linux-kernel - Re: [PATCH v10 5/7] samples: rust: Add debugfs sample driver

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Message-Id: <DC73760NQA0X.29E82RNZCV3FS@kernel.org>
Date: Wed, 20 Aug 2025 09:46:35 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Matthew Maurer" <mmaurer@...gle.com>, "Danilo Krummrich"
 <dakr@...nel.org>
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>, "Dirk Beheme" <dirk.behme@...bosch.com>,
 <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v10 5/7] samples: rust: Add debugfs sample driver

On Wed Aug 20, 2025 at 2:40 AM CEST, Matthew Maurer wrote:
> On Tue, Aug 19, 2025 at 5:34 PM Danilo Krummrich <dakr@...nel.org> wrote:
>>
>> On Wed Aug 20, 2025 at 12:53 AM CEST, Matthew Maurer wrote:
>> > Adds a new sample driver that demonstrates the debugfs APIs.
>> >
>> > The driver creates a directory in debugfs and populates it with a few
>> > files:
>> > - A read-only file that displays a fwnode property.
>> > - A read-write file that exposes an atomic counter.
>> > - A read-write file that exposes a custom struct.
>> >
>> > This sample serves as a basic example of how to use the `debugfs::Dir`
>> > and `debugfs::File` APIs to create and manage debugfs entries.
>> >
>> > Signed-off-by: Matthew Maurer <mmaurer@...gle.com>
>>
>> This is a great example, thanks! I really like how the API turned out.
>>
>> When it comes to the newly added Scope API - and I assume this does not come at
>> a surprise - I have some concerns.
>
> Yes, I expected this to be the case, but inspired by some of the
> comments about wanting to just create files off fields and forget
> about them, I wanted to take one more crack at it.
>
>>
>> But first, thanks a lot for posting the socinfo driver in both variants, with
>> and without the Scope API.
>>
>> I had a brief look at both of those and I can see why you want this.
>>
>> With the Scope thing you can indeed write things a bit more compressed (I think
>> in the patches the differences looks quite a bit bigger than it actually is,
>> because the scope-based one uses quite some code from the file-based one).
>>
>> I think the downsides are mainly:
>>
>>   - The degree of complexity added for a rather specific use-case, that is also
>>     perfectly representable with the file-based API.
> I don't *think* this is just for this use case - if I just wanted to
> improve the DebugFS use case, I'd mostly be looking at additional code
> for `pin-init` (adding an `Option` placement + a few ergonomic

`Option` is currently not possible to support, since we can't set solely
the discriminant (it must always be a write to the entire enum, thus
requiring a move), see [1].

[1]: https://github.com/Rust-for-Linux/pin-init/issues/59

> improvements to `pin_init` would slim off a large chunk of the code).

I'd be interested in what kinds of improvements you need, maybe they are
simple enough to just include :)

---
Cheers,
Benno

> The idea here was that a file might not always directly correspond to
> a field in a data structure, and the `File` API forces it to be one.
> We could decide that forcing every file to be a data structure field
> is a good idea, but I'm not certain it is.