rust: time: make Delta generic over its time unit

Delta hardcodes its value as i64 nanoseconds. A later patch adds a
jiffies span, whose natural representation is isize jiffies rather than
i64 nanoseconds, and a separate type per unit would duplicate the
arithmetic and comparison machinery.

Make Delta generic over its time unit so the jiffies span can reuse that
machinery. The nanosecond Delta keeps its current representation and API
via the default unit parameter, so no functional change.

Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://patch.msgid.link/20260808062839.1159990-2-tomo@flapping.org
[ Reworded to remove stray word. Added intra-doc links. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/time.rs

@@ -246,7 +246,7 @@ impl<C: ClockSource> ops::Sub for Instant<C> {
     #[inline]
     fn sub(self, other: Instant<C>) -> Delta {
         Delta {
-            nanos: self.inner - other.inner,
+            value: self.inner - other.inner,
         }
     }
 }
@@ -258,7 +258,7 @@ impl<T: ClockSource> ops::Add<Delta> for Instant<T> {
     fn add(self, rhs: Delta) -> Self::Output {
         // INVARIANT: With arithmetic over/underflow checks enabled, this will panic if we overflow
         // (e.g. go above `KTIME_MAX`)
-        let res = self.inner + rhs.nanos;
+        let res = self.inner + rhs.value;
 
         // INVARIANT: With overflow checks enabled, we verify here that the value is >= 0
         #[cfg(CONFIG_RUST_OVERFLOW_CHECKS)]
@@ -278,7 +278,7 @@ impl<T: ClockSource> ops::Sub<Delta> for Instant<T> {
     fn sub(self, rhs: Delta) -> Self::Output {
         // INVARIANT: With arithmetic over/underflow checks enabled, this will panic if we overflow
         // (e.g. go above `KTIME_MAX`)
-        let res = self.inner - rhs.nanos;
+        let res = self.inner - rhs.value;
 
         // INVARIANT: With overflow checks enabled, we verify here that the value is >= 0
         #[cfg(CONFIG_RUST_OVERFLOW_CHECKS)]
@@ -291,14 +291,38 @@ fn sub(self, rhs: Delta) -> Self::Output {
     }
 }
 
+mod private {
+    pub trait Sealed {}
+
+    impl Sealed for super::Nsec {}
+}
+
+/// A trait for time units.
+pub trait TimeUnit: private::Sealed {
+    /// The underlying representation of the time unit.
+    type Repr: Copy + Clone + PartialEq + PartialOrd + Eq + Ord + core::fmt::Debug;
+}
+
+/// A time unit of nanoseconds.
+///
+/// A [`Delta<Nsec>`] stores its value as [`i64`] nanoseconds and can represent
+/// any [`i64`] value, including negative, zero, and positive numbers.
+#[derive(Copy, Clone, PartialEq, PartialOrd, Eq, Ord, Debug)]
+pub enum Nsec {}
+
+impl TimeUnit for Nsec {
+    type Repr = i64;
+}
+
 /// 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.
+/// The span is stored in the unit given by the type parameter `U` (see
+/// [`TimeUnit`]); its value has type `U::Repr`. `U` defaults to [`Nsec`], so a
+/// plain [`Delta`] is a span in nanoseconds. The value can be negative, zero, or
+/// positive.
 #[derive(Copy, Clone, PartialEq, PartialOrd, Eq, Ord, Debug)]
-pub struct Delta {
-    nanos: i64,
+pub struct Delta<U: TimeUnit = Nsec> {
+    value: U::Repr,
 }
 
 impl ops::Add for Delta {
@@ -307,7 +331,7 @@ impl ops::Add for Delta {
     #[inline]
     fn add(self, rhs: Self) -> Self {
         Self {
-            nanos: self.nanos + rhs.nanos,
+            value: self.value + rhs.value,
         }
     }
 }
@@ -315,7 +339,7 @@ fn add(self, rhs: Self) -> Self {
 impl ops::AddAssign for Delta {
     #[inline]
     fn add_assign(&mut self, rhs: Self) {
-        self.nanos += rhs.nanos;
+        self.value += rhs.value;
     }
 }
 
