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

From: Andreas Hindborg <[email-masked]>
Date: Sat, 22 Mar 2025 09:50:48 +0100

FUJITA Tomonori <[email-masked]> 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 <[email-masked]>
> Reviewed-by: Andrew Lunn <[email-masked]>
> Reviewed-by: Alice Ryhl <[email-masked]>
> Reviewed-by: Gary Guo <[email-masked]>
> Reviewed-by: Fiona Behrens <[email-masked]>
> Signed-off-by: FUJITA Tomonori <[email-masked]>

Reviewed-by: Andreas Hindborg <[email-masked]>

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