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Date: Tue, 26 Aug 2025 16:12:44 +0200
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Cc: <a.hindborg@kernel.org>, <alex.gaynor@gmail.com>, <ojeda@kernel.org>,
 <aliceryhl@google.com>, <anna-maria@linutronix.de>,
 <bjorn3_gh@protonmail.com>, <boqun.feng@gmail.com>, <frederic@kernel.org>,
 <gary@garyguo.net>, <jstultz@google.com>, <linux-kernel@vger.kernel.org>,
 <lossin@kernel.org>, <lyude@redhat.com>, <rust-for-linux@vger.kernel.org>,
 <sboyd@kernel.org>, <tglx@linutronix.de>, <tmgross@umich.edu>,
 <acourbot@nvidia.com>, <daniel.almeida@collabora.com>
To: "FUJITA Tomonori" <fujita.tomonori@gmail.com>
From: "Danilo Krummrich" <dakr@kernel.org>
Subject: Re: [PATCH v1 2/2] rust: Add read_poll_timeout_atomic function
References: <20250821035710.3692455-1-fujita.tomonori@gmail.com>
 <20250821035710.3692455-3-fujita.tomonori@gmail.com>
In-Reply-To: <20250821035710.3692455-3-fujita.tomonori@gmail.com>

On Thu Aug 21, 2025 at 5:57 AM CEST, FUJITA Tomonori wrote:
> +pub fn read_poll_timeout_atomic<Op, Cond, T>(
> +    mut op: Op,
> +    mut cond: Cond,
> +    delay_delta: Delta,
> +    timeout_delta: Delta,
> +) -> Result<T>
> +where
> +    Op: FnMut() -> Result<T>,
> +    Cond: FnMut(&T) -> bool,
> +{
> +    let mut left_ns =3D timeout_delta.as_nanos();
> +    let delay_ns =3D delay_delta.as_nanos();
> +
> +    loop {
> +        let val =3D op()?;
> +        if cond(&val) {
> +            // Unlike the C version, we immediately return.
> +            // We know the condition is met so we don't need to check ag=
ain.
> +            return Ok(val);
> +        }
> +
> +        if left_ns < 0 {
> +            // Unlike the C version, we immediately return.
> +            // We have just called `op()` so we don't need to call it ag=
ain.
> +            return Err(ETIMEDOUT);
> +        }
> +
> +        if !delay_delta.is_zero() {
> +            udelay(delay_delta);
> +            left_ns -=3D delay_ns;
> +        }
> +
> +        cpu_relax();
> +        left_ns -=3D 1;

How do we know that each iteration costs 1ns? To make it even more obvious,=
 we
don't control the implementation of cond(). Shouldn't we use ktime for this=
?