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Thu, 29 Oct 2026 22:02:44 -0400 (EDT) From: FUJITA Tomonori To: fustini@kernel.org, guoren@kernel.org, ojeda@kernel.org, ukleinek@kernel.org, wefu@redhat.com Cc: a.hindborg@kernel.org, acourbot@nvidia.com, aliceryhl@google.com, bjorn3_gh@protonmail.com, boqun@kernel.org, chenhan0017.work@gmail.com, dakr@kernel.org, daniel.almeida@collabora.com, gary@garyguo.net, linux-riscv@lists.infradead.org, lossin@kernel.org, m.wilczynski@samsung.com, tamird@kernel.org, tmgross@umich.edu, work@onurozkan.dev, linux-pwm@vger.kernel.org, rust-for-linux@vger.kernel.org, FUJITA Tomonori Subject: [PATCH v3 1/2] rust: math: add div64_u64() and div64_u64_round_up() Date: Fri, 30 Oct 2026 11:02:08 +0900 Message-ID: <20261030020209.3833766-2-tomo@flapping.org> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20261030020209.3833766-1-tomo@flapping.org> References: <20261030020209.3833766-1-tomo@flapping.org> X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: FUJITA Tomonori On 32-bit architectures, dividing a u64 by a u64 with the `/` operator makes the compiler emit a call to a compiler runtime function, such as __aeabi_uldivmod() on arm. The kernel does not provide these functions, so the build fails when Rust code that is also built for 32-bit architectures uses `/` on u64. C code uses the helpers in include/linux/math64.h instead. Add a math module with Rust versions of two of them: - div64_u64(), the same as div64_u64() in C. - div64_u64_round_up(), the same as DIV64_U64_ROUND_UP() in C. On 64-bit architectures, both use `/`. On 32-bit architectures, div64_u64() calls the C function. Both panic if the divisor is zero, as `/` does. Assisted-by: LLM Signed-off-by: FUJITA Tomonori --- rust/kernel/lib.rs | 1 + rust/kernel/math.rs | 71 +++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 72 insertions(+) create mode 100644 rust/kernel/math.rs diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs index 1ab41129d557..482fef8be2e8 100644 --- a/rust/kernel/lib.rs +++ b/rust/kernel/lib.rs @@ -100,6 +100,7 @@ pub mod kunit; pub mod list; pub mod maple_tree; +pub mod math; pub mod mem; pub mod miscdevice; pub mod mm; diff --git a/rust/kernel/math.rs b/rust/kernel/math.rs new file mode 100644 index 000000000000..395816639af8 --- /dev/null +++ b/rust/kernel/math.rs @@ -0,0 +1,71 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! Integer arithmetic helpers. +//! +//! C header: [`include/linux/math64.h`](srctree/include/linux/math64.h). + +/// Divides a [`u64`] dividend by a [`u64`] divisor, rounding down. +/// +/// On 32-bit architectures, `/` on [`u64`] needs a function that the kernel does +/// not provide. Use this function in code that is also built for 32-bit +/// architectures. +/// +/// # Panics +/// +/// Panics if `divisor` is zero. +/// +/// # Examples +/// +/// ``` +/// use kernel::math::div64_u64; +/// +/// assert_eq!(div64_u64(7, 2), 3); +/// assert_eq!(div64_u64(u64::MAX, 1 << 32), u64::from(u32::MAX)); +/// ``` +#[inline] +pub fn div64_u64(dividend: u64, divisor: u64) -> u64 { + if divisor == 0 { + panic!("attempt to divide by zero"); + } + + #[cfg(CONFIG_64BIT)] + { + dividend / divisor + } + + #[cfg(not(CONFIG_64BIT))] + { + // SAFETY: `div64_u64()` requires a non-zero `divisor`, which is + // checked above. + unsafe { bindings::div64_u64(dividend, divisor) } + } +} + +/// Divides a [`u64`] dividend by a [`u64`] divisor, rounding up. +/// +/// # Panics +/// +/// Panics if `divisor` is zero. +/// +/// # Examples +/// +/// ``` +/// use kernel::math::div64_u64_round_up; +/// +/// assert_eq!(div64_u64_round_up(7, 2), 4); +/// assert_eq!(div64_u64_round_up(6, 2), 3); +/// assert_eq!(div64_u64_round_up(u64::MAX, 2), 1 << 63); +/// ``` +#[inline] +pub fn div64_u64_round_up(dividend: u64, divisor: u64) -> u64 { + let quotient = div64_u64(dividend, divisor); + + // `quotient * divisor` is not greater than `dividend`, so this does not + // overflow. + if dividend - quotient * divisor == 0 { + quotient + } else { + // A non-zero remainder means `divisor >= 2`, so this does not overflow. + quotient + 1 + } +} -- 2.43.0