rust: time: add Delta::to_jiffies_timeout() for timeout conversion

The boundaries of __msecs_to_jiffies() and nsecs_to_jiffies64() depend
on which HZ branch is compiled. With CONFIG_HZ_300
nsecs_to_jiffies64() overflows after 64.99 years, which is less than
the 292 years a Delta can hold. With HZ=1000 a jiffy is a millisecond,
so __msecs_to_jiffies() returns its argument unchanged and never caps
it at MAX_JIFFY_OFFSET.

Compute the conversion in Rust with mul_u64_add_u64_div_u64() instead. It
returns (a * b + c) / d, computing a * b internally in 128 bits, so
ceil(nanos * HZ / NSEC_PER_SEC) needs no input clamp and rounds once. The
bound then follows from the arithmetic: with HZ <= NSEC_PER_SEC, which a
static_assert() checks, the result is at most the nanosecond count.

Unless the result saturates, the value is rounded up, so the timeout is
never shorter than the requested span. It saturates at zero jiffies for a
negative span, i.e. an immediate timeout, and at MAX_JIFFY_OFFSET, the
upper bound the kernel uses for a jiffies span. Since MAX_JIFFY_OFFSET is
derived from long, only 32 bit can reach it, and a saturated timeout there
is finite, so it can be shorter than the requested span.

Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Link: https://msgid.link/20260924232530.446588-3-tomo@flapping.org
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

rust/helpers/helpers.c

@@ -73,6 +73,7 @@
 #include "kunit.c"
 #include "list.c"
 #include "maple_tree.c"
+#include "math.c"
 #include "mm.c"
 #include "mutex.c"
 #include "net/genetlink.c"

rust/helpers/math.c

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/math64.h>
+
+__rust_helper u64 rust_helper_mul_u64_add_u64_div_u64(u64 a, u64 b, u64 c, u64 d)
+{
+	return mul_u64_add_u64_div_u64(a, b, c, d);
+}

rust/kernel/Kconfig.test

@@ -83,4 +83,14 @@ config RUST_BITFIELD_KUNIT_TEST
 
 	  If unsure, say N.
 
+config RUST_TIME_KUNIT_TEST
+	bool "KUnit tests for the Rust time API" if !KUNIT_ALL_TESTS
+	default KUNIT_ALL_TESTS
+	help
+	  This option enables KUnit tests for the Rust time API.
+	  These are only for development and testing, not for regular
+	  kernel use cases.
+
+	  If unsure, say N.
+
 endif

rust/kernel/time.rs

@@ -39,6 +39,10 @@
 /// The number of nanoseconds per second.
 pub const NSEC_PER_SEC: i64 = bindings::NSEC_PER_SEC as i64;
 
+/// The C side `MAX_JIFFY_OFFSET`, i.e. `((LONG_MAX >> 1) - 1)`. It is the upper
+/// bound the kernel uses for a jiffies span, not a wait-forever value.
+const MAX_JIFFY_OFFSET: isize = (isize::MAX >> 1) - 1;
+
 /// The time unit of Linux kernel. One jiffy equals (1/HZ) second.
 pub type Jiffies = crate::ffi::c_ulong;
 
