linux-kernel - [PATCH v1] rust: time: Avoid 64-bit integer division

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Message-ID: <20250501015818.226376-1-fujita.tomonori@gmail.com>
Date: Thu,  1 May 2025 10:58:18 +0900
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: rust-for-linux@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	a.hindborg@...sung.com,
	boqun.feng@...il.com,
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Subject: [PATCH v1] rust: time: Avoid 64-bit integer division

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

The timer abstraction needs i64 / u32 division so C's div_s64() can be
used but ktime_to_ms() and ktime_to_us() provide a simpler solution
for this timer abstraction problem. On some architectures, there is
room to optimize the implementation of them, but such optimization can
be done if and when it becomes necessary.

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.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@...il.com>
---
 rust/helpers/helpers.c |  1 +
 rust/helpers/time.c    | 13 +++++++++++++
 rust/kernel/time.rs    | 10 ++++++----
 3 files changed, 20 insertions(+), 4 deletions(-)
 create mode 100644 rust/helpers/time.c

diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 1e7c84df7252..2ac088de050f 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -34,6 +34,7 @@
 #include "spinlock.c"
 #include "sync.c"
 #include "task.c"
+#include "time.c"
 #include "uaccess.c"
 #include "vmalloc.c"
 #include "wait.c"
diff --git a/rust/helpers/time.c b/rust/helpers/time.c
new file mode 100644
index 000000000000..0a5d1773a07c
--- /dev/null
+++ b/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_divns(kt, NSEC_PER_USEC);
+}
+
+s64 rust_helper_ktime_to_ms(const ktime_t kt)
+{
+	return ktime_divns(kt, NSEC_PER_MSEC);
+}
diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index a8089a98da9e..e3008f6324ea 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -228,13 +228,15 @@ pub const fn as_nanos(self) -> i64 {
     /// 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 {
+        // 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 {
+        // SAFETY: It is always safe to call `ktime_to_ms()` with any value.
+        unsafe { bindings::ktime_to_ms(self.nanos) }
     }
 }

base-commit: 679185904972421c570a1c337a8266835045012d
-- 
2.43.0