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Date: Sat, 14 Jun 2025 21:16:10 +0200
Message-Id: <DAMHWN6ML8A1.2AUE4UWR58KR2@kernel.org>
To: "Alexandre Courbot" <acourbot@nvidia.com>, "Miguel Ojeda"
 <ojeda@kernel.org>, "Alex Gaynor" <alex.gaynor@gmail.com>, "Boqun Feng"
 <boqun.feng@gmail.com>, "Gary Guo" <gary@garyguo.net>,
 =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, "Andreas
 Hindborg" <a.hindborg@kernel.org>, "Alice Ryhl" <aliceryhl@google.com>,
 "Trevor Gross" <tmgross@umich.edu>, "Danilo Krummrich" <dakr@kernel.org>,
 "David Airlie" <airlied@gmail.com>, "Simona Vetter" <simona@ffwll.ch>,
 "Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>, "Maxime Ripard"
 <mripard@kernel.org>, "Thomas Zimmermann" <tzimmermann@suse.de>
Cc: "John Hubbard" <jhubbard@nvidia.com>, "Ben Skeggs" <bskeggs@nvidia.com>,
 "Joel Fernandes" <joelagnelf@nvidia.com>, "Timur Tabi" <ttabi@nvidia.com>,
 "Alistair Popple" <apopple@nvidia.com>, <linux-kernel@vger.kernel.org>,
 <rust-for-linux@vger.kernel.org>, <nouveau@lists.freedesktop.org>,
 <dri-devel@lists.freedesktop.org>
Subject: Re: [PATCH v5 05/23] rust: num: add the `fls` operation
From: "Benno Lossin" <lossin@kernel.org>
X-Mailer: aerc 0.20.1
References: <20250612-nova-frts-v5-0-14ba7eaf166b@nvidia.com>
 <20250612-nova-frts-v5-5-14ba7eaf166b@nvidia.com>
In-Reply-To: <20250612-nova-frts-v5-5-14ba7eaf166b@nvidia.com>

On Thu Jun 12, 2025 at 4:01 PM CEST, Alexandre Courbot wrote:
> Add an equivalent to the `fls` (Find Last Set bit) C function to Rust
> unsigned types.

Have you tried to upstream this?

> It is to be first used by the nova-core driver.
>
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
> ---
>  rust/kernel/num.rs | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
>
> diff --git a/rust/kernel/num.rs b/rust/kernel/num.rs
> index ee0f67ad1a89e69f5f8d2077eba5541b472e7d8a..934afe17719f789c569dbd545=
34adc2e26fe59f2 100644
> --- a/rust/kernel/num.rs
> +++ b/rust/kernel/num.rs
> @@ -171,3 +171,34 @@ fn borrow(&self) -> &T {
>          &self.0
>      }
>  }
> +
> +macro_rules! impl_fls {
> +    ($($t:ty),+) =3D> {
> +        $(
> +            ::kernel::macros::paste! {
> +            /// Find Last Set Bit: return the 1-based index of the last =
(i.e. most significant) set
> +            /// bit in `v`.
> +            ///
> +            /// Equivalent to the C `fls` function.
> +            ///
> +            /// # Examples
> +            ///
> +            /// ```
> +            /// use kernel::num::fls_u32;
> +            ///
> +            /// assert_eq!(fls_u32(0x0), 0);
> +            /// assert_eq!(fls_u32(0x1), 1);
> +            /// assert_eq!(fls_u32(0x10), 5);
> +            /// assert_eq!(fls_u32(0xffff), 16);
> +            /// assert_eq!(fls_u32(0x8000_0000), 32);
> +            /// ```
> +            #[inline(always)]
> +            pub const fn [<fls_ $t>](v: $t) -> u32 {

Can we name this `find_last_set_bit_ $t`? When the upstream function
lands, we should also rename this one.

---
Cheers,
Benno

> +                $t::BITS - v.leading_zeros()
> +            }
> +            }
> +        )+
> +    };
> +}
> +
> +impl_fls!(usize, u8, u16, u32, u64, u128);