@@ -554,6 +558,62 @@ pub fn as_millis_ceil(self) -> i64 {
         }
     }
 
+    /// Convert this span to a [`Delta<Jiffy>`] suitable for use as a timeout.
+    ///
+    /// Unless the result saturates, the value is rounded up to the next whole
+    /// jiffy, so the resulting timeout is never shorter than `self`.
+    ///
+    /// A negative span saturates at zero jiffies, i.e. an immediate timeout.
+    ///
+    /// A span that does not fit saturates at the kernel's [`MAX_JIFFY_OFFSET`],
+    /// the upper bound for a jiffies span. That is a finite timeout, so a
+    /// saturated result can be shorter than the requested span. It is derived
+    /// from `long`, so only 32 bit can reach it, at about 12 days with `HZ=1000`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::time::Delta;
+    ///
+    /// // A negative span is an immediate timeout.
+    /// assert_eq!(Delta::from_millis(-1).to_jiffies_timeout().as_jiffies(), 0);
+    ///
+    /// // A span shorter than a jiffy still waits, i.e. the timeout is never
+    /// // shorter than the span.
+    /// assert!(Delta::from_nanos(1).to_jiffies_timeout().as_jiffies() >= 1);
+    /// ```
+    ///
+    /// [`MAX_JIFFY_OFFSET`]: srctree/include/linux/jiffies.h
+    #[inline]
+    pub fn to_jiffies_timeout(self) -> Delta<Jiffy> {
+        const HZ: u64 = bindings::HZ as u64;
+
+        // The quotient `(nsecs * HZ + NSEC_PER_SEC - 1) / NSEC_PER_SEC` has to fit in
+        // `u64`; `nsecs * HZ` does not. With `HZ <= NSEC_PER_SEC` the numerator is at
+        // most `(nsecs + 1) * NSEC_PER_SEC - 1`, so the quotient is at most `nsecs`.
+        crate::static_assert!(HZ <= NSEC_PER_SEC as u64);
+
+        // CAST: `i64::max()` makes the value non-negative, so the cast keeps it.
+        let nsecs = i64::max(self.as_nanos(), 0) as u64;
+
+        // SAFETY: `mul_u64_add_u64_div_u64()` must not be called with a zero divisor,
+        // and its result must fit in `u64`. `NSEC_PER_SEC` is a non-zero constant, and
+        // the assertion above bounds the quotient by `nsecs`.
+        let jiffies = unsafe {
+            bindings::mul_u64_add_u64_div_u64(
+                nsecs,
+                HZ,
+                (NSEC_PER_SEC - 1) as u64,
+                NSEC_PER_SEC as u64,
+            )
+        };
+
+        // CAST: `jiffies` is clamped to `MAX_JIFFY_OFFSET`, which is `<= isize::MAX`.
+        let jiffies = u64::min(jiffies, MAX_JIFFY_OFFSET as u64) as isize;
+
+        Delta::<Jiffy>::from_jiffies(jiffies)
+    }
+
     /// 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
@@ -580,3 +640,48 @@ pub fn rem_nanos(self, dividend: i32) -> Self {
         }
     }
 }
+
+#[cfg(CONFIG_RUST_TIME_KUNIT_TEST)]
+#[macros::kunit_tests(rust_kernel_time)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn to_jiffies_timeout_converts() {
+        const HZ: isize = bindings::HZ as isize;
+
+        // One second is exactly `HZ` jiffies, and the round-up must not add one.
+        assert_eq!(Delta::from_secs(1).to_jiffies_timeout().as_jiffies(), HZ);
+
+        // One nanosecond more has to round up to the next whole jiffy.
+        assert_eq!(
+            Delta::from_nanos(NSEC_PER_SEC + 1)
+                .to_jiffies_timeout()
+                .as_jiffies(),
+            HZ + 1
+        );
+    }
+
+    #[test]
+    fn to_jiffies_timeout_saturates() {
+        // The result never exceeds `MAX_JIFFY_OFFSET`. On 32 bit with `HZ=1000` this
+        // span is 2147483647 jiffies, so the clamp is what keeps it in range; on 64
+        // bit it fits and the check holds for every possible return value.
+        let clamped = Delta::from_millis(i64::from(i32::MAX)).to_jiffies_timeout();
+        assert!(clamped.as_jiffies() <= MAX_JIFFY_OFFSET);
+
+        // `MAX_JIFFY_OFFSET` is derived from `long`, so only 32 bit can reach it. On
+        // 64 bit `i64::MAX` nanoseconds is about 292 years, which is 9223372036855
+        // jiffies with `HZ=1000`, far below the limit.
+        #[cfg(not(CONFIG_64BIT))]
+        {
+            // An overlong span is clamped to `MAX_JIFFY_OFFSET`.
+            let overlong = Delta::from_nanos(i64::MAX).to_jiffies_timeout();
+            assert_eq!(overlong.as_jiffies(), MAX_JIFFY_OFFSET);
+        }
+
+        // A negative span is an immediate timeout, however long it is.
+        let negative = Delta::from_nanos(i64::MIN).to_jiffies_timeout();
+        assert_eq!(negative.as_jiffies(), 0);
+    }
+}