linux-kernel - Re: [PATCH v1 2/2] rust: Add read_poll_timeout

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Message-Id: <D8CE958C-508F-4A09-96BA-985B5A7C2BA7@collabora.com>
Date: Wed, 27 Aug 2025 09:14:04 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: FUJITA Tomonori <fujita.tomonori@...il.com>,
 a.hindborg@...nel.org,
 alex.gaynor@...il.com,
 ojeda@...nel.org,
 aliceryhl@...gle.com,
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 boqun.feng@...il.com,
 frederic@...nel.org,
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 jstultz@...gle.com,
 linux-kernel@...r.kernel.org,
 lossin@...nel.org,
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 rust-for-linux@...r.kernel.org,
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Subject: Re: [PATCH v1 2/2] rust: Add read_poll_timeout_atomic function

Hi Danilo,

[…}

> 
> Actually, let me put it in other words:
> 
> let val = read_poll_timeout_atomic(
>     || {
>         // Fetch the offset to read from from the HW.
>         let offset = io.read32(0x1000);
> 
>         // HW needs a break for some odd reason.
>         udelay(100);
> 
>         // Read the actual value.
>         io.try_read32(offset)
>     },
>     |val: &u32| *val == HW_READY,
>     Delta::from_micros(0),      // No delay, keep spinning.
>     Delta::from_millis(10),     // Timeout after 10ms.
> )?;
> 
> Seems like a fairly reasonable usage without knowing the implementation details
> of read_poll_timeout_atomic(), right?
> 
> Except that if the hardware does not become ready, this will spin for 16.67
> *minutes* -- in atomic context. Instead of the 10ms the user would expect.
> 
> This would be way less error prone if we do not provide a timeout value, but a
> retry count.
> 
>> Instead, I think it makes much more sense to provide a retry count as function
>> argument, such that the user can specify "I want a dealy of 100us, try it 100
>> times".
>> 
>> This way it is transparent to the caller that the timeout may be significantly
>> more than 10ms depending on the user's implementation.
>> 
>> As for doing this in C vs Rust: I don't think things have to align in every
>> implementation detail. If we can improve things on the Rust side from the
>> get-go, we should not stop ourselves from doing so, just because a similar C
>> implementation is hard to refactor, due to having a lot of users already.

I must say I do not follow. Can you expand yet some more on this?

— Daniel