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Date: Tue, 26 Aug 2025 19:15:42 +0200
Message-Id: <DCCJ26K4TBEG.5HLYXY68Y6QJ@kernel.org>
To: "Daniel Almeida" <daniel.almeida@collabora.com>
From: "Danilo Krummrich" <dakr@kernel.org>
Subject: Re: [PATCH v1 2/2] rust: Add read_poll_timeout_atomic function
Cc: "FUJITA Tomonori" <fujita.tomonori@gmail.com>, <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>
References: <20250821035710.3692455-1-fujita.tomonori@gmail.com>
 <20250821035710.3692455-3-fujita.tomonori@gmail.com>
 <DCCF63BESWQ9.9LC8MZK7NG1Y@kernel.org>
 <33725E09-3994-4306-AD4F-4A0253261DE1@collabora.com>
In-Reply-To: <33725E09-3994-4306-AD4F-4A0253261DE1@collabora.com>

On Tue Aug 26, 2025 at 6:59 PM CEST, Daniel Almeida wrote:
>
>
>> On 26 Aug 2025, at 11:12, Danilo Krummrich <dakr@kernel.org> wrote:
>>=20
>> 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 =
again.
>>> +            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 =
again.
>>> +            return Err(ETIMEDOUT);
>>> +        }
>>> +
>>> +        if !delay_delta.is_zero() {
>>> +            udelay(delay_delta);
>>> +            left_ns -=3D delay_ns;
>>> +        }
>>> +
>>> +        cpu_relax();
>>> +        left_ns -=3D 1;
>>=20
>> How do we know that each iteration costs 1ns? To make it even more obvio=
us, we
>> don't control the implementation of cond(). Shouldn't we use ktime for t=
his?
>
> I don=E2=80=99t think ktime can be used from an atomic context?

There's no problem calling things like ktime_get() from atomic context.