Rust timekeeping changes for v6.16

- Morph the rust hrtimer subsystem into the rust timekeeping subsystem,
    covering delay, sleep, timekeeping, timers. This new subsystem has all the
    relevant timekeeping C maintainers listed in the entry.
 
  - Replace `Ktime` with `Delta` and `Instant` types to represent a duration of
    time and a point in time.
 
  - Temporarily add `Ktime` to `hrtimer` module to allow `hrtimer` to delay
    converting to `Instant` and `Delta`.
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Merge tag 'rust-timekeeping-for-v6.16-v2' of https://github.com/Rust-for-Linux/linux into rust-next

Pull timekeeping updates from Andreas Hindborg:

 - Morph the Rust hrtimer subsystem into the Rust timekeeping subsystem,
   covering delay, sleep, timekeeping, timers. This new subsystem has
   all the relevant timekeeping C maintainers listed in the entry.

 - Replace 'Ktime' with 'Delta' and 'Instant' types to represent a
   duration of time and a point in time.

 - Temporarily add 'Ktime' to 'hrtimer' module to allow 'hrtimer' to
   delay converting to 'Instant' and 'Delta'.

* tag 'rust-timekeeping-for-v6.16-v2' of https://github.com/Rust-for-Linux/linux:
  MAINTAINERS: rust: Add a new section for all of the time stuff
  rust: time: Introduce Instant type
  rust: time: Introduce Delta type
  rust: time: Add PartialEq/Eq/PartialOrd/Ord trait to Ktime
  rust: hrtimer: Add Ktime temporarily

MAINTAINERS

@@ -10602,20 +10602,23 @@ F:	kernel/time/timer_list.c
 F:	kernel/time/timer_migration.*
 F:	tools/testing/selftests/timers/
 
-HIGH-RESOLUTION TIMERS [RUST]
+DELAY, SLEEP, TIMEKEEPING, TIMERS [RUST]
 M:	Andreas Hindborg <a.hindborg@kernel.org>
 R:	Boqun Feng <boqun.feng@gmail.com>
+R:	FUJITA Tomonori <fujita.tomonori@gmail.com>
 R:	Frederic Weisbecker <frederic@kernel.org>
 R:	Lyude Paul <lyude@redhat.com>
 R:	Thomas Gleixner <tglx@linutronix.de>
 R:	Anna-Maria Behnsen <anna-maria@linutronix.de>
+R:	John Stultz <jstultz@google.com>
+R:	Stephen Boyd <sboyd@kernel.org>
 L:	rust-for-linux@vger.kernel.org
 S:	Supported
 W:	https://rust-for-linux.com
 B:	https://github.com/Rust-for-Linux/linux/issues
-T:	git https://github.com/Rust-for-Linux/linux.git hrtimer-next
-F:	rust/kernel/time/hrtimer.rs
-F:	rust/kernel/time/hrtimer/
+T:	git https://github.com/Rust-for-Linux/linux.git timekeeping-next
+F:	rust/kernel/time.rs
+F:	rust/kernel/time/
 
 HIGH-SPEED SCC DRIVER FOR AX.25
 L:	linux-hams@vger.kernel.org

rust/kernel/time.rs

@@ -5,14 +5,36 @@
 //! This module contains the kernel APIs related to time and timers that
 //! have been ported or wrapped for usage by Rust code in the kernel.
 //!
+//! There are two types in this module:
+//!
+//! - The [`Instant`] type represents a specific point in time.
+//! - The [`Delta`] type represents a span of time.
+//!
+//! Note that the C side uses `ktime_t` type to represent both. However, timestamp
+//! and timedelta are different. To avoid confusion, we use two different types.
+//!
+//! A [`Instant`] object can be created by calling the [`Instant::now()`] function.
+//! It represents a point in time at which the object was created.
+//! By calling the [`Instant::elapsed()`] method, a [`Delta`] object representing
+//! the elapsed time can be created. The [`Delta`] object can also be created
+//! by subtracting two [`Instant`] objects.
+//!
+//! A [`Delta`] type supports methods to retrieve the duration in various units.
+//!
 //! C header: [`include/linux/jiffies.h`](srctree/include/linux/jiffies.h).
 //! C header: [`include/linux/ktime.h`](srctree/include/linux/ktime.h).
 
 pub mod hrtimer;
 
+/// 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;
 
@@ -27,59 +49,44 @@ pub fn msecs_to_jiffies(msecs: Msecs) -> Jiffies {
     unsafe { bindings::__msecs_to_jiffies(msecs) }
 }
 
-/// A Rust wrapper around a `ktime_t`.
+/// A specific point in time.
+///
+/// # Invariants
+///
+/// The `inner` value is in the range from 0 to `KTIME_MAX`.
 #[repr(transparent)]
-#[derive(Copy, Clone)]
-pub struct Ktime {
+#[derive(Copy, Clone, PartialEq, PartialOrd, Eq, Ord)]
+pub struct Instant {
     inner: bindings::ktime_t,
 }
 
