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

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Message-ID: <15e93515-400b-4cd3-8f35-afcba24fb32b@proton.me>
Date: Thu, 22 Aug 2024 20:27:48 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Daniel Almeida <daniel.almeida@...labora.com>, wedsonaf@...il.com, ojeda@...nel.org
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust: kernel: add support for bits/genmask macros

On 22.08.24 19:35, Daniel Almeida wrote:
> These macros were converted from their C equivalent.
> ---
> 
> Hey all, I did not see any patch for this floating in the mailing list.
> 
> Please let me know your thoughts. This one should be rather trivial.
> 
> 
>  rust/kernel/bits.rs | 32 ++++++++++++++++++++++++++++++++
>  rust/kernel/lib.rs  |  1 +
>  2 files changed, 33 insertions(+)
>  create mode 100644 rust/kernel/bits.rs
> 
> diff --git a/rust/kernel/bits.rs b/rust/kernel/bits.rs
> new file mode 100644
> index 000000000000..8ac142392086
> --- /dev/null
> +++ b/rust/kernel/bits.rs
> @@ -0,0 +1,32 @@
> +// 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.

For better markdown rendering, you should use backtics on `BIT`.

> +///
> +#[macro_export]
> +macro_rules! bit {
> +    ($n:expr) => {

Using `expr` here will allow things like `bit!(3 + 8)`. I am not sure if
we want that, is that the same behavior as in C?
If we don't want that, we can use `literal` instead.
I am not suggesting changing it, only if it makes sense.

> +        (1 << $n)
> +    };
> +}
> +
> +/// Create a contiguous bitmask starting at bit position `l` and ending at
> +/// position `h`, where h <= l.

Ditto here for `h <= l`.

> +///
> +/// For example genmask(39, 21) gives us the 64bit vector
> +/// 0x000000ffffe00000.

This can probably be an example (with ``` and `assert!`), because then
we also test the macro already :)

> +///
> +#[macro_export]
> +macro_rules! genmask {
> +    ($h:expr, $l:expr) => {{

Same question about `expr` vs `literal` here.

> +        const _: () = {
> +            assert!($h >= $l);

Would be nice to mention this in the documentation.

---
Cheers,
Benno

> +        };
> +        ((!0u64 - (1u64 << $l) + 1) & (!0u64 >> (64 - 1 - $h)))
> +    }};
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 274bdc1b0a82..3aaa1c410d2c 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;
> --
> 2.45.2
> 
>