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Message-Id: <20250804-num-v2-2-a96b9ca6eb02@nvidia.com>
Date: Mon, 04 Aug 2025 20:45:25 +0900
From: Alexandre Courbot <acourbot@...dia.com>
To: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
Björn Roy Baron <bjorn3_gh@...tonmail.com>,
Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>,
Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
Danilo Krummrich <dakr@...nel.org>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
nouveau@...ts.freedesktop.org, Alexandre Courbot <acourbot@...dia.com>
Subject: [PATCH v2 2/4] rust: add `Alignment` type
Alignment operations are very common in the kernel. Since they are
always performed using a power of two value, enforcing this invariant
through a dedicated type leads to less bugs and can lead to improved
generated code.
Introduce the `Alignment` type, inspired by the nightly Rust feature of
the same name. It provides the same interface as its upstream namesake,
while extending it with `align_up` and `align_down` operations that are
usable on any integer type.
Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
---
rust/kernel/lib.rs | 1 +
rust/kernel/ptr.rs | 213 +++++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 214 insertions(+)
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 2955f65da1278dd4cba1e4272ff178b8211a892c..0e66b55cde66ee1b274862cd78ad465a572dc5d9 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -100,6 +100,7 @@
pub mod platform;
pub mod prelude;
pub mod print;
+pub mod ptr;
pub mod rbtree;
pub mod revocable;
pub mod security;
diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs
new file mode 100644
index 0000000000000000000000000000000000000000..6d941db58944619ea5b05676af06981a3ceaaca8
--- /dev/null
+++ b/rust/kernel/ptr.rs
@@ -0,0 +1,213 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Types and functions to work with pointers and addresses.
+
+use core::fmt::Debug;
+use core::num::NonZero;
+use core::ops::{BitAnd, Not};
+
+use crate::build_assert;
+use crate::num::CheckedAdd;
+
+/// Type representing an alignment, which is always a power of two.
+///
+/// It be used to validate that a given value is a valid alignment, and to perform masking and
+/// align down/up operations. The alignment operations are done using the [`align_up!`] and
+/// [`align_down!`] macros.
+///
+/// Heavily inspired by the [`Alignment`] nightly feature from the Rust standard library, and
+/// hopefully to be eventually replaced by it.
+///
+/// [`Alignment`]: https://github.com/rust-lang/rust/issues/102070
+///
+/// # Invariants
+///
+/// An alignment is always a power of two.
+#[repr(transparent)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Alignment(NonZero<usize>);
+
+impl Alignment {
+ /// Validates that `align` is a power of two at build-time, and returns an [`Alignment`] of the
+ /// same value.
+ ///
+ /// A build error is triggered if `align` cannot be asserted to be a power of two.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// let v = Alignment::new(16);
+ /// assert_eq!(v.as_usize(), 16);
+ /// ```
+ #[inline(always)]
+ pub const fn new(align: usize) -> Self {
+ build_assert!(align.is_power_of_two());
+
+ // INVARIANT: `align` is a power of two.
+ // SAFETY: `align` is a power of two, and thus non-zero.
+ Self(unsafe { NonZero::new_unchecked(align) })
+ }
+
+ /// Validates that `align` is a power of two at runtime, and returns an
+ /// [`Alignment`] of the same value.
+ ///
+ /// [`None`] is returned if `align` was not a power of two.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::new_checked(16), Some(Alignment::new(16)));
+ /// assert_eq!(Alignment::new_checked(15), None);
+ /// assert_eq!(Alignment::new_checked(1), Some(Alignment::new(1)));
+ /// assert_eq!(Alignment::new_checked(0), None);
+ /// ```
+ #[inline(always)]
+ pub const fn new_checked(align: usize) -> Option<Self> {
+ if align.is_power_of_two() {
+ // INVARIANT: `align` is a power of two.
+ // SAFETY: `align` is a power of two, and thus non-zero.
+ Some(Self(unsafe { NonZero::new_unchecked(align) }))
+ } else {
+ None
+ }
+ }
+
+ /// Returns the alignment of `T`.
