[PATCH v4 2/7] rust: time: Introduce Delta type

[FUJITA Tomonori <fujita.tomonori@gmail.com>]

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 different from Delta.

Delta::from_[millis|secs] APIs take i64. When a span of time
overflows, i64::MAX is used.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
 rust/kernel/time.rs | 49 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 4a7c6037c256..574e72d3956b 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 = core::ffi::c_ulong;
 
@@ -81,3 +87,46 @@ fn sub(self, other: Ktime) -> Ktime {
         }
     }
 }
+
+/// A span of time.
+#[derive(Copy, Clone, PartialEq, PartialOrd, Eq, Ord, Debug)]
+pub struct Delta {
+    nanos: i64,
+}
+
+impl Delta {
+    /// Create a new `Delta` from a number of milliseconds.
+    #[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.
+    #[inline]
+    pub const fn from_secs(secs: i64) -> Self {
+        Self {
+            nanos: secs.saturating_mul(NSEC_PER_SEC),
+        }
+    }
+
+    /// Return `true` if the `Detla` spans no time.
+    #[inline]
+    pub fn is_zero(self) -> bool {
+        self.as_nanos() == 0
+    }
+
+    /// Return the number of nanoseconds in the `Delta`.
+    #[inline]
+    pub 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 fn as_micros_ceil(self) -> i64 {
+        self.as_nanos().saturating_add(NSEC_PER_USEC - 1) / NSEC_PER_USEC
+    }
+}
-- 
2.43.0