Re: [PATCH v11 3/8] rust: time: Introduce Delta type

Andreas Hindborg
2025-03-22T10:27:56Z

FUJITA Tomonori <fujita.tomonori@gmail.com> writes:

> Introduce a type representing a span of time. Define our own type
> because `core::time::Duration` is large and could panic during
> creation.
>
> time::Ktime could be also used for time duration but timestamp and
> timedelta are different so better to use a new type.
>
> i64 is used instead of u64 to represent a span of time; some C drivers
> uses negative Deltas and i64 is more compatible with Ktime using i64
> too (e.g., ktime_[us|ms]_delta() APIs return i64 so we create Delta
> object without type conversion.
>
> i64 is used instead of bindings::ktime_t because when the ktime_t
> type is used as timestamp, it represents values from 0 to
> KTIME_MAX, which is different from Delta.
>
> as_millis() method isn't used in this patchset. It's planned to be
> used in Binder driver.
>
> Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
> Reviewed-by: Andrew Lunn <andrew@lunn.ch>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Gary Guo <gary@garyguo.net>
> Reviewed-by: Fiona Behrens <me@kloenk.dev>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>

Two suggestions below, take or leave.

> ---
> rust/kernel/time.rs | 88 +++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 88 insertions(+)
>
> diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
> index 48b71e6641ce..622cd01e24d7 100644
> --- a/rust/kernel/time.rs
> +++ b/rust/kernel/time.rs
> @@ -8,9 +8,15 @@
> //! C header: [`include/linux/jiffies.h`](srctree/include/linux/jiffies.h).
> //! C header: [`include/linux/ktime.h`](srctree/include/linux/ktime.h).
>
> +/// The number of nanoseconds per microsecond.
> +pub const NSEC_PER_USEC: i64 = bindings::NSEC_PER_USEC as i64;
> +
> /// The number of nanoseconds per millisecond.
> pub const NSEC_PER_MSEC: i64 = bindings::NSEC_PER_MSEC as i64;
>
> +/// The number of nanoseconds per second.
> +pub const NSEC_PER_SEC: i64 = bindings::NSEC_PER_SEC as i64;
> +
> /// The time unit of Linux kernel. One jiffy equals (1/HZ) second.
> pub type Jiffies = crate::ffi::c_ulong;
>
> @@ -81,3 +87,85 @@ fn sub(self, other: Ktime) -> Ktime {
> }
> }
> }
> +
> +/// A span of time.
> +///
> +/// This struct represents a span of time, with its value stored as nanoseconds.
> +/// The value can represent any valid i64 value, including negative, zero, and
> +/// positive numbers.
> +#[derive(Copy, Clone, PartialEq, PartialOrd, Eq, Ord, Debug)]
> +pub struct Delta {
> + nanos: i64,
> +}
> +
> +impl Delta {
> + /// A span of time equal to zero.
> + pub const ZERO: Self = Self { nanos: 0 };
> +
> + /// Create a new [`Delta`] from a number of microseconds.
> + ///
> + /// The `micros` can range from -9_223_372_036_854_775 to 9_223_372_036_854_775.

To make these numbers truly useful, it would be nice to have them as
constants, for example `Delta::MAX_MICROS`, `Delta::MIN_MICROS`.

> + /// If `micros` is outside this range, `i64::MIN` is used for negative values,
> + /// and `i64::MAX` is used for positive values due to saturation.
> + #[inline]
> + pub const fn from_micros(micros: i64) -> Self {
> + Self {
> + nanos: micros.saturating_mul(NSEC_PER_USEC),
> + }
> + }
> +
> + /// Create a new [`Delta`] from a number of milliseconds.
> + ///
> + /// The `millis` can range from -9_223_372_036_854 to 9_223_372_036_854.
> + /// If `millis` is outside this range, `i64::MIN` is used for negative values,
> + /// and `i64::MAX` is used for positive values due to saturation.
> + #[inline]
> + pub const fn from_millis(millis: i64) -> Self {
> + Self {
> + nanos: millis.saturating_mul(NSEC_PER_MSEC),
> + }
> + }
> +
> + /// Create a new [`Delta`] from a number of seconds.
> + ///
> + /// The `secs` can range from -9_223_372_036 to 9_223_372_036.
> + /// If `secs` is outside this range, `i64::MIN` is used for negative values,
> + /// and `i64::MAX` is used for positive values due to saturation.
> + #[inline]
> + pub const fn from_secs(secs: i64) -> Self {
> + Self {
> + nanos: secs.saturating_mul(NSEC_PER_SEC),
> + }
> + }
> +
> + /// Return `true` if the [`Delta`] spans no time.
> + #[inline]
> + pub fn is_zero(self) -> bool {
> + self.as_nanos() == 0
> + }
> +
> + /// Return `true` if the [`Delta`] spans a negative amount of time.
> + #[inline]
> + pub fn is_negative(self) -> bool {
> + self.as_nanos() < 0
> + }
> +
> + /// Return the number of nanoseconds in the [`Delta`].
> + #[inline]
> + pub const fn as_nanos(self) -> i64 {
> + self.nanos
> + }
> +
> + /// Return the smallest number of microseconds greater than or equal
> + /// to the value in the [`Delta`].
> + #[inline]
> + pub const fn as_micros_ceil(self) -> i64 {
> + self.as_nanos().saturating_add(NSEC_PER_USEC - 1) / NSEC_PER_USEC
> + }
> +
> + /// Return the number of milliseconds in the [`Delta`].

We might consider adding "rounded towards zero" to this doc string.

Best regards,
Andreas Hindborg