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

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Message-Id: <D8KB8CPBOIY1.10ATK35TV2X79@nvidia.com>
Date: Wed, 19 Mar 2025 23:25:03 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Daniel Almeida" <daniel.almeida@...labora.com>, "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>, "Benno Lossin"
 <benno.lossin@...ton.me>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 "Danilo Krummrich" <dakr@...nel.org>
Cc: <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>, "Fiona
 Behrens" <me@...enk.dev>
Subject: Re: [PATCH v4] rust: kernel: add support for bits/genmask macros

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@...enk.dev>
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> Signed-off-by: Daniel Almeida <daniel.almeida@...labora.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.