@@ -325,7 +349,7 @@ impl ops::Sub for Delta {
     #[inline]
     fn sub(self, rhs: Self) -> Self::Output {
         Self {
-            nanos: self.nanos - rhs.nanos,
+            value: self.value - rhs.value,
         }
     }
 }
@@ -333,7 +357,7 @@ fn sub(self, rhs: Self) -> Self::Output {
 impl ops::SubAssign for Delta {
     #[inline]
     fn sub_assign(&mut self, rhs: Self) {
-        self.nanos -= rhs.nanos;
+        self.value -= rhs.value;
     }
 }
 
@@ -343,7 +367,7 @@ impl ops::Mul<i64> for Delta {
     #[inline]
     fn mul(self, rhs: i64) -> Self::Output {
         Self {
-            nanos: self.nanos * rhs,
+            value: self.value * rhs,
         }
     }
 }
@@ -351,7 +375,7 @@ fn mul(self, rhs: i64) -> Self::Output {
 impl ops::MulAssign<i64> for Delta {
     #[inline]
     fn mul_assign(&mut self, rhs: i64) {
-        self.nanos *= rhs;
+        self.value *= rhs;
     }
 }
 
@@ -362,25 +386,25 @@ impl ops::Div for Delta {
     fn div(self, rhs: Self) -> Self::Output {
         #[cfg(CONFIG_64BIT)]
         {
-            self.nanos / rhs.nanos
+            self.value / rhs.value
         }
 
         #[cfg(not(CONFIG_64BIT))]
         {
             // SAFETY: This function is always safe to call regardless of the input values
-            unsafe { bindings::div64_s64(self.nanos, rhs.nanos) }
+            unsafe { bindings::div64_s64(self.value, rhs.value) }
         }
     }
 }
 
 impl Delta {
     /// A span of time equal to zero.
-    pub const ZERO: Self = Self { nanos: 0 };
+    pub const ZERO: Self = Self { value: 0 };
 
     /// Create a new [`Delta`] from a number of nanoseconds.
     #[inline]
     pub const fn from_nanos(nanos: i64) -> Self {
-        Self { nanos }
+        Self { value: nanos }
     }
 
     /// Create a new [`Delta`] from a number of microseconds.
@@ -391,7 +415,7 @@ pub const fn from_nanos(nanos: i64) -> Self {
     #[inline]
     pub const fn from_micros(micros: i64) -> Self {
         Self {
-            nanos: micros.saturating_mul(NSEC_PER_USEC),
+            value: micros.saturating_mul(NSEC_PER_USEC),
         }
     }
 
@@ -403,7 +427,7 @@ pub const fn from_micros(micros: i64) -> Self {
     #[inline]
     pub const fn from_millis(millis: i64) -> Self {
         Self {
-            nanos: millis.saturating_mul(NSEC_PER_MSEC),
+            value: millis.saturating_mul(NSEC_PER_MSEC),
         }
     }
 
@@ -415,7 +439,7 @@ pub const fn from_millis(millis: i64) -> Self {
     #[inline]
     pub const fn from_secs(secs: i64) -> Self {
         Self {
-            nanos: secs.saturating_mul(NSEC_PER_SEC),
+            value: secs.saturating_mul(NSEC_PER_SEC),
         }
     }
 
@@ -434,7 +458,7 @@ pub fn is_negative(self) -> bool {
     /// Return the number of nanoseconds in the [`Delta`].
     #[inline]
     pub const fn as_nanos(self) -> i64 {
-        self.nanos
+        self.value
     }
 
     /// Return the smallest number of microseconds greater than or equal
@@ -487,7 +511,7 @@ pub fn rem_nanos(self, dividend: i32) -> Self {
         #[cfg(CONFIG_64BIT)]
         {
             Self {
-                nanos: self.as_nanos() % i64::from(dividend),
+                value: self.as_nanos() % i64::from(dividend),
             }
         }
 
@@ -499,7 +523,7 @@ pub fn rem_nanos(self, dividend: i32) -> Self {
             unsafe { bindings::div_s64_rem(self.as_nanos(), dividend, &mut rem) };
 
             Self {
-                nanos: i64::from(rem),
+                value: i64::from(rem),
             }
         }
     }