[PATCH v4 4/7] rust: time: add Delta::to_jiffies() for timeout conversion

[FUJITA Tomonori <tomo@flapping.org>]

From: FUJITA Tomonori <fujita.tomonori@gmail.com>

Add Delta<Nsec>::to_jiffies() conversion that rounds up so the
resulting timeout is never shorter than the requested span, clamps a
negative span to an immediate timeout, and saturates an overlong span
to the kernel's MAX_JIFFY_OFFSET "wait forever" value.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
 rust/kernel/time.rs | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index f890c2d03932..900481a3cba2 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -541,6 +541,27 @@ pub fn as_millis_ceil(self) -> i64 {
         }
     }
 
+    /// Convert this span to a [`Delta<Jiffy>`] suitable for use as a timeout.
+    ///
+    /// The value is rounded up to the next whole jiffy, so the resulting
+    /// timeout is never shorter than `self` (as `msecs_to_jiffies()` does).
+    /// A negative span clamps to zero jiffies (an immediate timeout).
+    #[inline]
+    pub fn to_jiffies(self) -> Delta<Jiffy> {
+        let msecs = self.as_millis_ceil();
+
+        // CAST: `msecs` is clamped to `0..=c_uint::MAX`, so it is non-negative and
+        // fits in `c_uint`.
+        let msecs = msecs.clamp(0, i64::from(crate::ffi::c_uint::MAX)) as crate::ffi::c_uint;
+
+        // SAFETY: `__msecs_to_jiffies()` is always safe to call.
+        let jiffies = unsafe { bindings::__msecs_to_jiffies(msecs) };
+
+        // CAST: `__msecs_to_jiffies()` returns a value in `0..=MAX_JIFFY_OFFSET`, i.e.
+        // `((LONG_MAX >> 1) - 1)`, which is non-negative and well within `isize`.
+        Delta::<Jiffy>::from_jiffies(jiffies as isize)
+    }
+
     /// 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
-- 
2.43.0