+ #[inline(always)]
+ pub const fn of<T>() -> Self {
+ // INVARIANT: `align_of` always returns a power of 2.
+ Self(unsafe { NonZero::new_unchecked(align_of::<T>()) })
+ }
+
+ /// Returns the base-2 logarithm of the alignment.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::of::<u8>().log2(), 0);
+ /// assert_eq!(Alignment::new(16).log2(), 4);
+ /// ```
+ #[inline(always)]
+ pub const fn log2(self) -> u32 {
+ self.0.ilog2()
+ }
+
+ /// Returns this alignment as a [`NonZero`].
+ ///
+ /// It is guaranteed to be a power of two.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::new(16).as_nonzero().get(), 16);
+ /// ```
+ #[inline(always)]
+ pub const fn as_nonzero(self) -> NonZero<usize> {
+ if !self.0.is_power_of_two() {
+ // SAFETY: per the invariants, `self.0` is always a power of two so this block will
+ // never be reached.
+ unsafe { core::hint::unreachable_unchecked() }
+ }
+ self.0
+ }
+
+ /// Returns this alignment as a `usize`.
+ ///
+ /// It is guaranteed to be a power of two.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::new(16).as_usize(), 16);
+ /// ```
+ #[inline(always)]
+ pub const fn as_usize(self) -> usize {
+ self.as_nonzero().get()
+ }
+
+ /// Returns the mask corresponding to `self.as_usize() - 1`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::new(0x10).mask(), 0xf);
+ /// ```
+ #[inline(always)]
+ pub const fn mask(self) -> usize {
+ // INVARIANT: `self.as_usize()` is guaranteed to be a power of two (i.e. non-zero), thus
+ // `1` can safely be substracted from it.
+ self.as_usize() - 1
+ }
+
+ /// Aligns `value` down to this alignment.
+ ///
+ /// If the alignment contained in `self` is too large for `T`, then `0` is returned, which
+ /// is correct as it is also the result that would have been returned if it did.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::new(0x10).align_down(0x2f), 0x20);
+ /// assert_eq!(Alignment::new(0x10).align_down(0x30), 0x30);
+ /// assert_eq!(Alignment::new(0x1000).align_down(0xf0u8), 0x0);
+ /// ```
+ #[inline(always)]
+ pub fn align_down<T>(self, value: T) -> T
+ where
+ T: TryFrom<usize> + BitAnd<Output = T> + Not<Output = T> + Default,
+ {
+ T::try_from(self.mask())
+ .map(|mask| value & !mask)
+ .unwrap_or(T::default())
+ }
+
+ /// Aligns `value` up to this alignment, returning `None` if aligning pushes the result above
+ /// the limits of `value`'s type.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use kernel::ptr::Alignment;
+ ///
+ /// assert_eq!(Alignment::new(0x10).align_up(0x4f), Some(0x50));
+ /// assert_eq!(Alignment::new(0x10).align_up(0x40), Some(0x40));
+ /// assert_eq!(Alignment::new(0x10).align_up(0x0), Some(0x0));
+ /// assert_eq!(Alignment::new(0x10).align_up(u8::MAX), None);
+ /// assert_eq!(Alignment::new(0x100).align_up(0x10u8), None);
+ /// assert_eq!(Alignment::new(0x100).align_up(0x0u8), Some(0x0));
+ /// ```
+ #[inline(always)]
+ pub fn align_up<T>(self, value: T) -> Option<T>
+ where
+ T: TryFrom<usize>
+ + BitAnd<Output = T>
+ + Not<Output = T>
+ + Default
+ + PartialEq
+ + Copy
+ + CheckedAdd,
+ {
+ let aligned_down = self.align_down(value);
+ if value == aligned_down {
+ Some(aligned_down)
+ } else {
+ T::try_from(self.as_usize())
+ .ok()
+ .and_then(|align| aligned_down.checked_add(align))
+ }
+ }
+}
--
2.50.1
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