assert_eq!(genmask_checked_u64(0..=63), Some(u64::MAX));
+    /// assert_eq!(genmask_checked_u64(21..=39), Some(0x0000_00ff_ffe0_0000));
+    ///
+    /// // `80` is out of the supported bit range.
+    /// assert_eq!(genmask_checked_u64(21..=80), None);
+    ///
+    /// // Invalid range where the start is bigger than the end.
+    /// assert_eq!(genmask_checked_u64(15..=8), None);
+    /// ```
+    ,
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::bits::genmask_u64;
+    /// assert_eq!(genmask_u64(21..=39), 0x0000_00ff_ffe0_0000);
+    /// assert_eq!(genmask_u64(0..=0), 0b1);
+    /// assert_eq!(genmask_u64(0..=63), u64::MAX);
+    /// ```
+);
+
+impl_genmask_fn!(
+    u32,
+    /// # Examples
+    ///
+    /// ```
+    /// # #![expect(clippy::reversed_empty_ranges)]
+    /// # use kernel::bits::genmask_checked_u32;
+    /// assert_eq!(genmask_checked_u32(0..=0), Some(0b1));
+    /// assert_eq!(genmask_checked_u32(0..=31), Some(u32::MAX));
+    /// assert_eq!(genmask_checked_u32(21..=31), Some(0xffe0_0000));
+    ///
+    /// // `40` is out of the supported bit range.
+    /// assert_eq!(genmask_checked_u32(21..=40), None);
+    ///
+    /// // Invalid range where the start is bigger than the end.
+    /// assert_eq!(genmask_checked_u32(15..=8), None);
+    /// ```
+    ,
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::bits::genmask_u32;
+    /// assert_eq!(genmask_u32(21..=31), 0xffe0_0000);
+    /// assert_eq!(genmask_u32(0..=0), 0b1);
+    /// assert_eq!(genmask_u32(0..=31), u32::MAX);
+    /// ```
+);
+
+impl_genmask_fn!(
+    u16,
+    /// # Examples
+    ///
+    /// ```
+    /// # #![expect(clippy::reversed_empty_ranges)]
+    /// # use kernel::bits::genmask_checked_u16;
+    /// assert_eq!(genmask_checked_u16(0..=0), Some(0b1));
+    /// assert_eq!(genmask_checked_u16(0..=15), Some(u16::MAX));
+    /// assert_eq!(genmask_checked_u16(6..=15), Some(0xffc0));
+    ///
+    /// // `20` is out of the supported bit range.
+    /// assert_eq!(genmask_checked_u16(6..=20), None);
+    ///
+    /// // Invalid range where the start is bigger than the end.
+    /// assert_eq!(genmask_checked_u16(10..=5), None);
+    /// ```
+    ,
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::bits::genmask_u16;
+    /// assert_eq!(genmask_u16(6..=15), 0xffc0);
+    /// assert_eq!(genmask_u16(0..=0), 0b1);
+    /// assert_eq!(genmask_u16(0..=15), u16::MAX);
+    /// ```
+);
+
+impl_genmask_fn!(
+    u8,
+    /// # Examples
+    ///
+    /// ```
+    /// # #![expect(clippy::reversed_empty_ranges)]
+    /// # use kernel::bits::genmask_checked_u8;
+    /// assert_eq!(genmask_checked_u8(0..=0), Some(0b1));
+    /// assert_eq!(genmask_checked_u8(0..=7), Some(u8::MAX));
+    /// assert_eq!(genmask_checked_u8(6..=7), Some(0xc0));
+    ///
+    /// // `10` is out of the supported bit range.
+    /// assert_eq!(genmask_checked_u8(6..=10), None);
+    ///
+    /// // Invalid range where the start is bigger than the end.
+    /// assert_eq!(genmask_checked_u8(5..=2), None);
+    /// ```
+    ,
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::bits::genmask_u8;
+    /// assert_eq!(genmask_u8(6..=7), 0xc0);
+    /// assert_eq!(genmask_u8(0..=0), 0b1);
+    /// assert_eq!(genmask_u8(0..=7), u8::MAX);
+    /// ```
+);

rust/kernel/lib.rs

@@ -54,6 +54,7 @@ pub use ffi;
 pub mod alloc;
 #[cfg(CONFIG_AUXILIARY_BUS)]
 pub mod auxiliary;
+pub mod bits;
 #[cfg(CONFIG_BLOCK)]
 pub mod block;
 #[doc(hidden)]