Hi Alex, > On 4 Aug 2025, at 08:45, Alexandre Courbot <[email protected]> wrote: > > 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 <[email protected]> > --- > 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
Nit: I’d go with “align up or align down operations” instead of using a slash. > +/// [`align_down!`] macros. > +/// > +/// Heavily inspired by the [`Alignment`] nightly feature from the Rust > standard library, and > +/// hopefully to be eventually replaced by it. It’s a bit hard to parse this. Also, I wonder if we should standardize some syntax for TODOs so we can parse them using a script? This way we can actually keep track and perhaps pipe them to our GitHub page as “good first issues” or just regular issues. I guess a simple "// TODO: “ here will do, for example. > +/// > +/// [`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. I half get this, but still: If it did what? > + /// > + /// # 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 > > Everything else looks fine, IMHO. — Daniel
