From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D47C41E521B; Mon, 9 Jun 2025 22:21:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749507701; cv=none; b=ld3og5q3LVOPwsn33QQ51uC+jR/B5n7YtdW5m/QJV/hrMk3E8xgkN3pgqSkuVUTXqqyJwKTB1scd9EJUimVuNgbvchHE6SMtMz9oLLFY0AV0f2fjb0lP1o1qcUx1+WQYOIcQ/qmLH2LcbZUbSLepy3X2EJ3Eg9kNPoZ+2kNcOeM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749507701; c=relaxed/simple; bh=55/zozL1WA2j69pcYw/eII5IJQRb9fqNA8Vih0PjwhQ=; h=Mime-Version:Content-Type:Date:Message-Id:Cc:Subject:From:To: References:In-Reply-To; b=ArLVGJ8MZK/IN+FL2qUvhLVC3dF9z3RgtCRw6LVccMhrD+5Y6iLHsxH1RVBBTH8D+Mx/W8PkesO03PBDSHICIFxnDR3J359DJypY88iPdpYd5CSxhtvWvNLCuJP7kEVbKoAsky2+0cELPiv/ZxIAx9e/r0MYN8sJn0thnvTXh1Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HZYnXFNE; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="HZYnXFNE" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 079C7C4CEEB; Mon, 9 Jun 2025 22:21:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1749507701; bh=55/zozL1WA2j69pcYw/eII5IJQRb9fqNA8Vih0PjwhQ=; h=Date:Cc:Subject:From:To:References:In-Reply-To:From; b=HZYnXFNEeuOHLuHHFTmSa2zdWu6nvFdvowgFWZoWqH5c/HzMe7XpMHOgq2VhHHYmK fGFlEWL4lhfuD5gPlwetQluNaf69tatEBwCBImO6gkZ2JejJZSnlrxRALq2e1yM4TX MQwIsgEylaaflARXQ9IyY5lmJ3JUhTw5Oi4v68RpDXw+aVtm6ohLhU0K0vkOgkmybB 5Qxo77lR6CteqwV0ksrWfbHdcdthBZCh63yxGKAbWvy47LqvRtSyKdg6cvIFCojR2E 0hltYstYaKf2mUaNk/7176LORS14RG+wLf7WvAWqGesFexfScK17WPaOw73R6Ca2pC 2tY0wLxPc99+A== Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Tue, 10 Jun 2025 00:21:37 +0200 Message-Id: Cc: , Subject: Re: [PATCH] rust: math: Add KernelMathExt trait with a mul_div helper From: "Benno Lossin" To: "Michal Wilczynski" , "Miguel Ojeda" , "Alex Gaynor" , "Boqun Feng" , "Gary Guo" , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , "Andreas Hindborg" , "Alice Ryhl" , "Trevor Gross" , "Danilo Krummrich" , "Marek Szyprowski" X-Mailer: aerc 0.20.1 References: <20250609-math-rust-v1-v1-1-285fac00031f@samsung.com> In-Reply-To: <20250609-math-rust-v1-v1-1-285fac00031f@samsung.com> On Mon Jun 9, 2025 at 11:53 PM CEST, Michal Wilczynski wrote: > The PWM subsystem and other kernel modules often need to perform a > 64 by 64-bit multiplication followed by a 64-bit division. Performing > this naively in Rust risks overflow on the intermediate multiplication. > The kernel provides the C helper 'mul_u64_u64_div_u64' for this exact > purpose. > > Introduce a safe Rust wrapper for this function to make it available to > Rust drivers. > > Following feedback from the mailing list [1], this functionality is You could turn this into a `Suggested-by`. > provided via a 'KernelMathExt' extension trait. This allows for > idiomatic, method style calls (e.g. val.mul_div()) and provides a > scalable pattern for adding helpers for other integer types in the > future. > > The safe wrapper is named 'mul_div' and not 'mul_u64_u64_div_u64' [2] > because its behavior differs from the underlying C function. The C > helper traps on a division by zero, whereas this safe wrapper returns > `None`, thus exhibiting different and safer behavior. > > This is required for the Rust PWM TH1520 driver [3]. > > [1] - https://lore.kernel.org/all/DAFQ19RBBSQL.3OGUXOQ0PA9YH@kernel.org/ > [2] - https://lore.kernel.org/all/CANiq72kVvLogBSVKz0eRg6V4LDB1z7b-6y1WPL= SQfXXLW7X3cw@mail.gmail.com/ > [3] - https://lore.kernel.org/all/20250524-rust-next-pwm-working-fan-for-= sending-v1-2-bdd2d5094ff7@samsung.com/ Please use the `Link: https://... [.]