linux-kernel - Re: [PATCH v1] rust: time: Avoid 64-bit integer division

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Message-Id: <20250501.114534.1953073574103349822.fujita.tomonori@gmail.com>
Date: Thu, 01 May 2025 11:45:34 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: rust-for-linux@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, a.hindborg@...sung.com,
 boqun.feng@...il.com, frederic@...nel.org, lyude@...hat.com,
 tglx@...utronix.de, anna-maria@...utronix.de, jstultz@...gle.com,
 sboyd@...nel.org, ojeda@...nel.org, alex.gaynor@...il.com,
 gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
 aliceryhl@...gle.com, tmgross@...ch.edu, chrisi.schrefl@...il.com,
 arnd@...db.de, linux@...linux.org.uk
Subject: Re: [PATCH v1] rust: time: Avoid 64-bit integer division

On Thu,  1 May 2025 10:58:18 +0900
FUJITA Tomonori <fujita.tomonori@...il.com> wrote:

> Avoid 64-bit integer division that 32-bit architectures don't
> implement generally. This uses ktime_to_ms() and ktime_to_us()
> instead.
> 
> The timer abstraction needs i64 / u32 division so C's div_s64() can be
> used but ktime_to_ms() and ktime_to_us() provide a simpler solution
> for this timer abstraction problem. On some architectures, there is
> room to optimize the implementation of them, but such optimization can
> be done if and when it becomes necessary.
> 
> One downside of calling the C's functions is that the as_micros/millis
> methods can no longer be const fn. We stick with the simpler approach
> unless there's a compelling need for a const fn.
> 
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@...il.com>
> ---
>  rust/helpers/helpers.c |  1 +
>  rust/helpers/time.c    | 13 +++++++++++++
>  rust/kernel/time.rs    | 10 ++++++----
>  3 files changed, 20 insertions(+), 4 deletions(-)
>  create mode 100644 rust/helpers/time.c
> 
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index 1e7c84df7252..2ac088de050f 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -34,6 +34,7 @@
>  #include "spinlock.c"
>  #include "sync.c"
>  #include "task.c"
> +#include "time.c"
>  #include "uaccess.c"
>  #include "vmalloc.c"
>  #include "wait.c"
> diff --git a/rust/helpers/time.c b/rust/helpers/time.c
> new file mode 100644
> index 000000000000..0a5d1773a07c
> --- /dev/null
> +++ b/rust/helpers/time.c
> @@ -0,0 +1,13 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/ktime.h>
> +
> +s64 rust_helper_ktime_to_us(const ktime_t kt)
> +{
> +	return ktime_divns(kt, NSEC_PER_USEC);
> +}
> +
> +s64 rust_helper_ktime_to_ms(const ktime_t kt)
> +{
> +	return ktime_divns(kt, NSEC_PER_MSEC);
> +}

Oops, they should call ktime_to_us() and ktime_to_ms() respectively.

I'll send v2 later.