linux-kernel - Re: [PATCH v2 2/5] rust: num: add `shr` and `shl` methods to `Bounded`

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Message-Id: <DFY76NDCE2HU.AJ6VJ1ETD944@nvidia.com>
Date: Mon, 26 Jan 2026 12:23:53 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Gary Guo" <gary@...yguo.net>
Cc: "Danilo Krummrich" <dakr@...nel.org>, "Alice Ryhl"
 <aliceryhl@...gle.com>, "Daniel Almeida" <daniel.almeida@...labora.com>,
 "Miguel Ojeda" <ojeda@...nel.org>, "Boqun Feng" <boqun.feng@...il.com>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Benno Lossin"
 <lossin@...nel.org>, "Andreas Hindborg" <a.hindborg@...nel.org>, "Trevor
 Gross" <tmgross@...ch.edu>, "Yury Norov" <yury.norov@...il.com>, "John
 Hubbard" <jhubbard@...dia.com>, "Alistair Popple" <apopple@...dia.com>,
 "Joel Fernandes" <joelagnelf@...dia.com>, "Timur Tabi" <ttabi@...dia.com>,
 "Edwin Peer" <epeer@...dia.com>, "Eliot Courtney" <ecourtney@...dia.com>,
 "Dirk Behme" <dirk.behme@...bosch.com>, "Steven Price"
 <steven.price@....com>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/5] rust: num: add `shr` and `shl` methods to
 `Bounded`

On Wed Jan 21, 2026 at 11:12 PM JST, Gary Guo wrote:
> On Wed Jan 21, 2026 at 7:23 AM GMT, Alexandre Courbot wrote:
>> Shifting a `Bounded` left or right changes the number of bits required
>> to represent the value. Add methods that perform the shift and return a
>> `Bounded` with the appropriately adjusted bit width.
>>
>> These methods are particularly useful for bitfield extraction.
>>
>> Suggested-by: Alice Ryhl <aliceryhl@...gle.com>
>> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
>> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
>> ---
>>  rust/kernel/num/bounded.rs | 40 ++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 40 insertions(+)
>>
>> diff --git a/rust/kernel/num/bounded.rs b/rust/kernel/num/bounded.rs
>> index f870080af8ac..8782535770f1 100644
>> --- a/rust/kernel/num/bounded.rs
>> +++ b/rust/kernel/num/bounded.rs
>> @@ -470,6 +470,46 @@ pub fn cast<U>(self) -> Bounded<U, N>
>>          // `N` bits, and with the same signedness.
>>          Bounded::__new(value)
>
> This patch doesn't apply cleanly. Looks like you send it from the wrong base
> commit.
>
> The __new call here is still safe while your code has `unsafe {}` in it.

The cover letter mentions that the base for this patchset is
`driver-core-next`, since `register!` lands in `kernel/rust/io` and we
need to rebase on top of Zhi's Io patchset, which will land there as
well.

This indeed creates a minor conflict with `rust-next` as it includes a
couple of patches for `bounded.rs`.

>
>>      }
>> +
>> +    /// Right-shifts `self` by `SHIFT` and returns the result as a `Bounded<_, { N - SHIFT }>`.
>
> The returned bound can be larger given the assert below?

Indeed, I forgot to update this comment - thanks!

>
>> +    ///
>> +    /// # Examples
>> +    ///
>> +    /// ```
>> +    /// use kernel::num::Bounded;
>> +    ///
>> +    /// let v = Bounded::<u32, 16>::new::<0xff00>();
>> +    /// let v_shifted: Bounded::<u32, 8> = v.shr::<8, _>();
>> +    ///
>> +    /// assert_eq!(v_shifted.get(), 0xff);
>> +    /// ```
>> +    pub fn shr<const SHIFT: u32, const RES: u32>(self) -> Bounded<T, RES> {
>> +        const { assert!(RES >= N - SHIFT) }
>
> Quite surprised that rustfmt didn't ask for the block to be expanded into
> multiple lines.
>
> I think we probably want to create a new assert macro for this pattern
> (obviously this doesn't block this patch).
>
>> +
>> +        // SAFETY: we shift the value right by `SHIFT`, reducing the number of bits needed to
>> +        // represent the shifted value by as much, and just asserted that `RES == N - SHIFT`.
>> +        unsafe { Bounded::__new(self.0 >> SHIFT) }
>> +    }
>> +
>> +    /// Left-shifts `self` by `SHIFT` and returns the result as a `Bounded<_, { N + SHIFT }>`.
>
> Same, the bound can be actually larger.

Fixed, thanks.