linux-kernel - Re: [PATCH v5 2/7] rust: num: add `shr` and `shl` methods to `Bounded`

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Message-Id: <D87FBE7D-C320-426A-96E2-DB23975CF895@collabora.com>
Date: Fri, 6 Feb 2026 15:23:44 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Danilo Krummrich <dakr@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Miguel Ojeda <ojeda@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>,
 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 v5 2/7] rust: num: add `shr` and `shl` methods to
 `Bounded`



> On 29 Jan 2026, at 10:32, Alexandre Courbot <acourbot@...dia.com> 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>
> Reviewed-by: Gary Guo <gary@...yguo.net>
> Tested-by: Dirk Behme <dirk.behme@...bosch.com>
> Acked-by: Miguel Ojeda <ojeda@...nel.org>
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
> ---
> rust/kernel/num/bounded.rs | 42 ++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 42 insertions(+)
> 
> diff --git a/rust/kernel/num/bounded.rs b/rust/kernel/num/bounded.rs
> index f870080af8ac..4b929762d5c2 100644
> --- a/rust/kernel/num/bounded.rs
> +++ b/rust/kernel/num/bounded.rs
> @@ -470,6 +470,48 @@ pub fn cast<U>(self) -> Bounded<U, N>
>         // `N` bits, and with the same signedness.
>         Bounded::__new(value)
>     }
> +
> +    /// Right-shifts `self` by `SHIFT` and returns the result as a `Bounded<_, RES>`, where `RES >=
> +    /// N - SHIFT`.
> +    ///
> +    /// # 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) }
> +
> +        // 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<_, RES>`, where `RES >=
> +    /// N + SHIFT`.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::num::Bounded;
> +    ///
> +    /// let v = Bounded::<u32, 8>::new::<0xff>();
> +    /// let v_shifted: Bounded::<u32, 16> = v.shl::<8, _>();
> +    ///
> +    /// assert_eq!(v_shifted.get(), 0xff00);
> +    /// ```
> +    pub fn shl<const SHIFT: u32, const RES: u32>(self) -> Bounded<T, RES> {
> +        const { assert!(RES >= N + SHIFT) }
> +
> +        // SAFETY: We shift the value left by `SHIFT`, augmenting 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) }
> +    }
> }
> 
> impl<T, const N: u32> Deref for Bounded<T, N>
> 
> -- 
> 2.52.0
> 
> 

Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>