linux-kernel - Re: [PATCH v9 5/5] rust: samples: Add debugfs sample

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Message-Id: <DB7UZCKSQ4G2.ZZBIWS6YJAOF@kernel.org>
Date: Wed, 09 Jul 2025 23:56:55 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Matthew Maurer" <mmaurer@...gle.com>
Cc: "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...il.com>, "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>, "Rafael J. Wysocki"
 <rafael@...nel.org>, "Sami Tolvanen" <samitolvanen@...gle.com>, "Timur
 Tabi" <ttabi@...dia.com>, "Benno Lossin" <lossin@...nel.org>,
 <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v9 5/5] rust: samples: Add debugfs sample

On Wed Jul 9, 2025 at 9:09 PM CEST, Matthew Maurer wrote:
> +// This data structure would be unlikely to be there in a real driver - it's to hook up mutation
> +// that would normally be driven by whatever the driver was actually servicing and show how that
> +// would work. We're assuming here that those methods would have access to a `&RustDebugFs`.

Please see also [1]. I think you're making this too complicated, and due to the
missing write() support you have to create workarounds because of that, which
serve as a bad reference.

Keep it simple, Create some driver private data in probe() and export a couple
of fields of this driver private data through debugfs.

If you really want to showcase that those values can change, queue some work and
modify the counter and / or the Inner type that is protected by a mutex.

[1] https://lore.kernel.org/lkml/DB7US8G7ISG0.20430M3P7I0K0@kernel.org/

> +#[pin_data]
> +struct Wrapper {
> +    _dir: Dir,
> +    #[pin]
> +    _wrapped: File<File<RustDebugFs>>,
> +}
> +
> +#[pin_data]
> +struct RustDebugFs {
> +    pdev: ARef<platform::Device>,
> +    // As we only hold these for drop effect (to remove the directory/files) we have a leading
> +    // underscore to indicate to the compiler that we don't expect to use this field directly.
> +    _debugfs: Dir,
> +    #[pin]
> +    _compatible: File<CString>,
> +    #[pin]
> +    counter: File<File<AtomicUsize>>,
> +    #[pin]
> +    inner: File<Mutex<Inner>>,
> +}
> +
> +#[derive(Debug)]
> +struct Inner {
> +    x: u32,
> +    y: u32,
> +}
> +
> +kernel::of_device_table!(
> +    OF_TABLE,
> +    MODULE_OF_TABLE,
> +    <Wrapper as platform::Driver>::IdInfo,
> +    [(of::DeviceId::new(c_str!("test,rust-debugfs-device")), ())]
> +);

I don't think we need both, ACPI should be much simpler with QEMU.

> +kernel::acpi_device_table!(
> +    ACPI_TABLE,
> +    MODULE_ACPI_TABLE,
> +    <Wrapper as platform::Driver>::IdInfo,
> +    [(acpi::DeviceId::new(c_str!("LNUXDEBF")), ())]
> +);
> +
> +impl platform::Driver for Wrapper {
> +    type IdInfo = ();
> +    const OF_ID_TABLE: Option<of::IdTable<Self::IdInfo>> = Some(&OF_TABLE);
> +    const ACPI_ID_TABLE: Option<acpi::IdTable<Self::IdInfo>> = Some(&ACPI_TABLE);
> +
> +    fn probe(
> +        pdev: &platform::Device<Core>,
> +        _info: Option<&Self::IdInfo>,
> +    ) -> Result<Pin<KBox<Self>>> {
> +        KBox::try_pin_init(Wrapper::new(RustDebugFs::new(pdev)), GFP_KERNEL)
> +    }
> +}