rust: time: Avoid 64-bit integer division on 32-bit architectures

Avoid 64-bit integer division that 32-bit architectures don't
implement generally. This uses ktime_to_us() and ktime_to_ms()
instead.

The time abstraction needs i64 / u32 division so C's div_s64() can be
used but ktime_to_us() and ktime_to_ms() provide a simpler solution
for this time abstraction problem on 32-bit architectures.

32-bit ARM is the only 32-bit architecture currently supported by
Rust. Using the cfg attribute, only 32-bit architectures will call
ktime_to_us() and ktime_to_ms(), while the other 64-bit architectures
will continue to use the current code as-is to avoid the overhead.

One downside of calling the C's functions is that the as_micros/millis
methods can no longer be const fn. We stick with the simpler approach
unless there's a compelling need for a const fn.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250502004524.230553-1-fujita.tomonori@gmail.com
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>

Author: fujita

rust/helpers/helpers.c

@@ -38,6 +38,7 @@
 #include "spinlock.c"
 #include "sync.c"
 #include "task.c"
+#include "time.c"
 #include "uaccess.c"
 #include "vmalloc.c"
 #include "wait.c"

rust/helpers/time.c

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/ktime.h>
+
+s64 rust_helper_ktime_to_us(const ktime_t kt)
+{
+	return ktime_to_us(kt);
+}
+
+s64 rust_helper_ktime_to_ms(const ktime_t kt)
+{
+	return ktime_to_ms(kt);
+}

rust/kernel/time.rs

@@ -228,13 +228,31 @@ impl Delta {
     /// Return the smallest number of microseconds greater than or equal
     /// to the value in the [`Delta`].
     #[inline]
-    pub const fn as_micros_ceil(self) -> i64 {
-        self.as_nanos().saturating_add(NSEC_PER_USEC - 1) / NSEC_PER_USEC
+    pub fn as_micros_ceil(self) -> i64 {
+        #[cfg(CONFIG_64BIT)]
+        {
+            self.as_nanos().saturating_add(NSEC_PER_USEC - 1) / NSEC_PER_USEC
+        }
+
+        #[cfg(not(CONFIG_64BIT))]
+        // SAFETY: It is always safe to call `ktime_to_us()` with any value.
+        unsafe {
+            bindings::ktime_to_us(self.as_nanos().saturating_add(NSEC_PER_USEC - 1))
+        }
     }
 
     /// Return the number of milliseconds in the [`Delta`].
     #[inline]
-    pub const fn as_millis(self) -> i64 {
-        self.as_nanos() / NSEC_PER_MSEC
+    pub fn as_millis(self) -> i64 {
+        #[cfg(CONFIG_64BIT)]
+        {
+            self.as_nanos() / NSEC_PER_MSEC
+        }
+
+        #[cfg(not(CONFIG_64BIT))]
+        // SAFETY: It is always safe to call `ktime_to_ms()` with any value.
+        unsafe {
+            bindings::ktime_to_ms(self.as_nanos())
+        }
     }
 }