[PATCH v3 2/2] rust: time: Implement basic arithmetic operations for Delta

[Lyude Paul <lyude@redhat.com>]

While rvkms is only going to be using a few of these, since Deltas are
basically the same as i64 it's easy enough to just implement all of the
basic arithmetic operations for Delta types.

Keep in mind there's one quirk here - the kernel has no support for
i64 % i64 on 32 bit platforms, the closest we have is i64 % i32 through
div_s64_rem(). So, instead of implementing ops::Rem or ops::RemAssign we
simply provide Delta::rem_nanos().

Signed-off-by: Lyude Paul <lyude@redhat.com>

---
V2:
* Don't forget to make sure that we inline all of these
* Drop ops::Rem and ops::RemAssign implementations for Delta, replace with
  Delta::rem_nanos() instead. It turns out that there's actually no way to
  perform i64 % i64 on 32 bit platforms in the kernel at the moment, the
  closest that we have is div_s64_rem() which only allows a 32 bit divisor.
* Actually use the kernel arithmetic helpers for division/remainders so
  that this code works on both 32 and 64 bit platforms.
V3:
* Change the output type for Div to i64, drop DivAssign
* Change Mul/MulAssign to accept i64, not another Delta
* Fix parameter name in rem_nanos (ns -> dividend)

 rust/kernel/time.rs | 98 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 98 insertions(+)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 4bd7a8a009f3e..e64c5b13152dd 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -265,6 +265,78 @@ pub struct Delta {
     nanos: i64,
 }
 
+impl ops::Add for Delta {
+    type Output = Self;
+
+    #[inline]
+    fn add(self, rhs: Self) -> Self {
+        Self {
+            nanos: self.nanos + rhs.nanos,
+        }
+    }
+}
+
+impl ops::AddAssign for Delta {
+    #[inline]
+    fn add_assign(&mut self, rhs: Self) {
+        self.nanos += rhs.nanos;
+    }
+}
+
+impl ops::Sub for Delta {
+    type Output = Self;
+
+    #[inline]
+    fn sub(self, rhs: Self) -> Self::Output {
+        Self {
+            nanos: self.nanos - rhs.nanos,
+        }
+    }
+}
+
+impl ops::SubAssign for Delta {
+    #[inline]
+    fn sub_assign(&mut self, rhs: Self) {
+        self.nanos -= rhs.nanos;
+    }
+}
+
+impl ops::Mul<i64> for Delta {
+    type Output = Self;
+
+    #[inline]
+    fn mul(self, rhs: i64) -> Self::Output {
+        Self {
+            nanos: self.nanos * rhs,
+        }
+    }
+}
+
+impl ops::MulAssign<i64> for Delta {
+    #[inline]
+    fn mul_assign(&mut self, rhs: i64) {
+        self.nanos *= rhs;
+    }
+}
+
+impl ops::Div for Delta {
+    type Output = i64;
+
+    #[inline]
+    fn div(self, rhs: Self) -> Self::Output {
+        #[cfg(CONFIG_64BIT)]
+        {
+            self.nanos / rhs.nanos
+        }
+
+        #[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) }
+        }
+    }
+}
+
 impl Delta {
     /// A span of time equal to zero.
     pub const ZERO: Self = Self { nanos: 0 };
@@ -353,4 +425,30 @@ pub fn as_millis(self) -> i64 {
             bindings::ktime_to_ms(self.as_nanos())
         }
     }
+
+    /// Return `self % dividend` where `dividend` is in nanoseconds.
+    ///
+    /// The kernel doesn't have any emulation for `s64 % s64` on 32 bit platforms, so this is
+    /// limited to 32 bit dividends.
+    #[inline]
+    pub fn rem_nanos(self, dividend: i32) -> Self {
+        #[cfg(CONFIG_64BIT)]
+        {
+            Self {
+                nanos: self.as_nanos() % i64::from(dividend),
+            }
+        }
+
+        #[cfg(not(CONFIG_64BIT))]
+        {
+            let mut rem = 0;
+
+            // SAFETY: `rem` is in the stack, so we can always provide a valid pointer to it.
+            unsafe { bindings::div_s64_rem(self.as_nanos(), dividend, &mut rem) };
+
+            Self {
+                nanos: i64::from(rem),
+            }
+        }
+    }
 }
-- 
2.50.0