Re: [PATCH v2 2/3] rust: ptr: add Alignment::from_u64() for DeviceSize constants

[Gary Guo]

On Thu Mar 12, 2026 at 3:15 AM GMT, John Hubbard wrote:
> Alignment::new() takes a const usize, which means callers that work
> with DeviceSize constants still need to import the usize SZ_*
> variants. Add from_u64() so callers can write
> Alignment::from_u64(u64::SZ_128K) and stay entirely in the
> DeviceSize world.
>
> Both asserts evaluate at compile time in const context, so there is
> no runtime cost.
>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> ---
>  rust/kernel/ptr.rs | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
>
> diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs
> index 5b6a382637fe..b06f6b404a46 100644
> --- a/rust/kernel/ptr.rs
> +++ b/rust/kernel/ptr.rs
> @@ -76,6 +76,41 @@ pub const fn new_checked(align: usize) -> Option<Self> {
>          }
>      }
>  
> +    /// Creates an [`Alignment`] from a [`u64`] value.
> +    ///
> +    /// This is useful when the alignment comes from a [`DeviceSize`] constant
> +    /// rather than a [`usize`] literal.
> +    ///
> +    /// A build error is triggered if `align` is not a power of two, or if it
> +    /// exceeds [`usize::MAX`].
> +    ///
> +    /// [`DeviceSize`]: crate::sizes::DeviceSize
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::ptr::Alignment;
> +    /// use kernel::sizes::DeviceSize;
> +    ///
> +    /// let v = Alignment::from_u64(u64::SZ_128K);
> +    /// assert_eq!(v.as_usize(), 0x0002_0000);
> +    /// ```
> +    #[inline(always)]
> +    pub const fn from_u64(align: u64) -> Self {
> +        assert!(
> +            align.is_power_of_two(),
> +            "Provided alignment is not a power of two."
> +        );

This should be build_assert! to match the doc.

Best,
Gary

> +        assert!(
> +            align <= usize::MAX as u64,
> +            "Provided alignment exceeds usize::MAX."
> +        );
> +
> +        // INVARIANT: `align` is a power of two.
> +        // SAFETY: `align` is a power of two, fits in usize, and thus non-zero.
> +        Self(unsafe { NonZero::new_unchecked(align as usize) })
> +    }
> +
>      /// Returns the alignment of `T`.
>      ///
>      /// This is equivalent to [`align_of`], but with the return value provided as an [`Alignment`].