Re: [PATCH v3] rust: kernel: add support for bits/genmask macros

[Fiona Behrens]

On 21 Jan 2025, at 16:29, 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.
>
> ---
> 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@xxxxxxxxxxxxx
>
> 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.
>
> Signed-off-by: Daniel Almeida <daniel.almeida@xxxxxxxxxxxxx>

Reviewed-by: Fiona Behrens <me@xxxxxxxxxx>

> ---
>  rust/kernel/bits.rs | 42 ++++++++++++++++++++++++++++++++++++++++++
>  rust/kernel/lib.rs  |  1 +
>  2 files changed, 43 insertions(+)
>
> diff --git a/rust/kernel/bits.rs b/rust/kernel/bits.rs
> new file mode 100644
> index 0000000000000000000000000000000000000000..32049abd2078525be25b57b8e6b5948d1eecbf9f
> --- /dev/null
> +++ b/rust/kernel/bits.rs
> @@ -0,0 +1,42 @@
> +// 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(n: u32) -> u64 {
> +    1 << n
> +}
> +
> +/// 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))
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index b5f4b3ce6b48203507f89bcc4b0bf7b076be6247..4f256941ac8fbd1263d5c014a0ce2f5ffb9d1849 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -27,6 +27,7 @@
>  extern crate self as kernel;
>
>  pub mod alloc;
> +pub mod bits;
>  #[cfg(CONFIG_BLOCK)]
>  pub mod block;
>  mod build_assert;
>
> ---
> base-commit: 91e21479c81dd4e9e22a78d7446f92f6b96a7284
> change-id: 20241023-topic-panthor-rs-genmask-fabc573fef43
>
> Best regards,
> -- 
> Daniel Almeida <daniel.almeida@xxxxxxxxxxxxx>