Re: [PATCH 7/9] rust: time: add arch_timer_get_rate wrapper

[FUJITA Tomonori <tomo@flapping.org>]

On Tue, 15 Sep 2026 12:57:40 +0200
Laura Nao <laura.nao@collabora.com> wrote:

> From: Deborah Brouwer <deborah.brouwer@collabora.com>
> 
> Provide a safe Rust wrapper for arch_timer_get_rate().
> 
> The Rust binding calls a C helper that returns 0 when the ARM
> architectural timer is not available or not yet initialized. Map this to
> Option<u32> to make the absence of a valid rate explicit to Rust callers.
> 
> This allows Rust drivers to query the system timer frequency and
> select appropriate time sources when programming hardware timeouts.
> 
> Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>
> Signed-off-by: Laura Nao <laura.nao@collabora.com>
> ---
>  rust/helpers/time.c |  6 ++++++
>  rust/kernel/time.rs | 30 ++++++++++++++++++++++++++++++
>  2 files changed, 36 insertions(+)

(snip)

> +/// Returns the ARM architecture timer frequency in Hz, if available.
> +///
> +/// This function queries the system-wide ARM architecture timer frequency.
> +/// The architecture timer provides a consistent time source across all CPU cores.
> +///
> +/// Returns `None` if:
> +/// - The ARM architecture timer is not available (`CONFIG_ARM_ARCH_TIMER` not enabled)
> +/// - The timer rate is zero (not initialized)
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::time::arch_timer_get_rate;
> +///
> +/// if let Some(rate) = arch_timer_get_rate() {
> +///     // Use `rate`.
> +/// }
> +/// ```
> +pub fn arch_timer_get_rate() -> Option<u32> {
> +    // SAFETY: The C helper is available in all configs; it calls
> +    // `arch_timer_get_rate()`, which falls back to an inline stub returning 0
> +    // when CONFIG_ARM_ARCH_TIMER is disabled.

A SAFETY comment states why the unsafe call is sound. This one
explains why the helper is available in all configs.

How about:

// SAFETY: It is always safe to call `arch_timer_get_rate()`.

It just returns a variable without any lock. rust/kernel/time.rs
already does this for `ktime_get()` and `ktime_to_us()`.


> +    let rate = unsafe { bindings::arch_timer_get_rate() };
> +    if rate == 0 {
> +        None
> +    } else {
> +        Some(rate)
> +    }
> +}
> +
>  impl Delta {
>      /// A span of time equal to zero.
>      pub const ZERO: Self = Self { value: 0 };
> 

This puts the new function between `impl ops::Div for Delta` and
`impl Delta`, so it splits the `Delta` blocks.

`msecs_to_jiffies()` is the only other free function in this file and it
is at the top, next to the type aliases. Please put
`arch_timer_get_rate()` there,