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

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Message-ID: <aBN1BzCawU0a9Nx9@Mac.home>
Date: Thu, 1 May 2025 06:20:07 -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 06:12:02AM -0700, Boqun Feng wrote:
> 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) }

Copy-paste errors:

	    unsafe { bindings::ktime_to_ms(ns) }

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

	    ns / NSEC_PER_MSEC

;-)

Regards,
Boqun

> 	}
> 
> 	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
> >     }
> > }
> > 
>