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

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Message-Id: <DCD7DP6A72A8.2HAYR7K7Z14UO@kernel.org>
Date: Wed, 27 Aug 2025 14:19:11 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Daniel Almeida" <daniel.almeida@...labora.com>
Cc: "FUJITA Tomonori" <fujita.tomonori@...il.com>, <a.hindborg@...nel.org>,
 <alex.gaynor@...il.com>, <ojeda@...nel.org>, <aliceryhl@...gle.com>,
 <anna-maria@...utronix.de>, <bjorn3_gh@...tonmail.com>,
 <boqun.feng@...il.com>, <frederic@...nel.org>, <gary@...yguo.net>,
 <jstultz@...gle.com>, <linux-kernel@...r.kernel.org>, <lossin@...nel.org>,
 <lyude@...hat.com>, <rust-for-linux@...r.kernel.org>, <sboyd@...nel.org>,
 <tglx@...utronix.de>, <tmgross@...ch.edu>, <acourbot@...dia.com>
Subject: Re: [PATCH v1 2/2] rust: Add read_poll_timeout_atomic function

On Wed Aug 27, 2025 at 2:14 PM CEST, Daniel Almeida wrote:
> 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?

Sure, but it would help if you could clarify which aspect you want me to expand
on. :)