linux-kernel - Re: [PATCH 3/3] samples: rust: pci: take advantage of Devres::access

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Message-ID: <D9HAO06XMT9X.1NL63T3GBQG7B@proton.me>
Date: Sun, 27 Apr 2025 08:56:29 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>
Cc: gregkh@...uxfoundation.org, rafael@...nel.org, bhelgaas@...gle.com, kwilczynski@...nel.org, zhiw@...dia.com, cjia@...dia.com, jhubbard@...dia.com, bskeggs@...dia.com, acurrid@...dia.com, joelagnelf@...dia.com, ttabi@...dia.com, acourbot@...dia.com, ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu, linux-pci@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] samples: rust: pci: take advantage of Devres::access_with()

On Sat Apr 26, 2025 at 11:26 PM CEST, Danilo Krummrich wrote:
> On Sat, Apr 26, 2025 at 08:30:39PM +0000, Benno Lossin wrote:
>> On Sat Apr 26, 2025 at 3:30 PM CEST, Danilo Krummrich wrote:
>> > For the I/O operations executed from the probe() method, take advantage
>> > of Devres::access_with(), avoiding the atomic check and RCU read lock
>> > required otherwise entirely.
>> >
>> > Signed-off-by: Danilo Krummrich <dakr@...nel.org>
>> > ---
>> >  samples/rust/rust_driver_pci.rs | 12 ++++++------
>> >  1 file changed, 6 insertions(+), 6 deletions(-)
>> >
>> > diff --git a/samples/rust/rust_driver_pci.rs b/samples/rust/rust_driver_pci.rs
>> > index 9ce3a7323a16..3e1569e5096e 100644
>> > --- a/samples/rust/rust_driver_pci.rs
>> > +++ b/samples/rust/rust_driver_pci.rs
>> > @@ -83,12 +83,12 @@ fn probe(pdev: &pci::Device<Core>, info: &Self::IdInfo) -> Result<Pin<KBox<Self>
>> >              GFP_KERNEL,
>> >          )?;
>> >  
>> > -        let res = drvdata
>> > -            .bar
>> > -            .try_access_with(|b| Self::testdev(info, b))
>> > -            .ok_or(ENXIO)??;
>> > -
>> > -        dev_info!(pdev.as_ref(), "pci-testdev data-match count: {}\n", res);
>> > +        let bar = drvdata.bar.access_with(pdev.as_ref())?;
>> 
>> Since this code might inspire other code, I don't think that we should
>> return `EINVAL` here (bubbled up from `access_with`). Not sure what the
>> correct thing here would be though...
>
> I can't think of any other error code that would match better, EINVAL seems to
> be the correct thing. Maybe one could argue for ENODEV, but I still think EINVAL
> fits better.

The previous iteration of the sample used the ENXIO error code.

In this sample it should be impossible to trigger the error path. But
others might copy the code into a context where that is not the case and
then might have a horrible time debugging where the `EINVAL` came from.
I don't know if our answer to that should be "improve debugging errors
in general" or "improve the error handling in this case". I have no
idea how the former could look like, maybe something around
`#[track_caller]` and noting the lines where an error was created? For
the latter case, we could do:

    let bar = match drvdata.bar.access_with(pdev.as_ref()) {
        Ok(bar) => bar,
        Err(_) => {
            // `bar` was just created using the `pdev` above, so this should never happen.
            return Err(ENXIO);
        }
    };

---
Cheers,
Benno