-impl Ktime {
-    /// Create a `Ktime` from a raw `ktime_t`.
-    #[inline]
-    pub fn from_raw(inner: bindings::ktime_t) -> Self {
-        Self { inner }
-    }
-
+impl Instant {
     /// Get the current time using `CLOCK_MONOTONIC`.
     #[inline]
-    pub fn ktime_get() -> Self {
-        // SAFETY: It is always safe to call `ktime_get` outside of NMI context.
-        Self::from_raw(unsafe { bindings::ktime_get() })
-    }
-
-    /// Divide the number of nanoseconds by a compile-time constant.
-    #[inline]
-    fn divns_constant<const DIV: i64>(self) -> i64 {
-        self.to_ns() / DIV
-    }
-
-    /// Returns the number of nanoseconds.
-    #[inline]
-    pub fn to_ns(self) -> i64 {
-        self.inner
-    }
-
-    /// Returns the number of milliseconds.
-    #[inline]
-    pub fn to_ms(self) -> i64 {
-        self.divns_constant::<NSEC_PER_MSEC>()
-    }
-}
-
-/// Returns the number of milliseconds between two ktimes.
-#[inline]
-pub fn ktime_ms_delta(later: Ktime, earlier: Ktime) -> i64 {
-    (later - earlier).to_ms()
-}
-
-impl core::ops::Sub for Ktime {
-    type Output = Ktime;
-
-    #[inline]
-    fn sub(self, other: Ktime) -> Ktime {
+    pub fn now() -> Self {
+        // INVARIANT: The `ktime_get()` function returns a value in the range
+        // from 0 to `KTIME_MAX`.
         Self {
-            inner: self.inner - other.inner,
+            // SAFETY: It is always safe to call `ktime_get()` outside of NMI context.
+            inner: unsafe { bindings::ktime_get() },
+        }
+    }
+
+    /// Return the amount of time elapsed since the [`Instant`].
+    #[inline]
+    pub fn elapsed(&self) -> Delta {
+        Self::now() - *self
+    }
+}
+
+impl core::ops::Sub for Instant {
+    type Output = Delta;
+
+    // By the type invariant, it never overflows.
+    #[inline]
+    fn sub(self, other: Instant) -> Delta {
+        Delta {
+            nanos: self.inner - other.inner,
         }
     }
 }
@@ -149,3 +156,85 @@ impl ClockId {
         self as bindings::clockid_t
     }
 }
+
+/// 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.
+    /// 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`].
+    #[inline]
+    pub const fn as_millis(self) -> i64 {
+        self.as_nanos() / NSEC_PER_MSEC
+    }
+}

rust/kernel/time/hrtimer.rs

@@ -68,10 +68,26 @@
 //! `start` operation.
 
 use super::ClockId;
-use crate::{prelude::*, time::Ktime, types::Opaque};
+use crate::{prelude::*, types::Opaque};
 use core::marker::PhantomData;
 use pin_init::PinInit;
 
+/// A Rust wrapper around a `ktime_t`.
+// NOTE: Ktime is going to be removed when hrtimer is converted to Instant/Delta.
+#[repr(transparent)]
+#[derive(Copy, Clone, PartialEq, PartialOrd, Eq, Ord)]
+pub struct Ktime {
+    inner: bindings::ktime_t,
+}
+
+impl Ktime {
+    /// Returns the number of nanoseconds.
+    #[inline]
+    pub fn to_ns(self) -> i64 {
+        self.inner
+    }
+}
+
 /// A timer backed by a C `struct hrtimer`.
 ///
 /// # Invariants

rust/kernel/time/hrtimer/arc.rs

@@ -5,10 +5,10 @@ use super::HrTimer;
 use super::HrTimerCallback;
 use super::HrTimerHandle;
 use super::HrTimerPointer;
+use super::Ktime;
 use super::RawHrTimerCallback;
 use crate::sync::Arc;
 use crate::sync::ArcBorrow;
-use crate::time::Ktime;
 
 /// A handle for an `Arc<HasHrTimer<T>>` returned by a call to
 /// [`HrTimerPointer::start`].

rust/kernel/time/hrtimer/pin.rs

@@ -4,9 +4,9 @@ use super::HasHrTimer;
 use super::HrTimer;
 use super::HrTimerCallback;
 use super::HrTimerHandle;
+use super::Ktime;
 use super::RawHrTimerCallback;
 use super::UnsafeHrTimerPointer;
-use crate::time::Ktime;
 use core::pin::Pin;
 
 /// A handle for a `Pin<&HasHrTimer>`. When the handle exists, the timer might be

rust/kernel/time/hrtimer/pin_mut.rs

@@ -1,9 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0
 
 use super::{
-    HasHrTimer, HrTimer, HrTimerCallback, HrTimerHandle, RawHrTimerCallback, UnsafeHrTimerPointer,
+    HasHrTimer, HrTimer, HrTimerCallback, HrTimerHandle, Ktime, RawHrTimerCallback,
+    UnsafeHrTimerPointer,
 };
-use crate::time::Ktime;
 use core::{marker::PhantomData, pin::Pin, ptr::NonNull};
 
 /// A handle for a `Pin<&mut HasHrTimer>`. When the handle exists, the timer might

rust/kernel/time/hrtimer/tbox.rs

@@ -5,9 +5,9 @@ use super::HrTimer;
 use super::HrTimerCallback;
 use super::HrTimerHandle;
 use super::HrTimerPointer;
+use super::Ktime;
 use super::RawHrTimerCallback;
 use crate::prelude::*;
-use crate::time::Ktime;
 use core::ptr::NonNull;
 
 /// A handle for a [`Box<HasHrTimer<T>>`] returned by a call to