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

[Alexandre Courbot]

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 at 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..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).