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

["Gary Guo" <gary@garyguo.net>]

On Wed Mar 4, 2026 at 3:47 AM GMT, John Hubbard wrote:
> On 2/23/26 6:20 AM, Danilo Krummrich wrote:
>> On Mon Feb 23, 2026 at 3:16 PM CET, Gary Guo wrote:
>>> On 2026-02-23 11:07, Danilo Krummrich wrote:
>>>> On Sun Feb 22, 2026 at 8:04 PM CET, John Hubbard wrote:
>>>>> On 2/21/26 11:46 PM, Gary Guo wrote:
>>>>>> 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.
>>>>>>>
>>>>> ...
>>>>>
>>>>>>> +#[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.
>>>>
>>>> I think the most common case is that ALIGN is const, but value is not.
>>>>
>>>> What about keeping the function as is (with the panic() replaced with a Result)
>>>> and also add
>>>>
>>>> 	#[inline(always)]
>>>> 	pub const fn const_expect<T: Copy>(opt: Result<T>, &'static str) -> T {
>>>> 	    match opt {
>>>> 	        Ok(v) => v,
>>>> 	        Err(_) => panic!(""),
>>>> 	    }
>>>> 	}
>>>>
>>>
>>> We already have `Alignable::align_up` for non-const cases, so this would only be used
>>> in const context and I don't see the need of having explicit const_expect?
>> 
>> Fair enough -- unfortunate we can't call this from const context.
>
> OK, so after the dust settled on this discussion, I *think* we ended up
> at this, which I have staged for an upcoming v6. Did I understand you both
> correctly?
>
> commit 1360a440272976df697140361537c5b697d602e0
> Author: John Hubbard <jhubbard@nvidia.com>
> Date:   Thu Feb 19 14:44:02 2026 -0800
>
>     rust: ptr: add const_align_up()
>     
>     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.
>     
>     Overflow causes a panic, which becomes a compile-time error in const
>     context. For runtime alignment with fallible overflow handling, callers
>     should use Alignable::align_up() instead.
>     
>     Suggested-by: Danilo Krummrich <dakr@kernel.org>
>     Suggested-by: Miguel Ojeda <ojeda@kernel.org>
>     Signed-off-by: John Hubbard <jhubbard@nvidia.com>
>
> diff --git a/rust/kernel/ptr.rs b/rust/kernel/ptr.rs
> index 5b6a382637fe..b8b0cb1e0b6b 100644
> --- a/rust/kernel/ptr.rs
> +++ b/rust/kernel/ptr.rs
> @@ -225,3 +225,31 @@ 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 in const context.
> +/// For runtime alignment with fallible overflow handling, use
> +/// [`Alignable::align_up`] instead.
> +///
> +/// # 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"),
> +    }
> +}

The implementation doesn't address any of my original comment and all my points
still apply.

Best,
Gary

> <blueforge> linux-github (nova-core-blackwell-v6)$ 
>
>
> thanks,