` format. > > Signed-off-by: Michal Wilczynski > --- > rust/kernel/lib.rs | 1 + > rust/kernel/math.rs | 34 ++++++++++++++++++++++++++++++++++ > 2 files changed, 35 insertions(+) > > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs > index 6b4774b2b1c37f4da1866e993be6230bc6715841..d652c92633b82525f37e5cd8a= 040d268e0c191d1 100644 > --- a/rust/kernel/lib.rs > +++ b/rust/kernel/lib.rs > @@ -85,6 +85,7 @@ > #[cfg(CONFIG_KUNIT)] > pub mod kunit; > pub mod list; > +pub mod math; > pub mod miscdevice; > pub mod mm; > #[cfg(CONFIG_NET)] > diff --git a/rust/kernel/math.rs b/rust/kernel/math.rs > new file mode 100644 > index 0000000000000000000000000000000000000000..b89e23f9266117dcf96561fbf= 13b1c66a4851b48 > --- /dev/null > +++ b/rust/kernel/math.rs > @@ -0,0 +1,34 @@ > +// SPDX-License-Identifier: GPL-2.0 > +// Copyright (c) 2025 Samsung Electronics Co., Ltd. > +// Author: Michal Wilczynski > + > +//! Safe wrappers for kernel math helpers. I wouldn't stress the fact that these are safe, they better be! > +//! > +//! This module provides safe, idiomatic Rust wrappers for C functions, = whose > +//! FFI bindings are auto-generated in the `bindings` crate. > + > +use crate::bindings; > + > +/// An extension trait that provides access to kernel math helpers on pr= imitive integer types. > +pub trait KernelMathExt: Sized { We should add this trait to the prelude. > + /// Multiplies self by `multiplier and divides by divisor. > + /// > + /// This wrapper around the kernel's `mul_u64_u64_div_u64` C helper = ensures that no The trait shouldn't make a reference to `u64`. > + /// overflow occurs during the intermediate multiplication. > + /// > + /// # Returns > + /// * Some(result) if the division is successful. Use backtics (`) for code in documentation and comments. > + /// * None if the divisor is zero. > + fn mul_div(self, multiplier: Self, divisor: Self) -> Option; > +} > + > +impl KernelMathExt for u64 { Can you also implement this for the other types that have a `mul_..._div` function in the kernel? > + fn mul_div(self, multiplier: u64, divisor: u64) -> Option { > + if divisor =3D=3D 0 { > + return None; > + } > + // SAFETY: The C function `mul_u64_u64_div_u64` is safe to call = because the divisor > + // is guaranteed to be non-zero. The FFI bindings use `u64`, mat= ching our types. Let's not talk about "safe to call", but rather say it like this: // SAFETY: `mul_u64_u64_div_u64` requires the divisor to be non-zero // which is checked above. It has no other requirements. --- Cheers, Benno > + Some(unsafe { bindings::mul_u64_u64_div_u64(self, multiplier, di= visor) }) > + } > +} > > --- > base-commit: 19272b37aa4f83ca52bdf9c16d5d81bdd1354494 > change-id: 20250609-math-rust-v1-d3989515e32e > > Best regards,