netdev - Re: [PATCH net-next v3 4/8] rust: time: Implement addition of Ktime and Delta

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Message-ID: <ZxwD0fG3hTT1Nkf3@Boquns-Mac-mini.local>
Date: Fri, 25 Oct 2024 13:47:13 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc: FUJITA Tomonori <fujita.tomonori@...il.com>, netdev@...r.kernel.org,
	rust-for-linux@...r.kernel.org, andrew@...n.ch,
	hkallweit1@...il.com, tmgross@...ch.edu, ojeda@...nel.org,
	alex.gaynor@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
	benno.lossin@...ton.me, a.hindborg@...sung.com,
	aliceryhl@...gle.com, anna-maria@...utronix.de, frederic@...nel.org,
	tglx@...utronix.de, arnd@...db.de, jstultz@...gle.com,
	sboyd@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v3 4/8] rust: time: Implement addition of Ktime
 and Delta

On Thu, Oct 17, 2024 at 08:58:48PM +0200, Miguel Ojeda wrote:
> On Thu, Oct 17, 2024 at 7:03 PM Boqun Feng <boqun.feng@...il.com> wrote:
> >
> > but one thing I'm not sure is since it looks like saturating to
> > KTIME_SEC_MAX is the current C choice, if we want to do the same, should
> > we use the name `add_safe()` instead of `saturating_add()`? FWIW, it
> > seems harmless to saturate at KTIME_MAX to me. So personally, I like
> 
> Wait -- `ktime_add_safe()` calls `ktime_set(KTIME_SEC_MAX, 0)` which
> goes into the conditional that returns `KTIME_MAX`, not `KTIME_SEC_MAX
> * NSEC_PER_SEC` (which is what I guess you were saying).
> 

Yeah.. this is very interesting ;-) I missed that.

> So I am confused -- it doesn't saturate to `KTIME_SEC_MAX` (scaled)
> anyway. Which is confusing in itself.
> 

Then I think it suffices to say ktime_add_safe() is just a
saturating_add() for i64? ;-)

> In fact, it means that `ktime_add_safe()` allows you to get any value
> whatsoever as long as you don't overflow, but `ktime_set` does not
> allow you to -- unless you use enough nanoseconds to get you there
> (i.e. over a second in nanoseconds).
> 

That seems to the be case.

Regards,
Boqun

> Cheers,
> Miguel
>