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Date: Mon, 23 Feb 2026 15:20:40 +0100
Message-Id: <DGMEORS8PL1T.242O5BRE6ZSM6@kernel.org>
Subject: Re: [PATCH v5 21/38] rust: ptr: add const_align_up() and enable
 inline_const feature
Cc: "John Hubbard" <jhubbard@nvidia.com>, "Alexandre Courbot"
 <acourbot@nvidia.com>, "Joel Fernandes" <joelagnelf@nvidia.com>, "Timur
 Tabi" <ttabi@nvidia.com>, "Alistair Popple" <apopple@nvidia.com>, "Eliot
 Courtney" <ecourtney@nvidia.com>, "Zhi Wang" <zhiw@nvidia.com>, "David
 Airlie" <airlied@gmail.com>, "Simona Vetter" <simona@ffwll.ch>, "Bjorn
 Helgaas" <bhelgaas@google.com>, "Miguel Ojeda" <ojeda@kernel.org>, "Alex
 Gaynor" <alex.gaynor@gmail.com>, "Boqun Feng" <boqun.feng@gmail.com>,
 =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, "Benno Lossin"
 <lossin@kernel.org>, "Andreas Hindborg" <a.hindborg@kernel.org>, "Alice
 Ryhl" <aliceryhl@google.com>, "Trevor Gross" <tmgross@umich.edu>,
 <nouveau@lists.freedesktop.org>, <rust-for-linux@vger.kernel.org>, "LKML"
 <linux-kernel@vger.kernel.org>
To: "Gary Guo" <gary@garyguo.net>
From: "Danilo Krummrich" <dakr@kernel.org>
References: <20260221020952.412352-1-jhubbard@nvidia.com>
 <20260221020952.412352-22-jhubbard@nvidia.com>
 <ee76d46cc0cab88ce6dbdeda6a8850dc@garyguo.net>
 <89b240a4-33ec-44dc-8f7c-1a3f8d379e0d@nvidia.com>
 <DGMAKO04ZM3J.1NJDI9SYE354S@kernel.org>
 <ba7aec533e083622be9055e77c30fee0@garyguo.net>
In-Reply-To: <ba7aec533e083622be9055e77c30fee0@garyguo.net>

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 valida=
te
>>>>> the alignment at compile time with a clear error message.
>>>>>
>>> ...
>>>
>>>>> +#[inline(always)]
>>>>> +pub const fn const_align_up<const ALIGN: usize>(value: usize) -> usi=
ze {
>>>>> +    const { assert!(ALIGN.is_power_of_two(), "ALIGN must be a power =
of two") };
>>>>> +    match value.checked_add(ALIGN - 1) {
>>>>> +        Some(v) =3D> v & !(ALIGN - 1),
>>>>> +        None =3D> panic!("const_align_up: overflow"),
>>>>=20
>>>> This is wrong. Either this function is always used in const context, i=
n which case
>>>> you take `ALIGN` as normal function parameter and use `build_assert` a=
nd `build_error`,
>>>> or this function can be called from runtime and you shouldn't have a p=
anic call here.
>>=20
>> I think the most common case is that ALIGN is const, but value is not.
>>=20
>> What about keeping the function as is (with the panic() replaced with a =
Result)
>> and also add
>>=20
>> 	#[inline(always)]
>> 	pub const fn const_expect<T: Copy>(opt: Result<T>, &'static str) -> T {
>> 	    match opt {
>> 	        Ok(v) =3D> v,
>> 	        Err(_) =3D> panic!(""),
>> 	    }
>> 	}
>>=20
>
> 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.