linux-kernel - Re: [PATCH v8 6/6] rust: samples: Add debugfs sample

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Message-ID: <aGQcODIzMiB46gKF@pollux>
Date: Tue, 1 Jul 2025 19:34:48 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Matthew Maurer <mmaurer@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	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>,
	"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,
	Dirk Behme <dirk.behme@...bosch.com>
Subject: Re: [PATCH v8 6/6] rust: samples: Add debugfs sample

On Tue, Jul 01, 2025 at 10:24:04AM -0700, Matthew Maurer wrote:
> On Tue, Jul 1, 2025 at 7:03 AM Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
> >
> > On Fri, Jun 27, 2025 at 11:18:29PM +0000, Matthew Maurer wrote:
> > > +        // An `Arc<Mutex<usize>>` doesn't implement display, so let's give explicit instructions on
> > > +        // how to print it
> > > +        let file_2 = sub.fmt_file(c_str!("arc_backed"), my_arc.clone(), &|val, f| {
> > > +            writeln!(f, "locked value: {:#010x}", *val.lock())
> > > +        });
> >
> > While cute, is this really going to be the way to describe all "custom"
> > debugfs function callbacks?  No other way to point to a function itself
> > instead?  Look at "fun" debugfs functions like qh_lines() in
> > drivers/usb/host/ehci-dbg.c that is dumping tons of data out.  Putting
> > that inline here is going to be a bit ackward :)
> 
> Good news, function pointers are legal to pass in here as well
> already, I can add that usage to make it clear.
> 
> >
> > So can you show an example of a "traditional" debugfs file output with
> > multiple lines that is dealing with a dynamically allocated device that
> > is associated with the module (not the static example you showed here),
> > as that's going to be the real way this is used, not with static
> > variables.
> 
> Sure, do we want to:
> * Finish creating the driver struct early in `init`, then call dynamic
> `.create(&str)` or `.destroy(&str)` `.modify(&str)` type things on it
> in `init` to show how it would work
> * Actually wire up an input source to drive create/destroy/modify
> dynamically (e.g. I could implement a miscdevice) - if you want this
> one, do you have a preference on where I get my input signal from?

I think the idea was to show how it works in a real driver context, e.g. a
platform driver, just like what samples/rust/rust_driver_platform.rs does. Not a
miscdevice registered from a module, which is a rather rare use-case.

If you rebase on the latest driver-core-next, you can write a platform driver
with an ACPI ID table, which can easily probed by passing
`-acpitable file=ssdt.aml` to qemu, i.e. no need to mess with OF.