linux-kernel - Re: [PATCH] rust: time: New module for timekeeping functions

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Message-ID: <cZTGCCR7h2Foi4V_jJVIBGj74UpVRYHnay_ia7nqy_1MBNIj8Ntlux3waD0xJAhvSp4wkBfZ_fo7t90PPfZdfzUIUTa2To27EMfLzY4qUTk=@protonmail.com>
Date:   Tue, 21 Feb 2023 11:23:09 +0000
From:   Björn Roy Baron <bjorn3_gh@...tonmail.com>
To:     Asahi Lina <lina@...hilina.net>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org,
        rust-for-linux@...r.kernel.org, asahi@...ts.linux.dev
Subject: Re: [PATCH] rust: time: New module for timekeeping functions

On Tuesday, February 21st, 2023 at 08:06, Asahi Lina <lina@...hilina.net> wrote:

> This module is intended to contain functions related to kernel
> timekeeping and time. Initially, this just wraps ktime_get() and
> ktime_get_boottime() and returns them as core::time::Duration instances.
> This is useful for drivers that need to implement simple retry loops and
> timeouts.
> 
> Signed-off-by: Asahi Lina lina@...hilina.net
> 
> ---
>  rust/bindings/bindings_helper.h |  4 +++-
>  rust/kernel/lib.rs              |  1 +
>  rust/kernel/time.rs             | 25 +++++++++++++++++++++++++
>  3 files changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 75d85bd6c592..587f3d1c0c9f 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -6,8 +6,10 @@
>   * Sorted alphabetically.
>   */
> 
> -#include <linux/slab.h>
> +#include <linux/ktime.h>
>  #include <linux/refcount.h>
> +#include <linux/slab.h>
> +#include <linux/timekeeping.h>
> 
>  /* `bindgen` gets confused at certain things. */
>  const gfp_t BINDINGS_GFP_KERNEL = GFP_KERNEL;
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 223564f9f0cc..371b1b17570e 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -37,6 +37,7 @@ mod static_assert;
>  pub mod std_vendor;
>  pub mod str;
>  pub mod sync;
> +pub mod time;
>  pub mod types;
> 
>  #[doc(hidden)]
> diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
> new file mode 100644
> index 000000000000..02844db47d34
> --- /dev/null
> +++ b/rust/kernel/time.rs
> @@ -0,0 +1,25 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Timekeeping functions.
> +//!
> +//! C header: [`include/linux/ktime.h`](../../../../include/linux/ktime.h)
> +//! C header: [`include/linux/timekeeping.h`](../../../../include/linux/timekeeping.h)
> +
> +use crate::bindings;
> +use core::time::Duration;
> +
> +/// Returns the kernel time elapsed since boot, excluding time spent sleeping, as a [`Duration`].
> +pub fn ktime_get() -> Duration {
> +    // SAFETY: Function has no side effects and no inputs.
> +    Duration::from_nanos(unsafe { bindings::ktime_get() }.try_into().unwrap())

ktime_t seems to be a signed 64bit int, while Duration::from_nanos expects an unsigned 64bit int. Based on https://lore.kernel.org/all/alpine.DEB.2.21.1903231125480.2157@nanos.tec.linutronix.de/T/#u I think it is fine to cast directly to u64 without doing the overflow check of try_into. This would eliminate a potential BUG() call. I think it would be fine to keep as is if you prefer though. As such:

Reviewed-by: Björn Roy Baron <bjorn3_gh@...tonmail.com>

Cheers,
Bjorn

> +}
> +
> +/// Returns the kernel time elapsed since boot, including time spent sleeping, as a [`Duration`].
> +pub fn ktime_get_boottime() -> Duration {
> +    Duration::from_nanos(
> +        // SAFETY: Function has no side effects and no variable inputs.
> +        unsafe { bindings::ktime_get_with_offset(bindings::tk_offsets_TK_OFFS_BOOT) }
> +            .try_into()
> +            .unwrap(),
> +    )
> +}
> 
> ---
> base-commit: 89f5349e0673322857bd432fa23113af56673739
> change-id: 20230221-gpu-up-time-ea9412204c3b
> 
> Thank you,
> ~~ Lina