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

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Message-ID: <aBNzIp9UF7GZVYLs@Mac.home>
Date: Thu, 1 May 2025 06:12:02 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
	a.hindborg@...sung.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, May 01, 2025 at 10:07:17PM +0900, FUJITA Tomonori wrote:
> On Thu, 1 May 2025 05:26:54 -0700
> Boqun Feng <boqun.feng@...il.com> wrote:
> 
> > On Thu, May 01, 2025 at 10:58:18AM +0900, FUJITA Tomonori 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.
> >> 
> > 
> > Nacked-by: Boqun Feng <boqun.feng@...il.com>
> > 
> > As I said a few times, we should rely on compiler's optimization when
> > available, i.e. it's a problem that ARM compiler doesn't have this
> > optimization, don't punish other architecture of no reason.
> 
> Did you mean that we should do something like the following?
> 

Yes, or

	#[cfg(CONFIG_ARM)]
	fn ns_to_ms(ns: i64) -> i64 {
	    // SAFETY: It is always safe to call `ktime_to_ms()` with any value.
	    unsafe { bindings::ktime_to_ms(self.nanos) }
	}

	#[cfg(not(CONFIG_ARM))]
	fn ns_to_ms(ns: i64) -> i64 {
	    self.as_nanos() / NSEC_PER_MSEC
	}

	pub fn as_millis(self) -> i64 {
	    ns_to_ms(self.as_nanos())
	}

Regards,
Boqun

> pub fn as_millis(self) -> i64 {
>     #[cfg(CONFIG_ARM)]
>     {
>         // SAFETY: It is always safe to call `ktime_to_ms()` with any value.
>         unsafe { bindings::ktime_to_ms(self.nanos) }
>     }
>     #[cfg(not(CONFIG_ARM))]
>     {
>         self.as_nanos() / NSEC_PER_MSEC
>     }
> }
>