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

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Message-Id: <DANR43CR8X87.1YWHJK7P75TPQ@nvidia.com>
Date: Mon, 16 Jun 2025 15:41:44 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Benno Lossin" <lossin@...nel.org>, "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>
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>
Subject: Re: [PATCH v5 05/23] rust: num: add the `fls` operation

On Sun Jun 15, 2025 at 4:16 AM JST, Benno Lossin wrote:
> 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?

I will consider alongside `prev_multiple_of` that we discussed during v4. :)

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

We can - but as for `align_up`/`next_multiple_of`, I am not sure which
naming scheme (kernel-like or closer to Rust conventions) is favored in
such cases, and so far it seems to come down to personal preference. I
tend to think that staying close to kernel conventions make it easier to
understand when a function is the equivalent of a C one, but whichever
policy we adopt it would be nice to codify it somewhere (apologies if it
is already and I missed it).