linux-kernel - Re: [PATCH v9 0/5] rust: DebugFS Bindings

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Message-ID: <2025071014-radiantly-dreamland-4017@gregkh>
Date: Thu, 10 Jul 2025 07:27:49 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Matthew Maurer <mmaurer@...gle.com>, 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
Subject: Re: [PATCH v9 0/5] rust: DebugFS Bindings

On Thu, Jul 10, 2025 at 12:12:15AM +0200, Danilo Krummrich wrote:
> On Wed, Jul 09, 2025 at 03:04:51PM -0700, Matthew Maurer wrote:
> > On Wed, Jul 9, 2025 at 2:59 PM Danilo Krummrich <dakr@...nel.org> wrote:
> > >
> > > On Wed Jul 9, 2025 at 11:53 PM CEST, Matthew Maurer wrote:
> > > > On Wed, Jul 9, 2025 at 2:47 PM Danilo Krummrich <dakr@...nel.org> wrote:
> > > >>
> > > >> On Wed Jul 9, 2025 at 9:09 PM CEST, Matthew Maurer wrote:
> > > >> > This series provides safe DebugFS bindings for Rust, with a sample
> > > >> > module using them.
> > > >> >
> > > >> > Example interaction with the sample driver:
> > > >>
> > > >> I understand what you're trying to do here, i.e. showcase that values exported
> > > >> via debugfs can be altered.
> > > >>
> > > >> The problem is that the current abstractions only implement read(), but not
> > > >> write().
> > > >
> > > > I was trying to keep the initial bindings simple. Adding `write` is
> > > > definitely something we could do, but I thought maybe that could be in
> > > > a subsequent patch.
> > >
> > > Absolutely, yes! I didn't mean to ask to add it now. :)
> > >
> > > >> If you really want to showcase changing values, you can, for instance, create a
> > > >> workqueue inside the sample driver and modify the counter periodically.
> > > >
> > > > This is supposed to be sample code, so ideally it should be as narrow
> > > > as is reasonable in what subsystems it touches, no? If people would
> > > > really prefer the sample schedule a ticking counter I can do that, but
> > > > it already felt weird to be registering a platform driver in a debugfs
> > > > sample.
> > >
> > > I'm not asking to do that. If the values don't change for now, because
> > > there's no write() yet, that's perfectly fine with me. :)
> > 
> > Potentially I misinterpreted Greg[1], I thought he wanted to see how
> > mutation would work.
> > If we don't need mutation, I'm fine simplifying the driver to not have
> > any mutation triggers and just export a few random things.
> 
> I mean, the most simple way would be to create the debugfs entries in probe()
> and mutate them - still in probe() - right afterwards once. I think we should
> do in any case. And AFAICT, this also covers [1].
> 
> > [1]: https://lore.kernel.org/all/2025070349-tricky-arguable-5362@gregkh/

Ugh.

Yes we need write.  And read, and custom file-ops, and the like as
that's what debugfs is doing today for C code!  We need this to be as
simple as, or almost as simple as, what we have today in C or no one is
going to use this stuff and go off and attempt to write their own mess.

While I would love to have something as simple as:
	void debugfs_create_u8(const char *name, umode_t mode, struct dentry *parent, u8 *value);
like we do today.  I understand that this makes all sorts of
"assumptions" that Rust really doesn't like (i.e. lifetime of *value and
the like), BUT we MUST have something like this for Rust users, as
that's going to ensure that people actually use this api.

Look at an in-kernel function today, like ath9k_init_debug() that
creates a metric-ton of debugfs files and binds them to different
variables that are owned by a structure and more complex data structures
and memory dumps and other random file interactions.  We need, in Rust,
a way to do everything that that function can do today, in a SIMPLE
manner that reads just as easily as ath9k_init_debug() does.

So no "we will add write support later" stuff, sorry, real drivers
require write support in debugfs.

thanks,

greg k-h