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

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, May 01, 2025 at 10:58:18AM +0900, FUJITA Tomonori wrote:
> 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.
> 

Nacked-by: Boqun Feng <boqun.feng@gmail.com>

As I said a few times, we should rely on compiler's optimization when
available, i.e. it's a problem that ARM compiler doesn't have this
optimization, don't punish other architecture of no reason.

Regards,
Boqun

> 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@gmail.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
> 
>