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

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Message-ID: <CAGSQo02tdxoqTE1z5_M8g0PpXYEdWr4Yy31yaoPK8C_O2G_QoQ@mail.gmail.com>
Date: Tue, 1 Jul 2025 11:32:42 -0700
From: Matthew Maurer <mmaurer@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
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 1, 2025 at 10:34 AM Danilo Krummrich <dakr@...nel.org> wrote:
>
> 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.

I'm confused as to how registering as a platform driver would result
in an input source that would let me trigger the creation/destruction
of DebugFS files. I need some kind of input stream to do that. Is
there some input stream that's available to a platform driver that I'm
missing, or are you suggesting that the input stream would effectively
be the probe's `id_info` field? If I did that, wouldn't I still have a
static arrangement of DebugFS files in my driver struct?

I could have misunderstood, but I don't think that's what Greg is
asking for - I think he wants to see how at a data structure level, I
can handle creating and destroying DebugFS files that correspond to
some kind of object being created and destroyed, rather than just
having a static list of slots in my driver struct for keeping them
alive.