linux-kernel - Re: [PATCH v8 3/7] rust: time: Introduce Instant type

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Message-ID: <20250122165837.787867ba.gary@garyguo.net>
Date: Wed, 22 Jan 2025 16:58:37 +0000
From: Gary Guo <gary@...yguo.net>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: aliceryhl@...gle.com, linux-kernel@...r.kernel.org,
 boqun.feng@...il.com, rust-for-linux@...r.kernel.org,
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Subject: Re: [PATCH v8 3/7] rust: time: Introduce Instant type

On Wed, 22 Jan 2025 22:46:35 +0900 (JST)
FUJITA Tomonori <fujita.tomonori@...il.com> wrote:

> On Wed, 22 Jan 2025 13:51:21 +0100
> Alice Ryhl <aliceryhl@...gle.com> wrote:
> 
> > On Wed, Jan 22, 2025 at 1:49 PM FUJITA Tomonori
> > <fujita.tomonori@...il.com> wrote:  
> >>
> >> On Thu, 16 Jan 2025 21:06:44 +0900 (JST)
> >> FUJITA Tomonori <fujita.tomonori@...il.com> wrote:
> >>  
> >> > On Thu, 16 Jan 2025 10:32:45 +0100
> >> > Alice Ryhl <aliceryhl@...gle.com> wrote:
> >> >  
> >> >>> -impl Ktime {
> >> >>> -    /// Create a `Ktime` from a raw `ktime_t`.
> >> >>> +impl Instant {
> >> >>> +    /// Create a `Instant` from a raw `ktime_t`.
> >> >>>      #[inline]
> >> >>> -    pub fn from_raw(inner: bindings::ktime_t) -> Self {
> >> >>> +    fn from_raw(inner: bindings::ktime_t) -> Self {
> >> >>>          Self { inner }
> >> >>>      }  
> >> >>
> >> >> Please keep this function public.  
> >> >
> >> > Surely, your driver uses from_raw()?  
> >>
> >> I checked out the C version of Binder driver and it doesn't seem like
> >> the driver needs from_raw function. The Rust version [1] also doesn't
> >> seem to need the function. Do you have a different use case?  
> > 
> > Not for this particular function, but I've changed functions called
> > from_raw and similar from private to public so many times at this
> > point that I think it should be the default.  
> 
> Then can we remove from_raw()?
> 
> We don't use Instant to represent both a specific point in time and a
> span of time (we add Delta) so a device driver don't need to create an
> Instant from an arbitrary value, I think.
> 
> If we allow a device driver to create Instant via from_raw(), we need
> to validate a value from the driver. If we create ktime_t only via
> ktime_get(), we don't need the details of ktime like a valid range of
> ktime_t.

Yeah, I agree on this. If we specify the range 0..KTIME_MAX as
invariant then this either has to check or has to be unsafe. I don't
think that's worth adding unless there's a concrete need for it.

Best,
Gary