linux-kernel - [PATCH v5 05/23] rust: num: add the `fls` operation

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Message-Id: <20250612-nova-frts-v5-5-14ba7eaf166b@nvidia.com>
Date: Thu, 12 Jun 2025 23:01:33 +0900
From: Alexandre Courbot <acourbot@...dia.com>
To: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
 Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, 
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
 Benno Lossin <lossin@...nel.org>
Cc: John Hubbard <jhubbard@...dia.com>, Ben Skeggs <bskeggs@...dia.com>, 
 Joel Fernandes <joelagnelf@...dia.com>, Timur Tabi <ttabi@...dia.com>, 
 Alistair Popple <apopple@...dia.com>, linux-kernel@...r.kernel.org, 
 rust-for-linux@...r.kernel.org, nouveau@...ts.freedesktop.org, 
 dri-devel@...ts.freedesktop.org, Alexandre Courbot <acourbot@...dia.com>
Subject: [PATCH v5 05/23] rust: num: add the `fls` operation

Add an equivalent to the `fls` (Find Last Set bit) C function to Rust
unsigned types.

It is to be first used by the nova-core driver.

Signed-off-by: Alexandre Courbot <acourbot@...dia.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..934afe17719f789c569dbd54534adc2e26fe59f2 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),+) => {
+        $(
+            ::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 {
+                $t::BITS - v.leading_zeros()
+            }
+            }
+        )+
+    };
+}
+
+impl_fls!(usize, u8, u16, u32, u64, u128);

-- 
2.49.0