Re: [PATCH v5 21/38] rust: ptr: add const_align_up() and enable inline_const feature

[Gary Guo <gary@garyguo.net>]

On 2026-02-21 02:09, John Hubbard wrote:
> Add const_align_up<ALIGN>() to kernel::ptr as the const-compatible
> equivalent of Alignable::align_up(). This uses inline_const to validate
> the alignment at compile time with a clear error message.
> 
> Add inline_const to rust_allowed_features in scripts/Makefile.build,
> following the approach in [1].
> 
> [1] https://lore.kernel.org/rust-for-linux/20260206171253.2704684-2-gary@kernel.org/
> 
> Suggested-by: Danilo Krummrich <dakr@kernel.org>
> Suggested-by: Miguel Ojeda <ojeda@kernel.org>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
>  rust/kernel/ptr.rs     | 27 +++++++++++++++++++++++++++
>  scripts/Makefile.build |  2 +-
>  2 files changed, 28 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs
> index 5b6a382637fe..b3509caa5ad7 100644
> --- a/rust/kernel/ptr.rs
> +++ b/rust/kernel/ptr.rs
> @@ -225,3 +225,30 @@ fn align_up(self, alignment: Alignment) -> Option<Self> {
>  }
>  
>  impl_alignable_uint!(u8, u16, u32, u64, usize);
> +
> +/// Aligns `value` up to `ALIGN` at compile time.
> +///
> +/// This is the const-compatible equivalent of [`Alignable::align_up`].
> +/// `ALIGN` must be a power of two (enforced at compile time).
> +///
> +/// Panics on overflow, which becomes a compile-time error when called in a
> +/// const context.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::ptr::const_align_up;
> +/// use kernel::sizes::SZ_4K;
> +///
> +/// assert_eq!(const_align_up::<16>(0x4f), 0x50);
> +/// assert_eq!(const_align_up::<16>(0x40), 0x40);
> +/// assert_eq!(const_align_up::<SZ_4K>(1), SZ_4K);
> +/// ```
> +#[inline(always)]
> +pub const fn const_align_up<const ALIGN: usize>(value: usize) -> usize {
> +    const { assert!(ALIGN.is_power_of_two(), "ALIGN must be a power of two") };
> +    match value.checked_add(ALIGN - 1) {
> +        Some(v) => v & !(ALIGN - 1),
> +        None => panic!("const_align_up: overflow"),

This is wrong. Either this function is always used in const context, in which case
you take `ALIGN` as normal function parameter and use `build_assert` and `build_error`,
or this function can be called from runtime and you shouldn't have a panic call here.

Best,
Gary

> +    }
> +}
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 32e209bc7985..a58a7d079710 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -319,7 +319,7 @@ $(obj)/%.lst: $(obj)/%.c FORCE
>  #
>  # Please see https://github.com/Rust-for-Linux/linux/issues/2 for details on
>  # the unstable features in use.
> -rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons,offset_of_nested,raw_ref_op,used_with_arg
> +rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,inline_const,lint_reasons,offset_of_nested,raw_ref_op,used_with_arg
>  
>  # `--out-dir` is required to avoid temporaries being created by `rustc` in the
>  # current working directory, which may be not accessible in the out-of-tree