Re: [PATCH v4] rust: kernel: add support for bits/genmask macros

["Alexandre Courbot" <acourbot@nvidia.com>]

On Wed Mar 19, 2025 at 11:25 PM JST, Alexandre Courbot wrote:
> Hi Daniel,
>
> On Wed Mar 19, 2025 at 12:34 AM JST, Daniel Almeida wrote:
>> In light of bindgen being unable to generate bindings for macros,
>> manually define the bit and genmask C macros in Rust.
>>
>> Bit and genmask are frequently used in drivers, and are simple enough to
>> just be redefined. Their implementation is also unlikely to ever change.
>>
>> These macros are converted from their kernel equivalent. Since
>> genmask_u32 and genmask_u64 are thought to suffice, these two versions
>> are implemented as const fns instead of declarative macros.
>>
>> Reviewed-by: Fiona Behrens <me@kloenk.dev>
>> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
>> Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
>
> Cool, I'll probably depend on this for the next version of my register
> layout patch [1].
>
> [1]
> https://lore.kernel.org/rust-for-linux/20250313-registers-v1-1-8d498537e8b2@nvidia.com/
>
>> ---
>> Changes in v4:
>> - Split bits into bits_u32 and bits_u64
>> - Added r-b's
>> - Rebased on top of rust-next
>> - Link to v3: https://lore.kernel.org/r/20250121-topic-panthor-rs-genmask-v3-1-5c3bdf21ce05@collabora.com
>>
>> Changes in v3:
>> - Changed from declarative macro to const fn
>> - Added separate versions for u32 and u64
>> - Link to v2: https://lore.kernel.org/r/20241024-topic-panthor-rs-genmask-v2-1-85237c1f0cea@collabora.com
>>
>> Changes in v2:
>>
>> - Added ticks around `BIT`, and `h >=l` (Thanks, Benno).
>> - Decided to keep the arguments as `expr`, as I see no issues with that
>> - Added a proper example, with an assert_eq!() (Thanks, Benno)
>> - Fixed the condition h <= l, which should be h >= l.
>> - Checked that the assert for the condition above is described in the
>>   docs.
>> ---
>>  rust/kernel/bits.rs | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
>>  rust/kernel/lib.rs  |  1 +
>>  2 files changed, 50 insertions(+)
>>
>> diff --git a/rust/kernel/bits.rs b/rust/kernel/bits.rs
>> new file mode 100644
>> index 0000000000000000000000000000000000000000..ec13bb480082de9584b7d23c78df0e76d0fbf132
>> --- /dev/null
>> +++ b/rust/kernel/bits.rs
>> @@ -0,0 +1,49 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +//! Bit manipulation macros.
>> +//!
>> +//! C header: [`include/linux/bits.h`](srctree/include/linux/bits.h)
>> +
>> +/// Produces a literal where bit `n` is set.
>> +///
>> +/// Equivalent to the kernel's `BIT` macro.
>> +pub const fn bit_u32(n: u32) -> u32 {
>> +    1 << n
>> +}
>> +
>> +/// Produces a literal where bit `n` is set.
>> +///
>> +/// Equivalent to the kernel's `BIT` macro.
>> +pub const fn bit_u64(n: u32) -> u64 {
>> +    1u64 << n as u64
>> +}
>> +
>> +/// Create a contiguous bitmask starting at bit position `l` and ending at
>> +/// position `h`, where `h >= l`.
>> +///
>> +/// # Examples
>> +/// ```
>> +///     use kernel::bits::genmask_u64;
>> +///     let mask = genmask_u64(39, 21);
>> +///     assert_eq!(mask, 0x000000ffffe00000);
>> +/// ```
>> +///
>> +pub const fn genmask_u64(h: u32, l: u32) -> u64 {
>> +    assert!(h >= l);
>> +    (!0u64 - (1u64 << l) + 1) & (!0u64 >> (64 - 1 - h))
>> +}
>> +
>> +/// Create a contiguous bitmask starting at bit position `l` and ending at
>> +/// position `h`, where `h >= l`.
>> +///
>> +/// # Examples
>> +/// ```
>> +///     use kernel::bits::genmask_u32;
>> +///     let mask = genmask_u32(9, 0);
>> +///     assert_eq!(mask, 0x000003ff);
>> +/// ```
>> +///
>> +pub const fn genmask_u32(h: u32, l: u32) -> u32 {
>> +    assert!(h >= l);
>> +    (!0u32 - (1u32 << l) + 1) & (!0u32 >> (32 - 1 - h))
>> +}
>
> Would it make sense to also have u16 and u8 variants? I guess one can
> always use genmask_u32 and then cast to the desired type though.

Forgot to mention:

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>