linux-kernel - Re: [PATCH v3 4/4] rust: samples: Add debugfs sample

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Message-ID: <aBRtrfTaaC3Vk9fL@pollux>
Date: Fri, 2 May 2025 09:01:01 +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>,
	Benno Lossin <benno.lossin@...ton.me>,
	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>, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v3 4/4] rust: samples: Add debugfs sample

On Thu, May 01, 2025 at 10:47:44PM +0000, Matthew Maurer wrote:
> +struct RustDebugFs {
> +    // As we only hold this for drop effect (to remove the directory) we have a leading underscore
> +    // to indicate to the compiler that we don't expect to use this field directly.

This feels like an introduction to Rust in the kernel for Rust beginners, which
is a great thing!

However, I wonder if, instead, we may want a dedicated (or even multiple) sample
modules or sample drivers, where we go into such detail and leave those samples
to focus only on the corresponding API?

> +    _debugfs: Dir,
> +}
> +
> +static EXAMPLE: AtomicU32 = AtomicU32::new(8);
> +
> +impl kernel::Module for RustDebugFs {
> +    fn init(_this: &'static ThisModule) -> Result<Self> {
> +        // Create a debugfs directory in the root of the filesystem called "sample_debugfs".
> +        let debugfs = Dir::new(c_str!("sample_debugfs"));
> +
> +        // Create a subdirectory, so "sample_debugfs/subdir" now exists.
> +        let sub = debugfs.subdir(c_str!("subdir"));
> +
> +        // Create a single file in the subdirectory called "example" that will read from the
> +        // `EXAMPLE` atomic variable.
> +        sub.fmt_file(c_str!("example"), &EXAMPLE, &|example, f| {
> +            writeln!(f, "Reading atomic: {}", example.load(Ordering::Relaxed))
> +        });
> +        // Now, "sample_debugfs/subdir/example" will print "Reading atomic: 8\n" when read.
> +
> +        // Change the value in the variable displayed by the file. This is intended to demonstrate
> +        // that the module can continue to change the value used by the file.
> +        EXAMPLE.store(10, Ordering::Relaxed);
> +        // Now, "sample_debugfs/subdir/example" will print "Reading atomic: 10\n" when read.
> +
> +        // Save the root debugfs directory we created to our module object. It will be
> +        // automatically cleaned up when our module is unloaded because dropping the module object
> +        // will drop the `Dir` handle. The base directory, the subdirectory, and the file will all
> +        // continue to exist until the module is unloaded.

Same with this comment.

@Greg: I know you proposed this one; for educational purposes I suppose. What's
your take on the above?