linux-kernel - Re: [PATCH v2 5/6] rust: debugfs: Allow access to device in Devres-wrapped scopes

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Message-ID: <CAGSQo03g8bjRosYVPzxhK9FpSW1ZfW52unPaoxpcn=pXaDUriQ@mail.gmail.com>
Date: Tue, 3 Feb 2026 10:04:03 -0800
From: Matthew Maurer <mmaurer@...gle.com>
To: Gary Guo <gary@...yguo.net>
Cc: Bjorn Andersson <andersson@...nel.org>, Konrad Dybcio <konradybcio@...nel.org>, 
	Satya Durga Srinivasu Prabhala <satyap@...cinc.com>, Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Daniel Almeida <daniel.almeida@...labora.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Michal Wilczynski <m.wilczynski@...sung.com>, Dave Ertman <david.m.ertman@...el.com>, 
	Ira Weiny <ira.weiny@...el.com>, Leon Romanovsky <leon@...nel.org>, Trilok Soni <tsoni@...cinc.com>, 
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, driver-core@...ts.linux.dev, 
	dri-devel@...ts.freedesktop.org, linux-pwm@...r.kernel.org
Subject: Re: [PATCH v2 5/6] rust: debugfs: Allow access to device in
 Devres-wrapped scopes

On Tue, Feb 3, 2026 at 8:48 AM Gary Guo <gary@...yguo.net> wrote:
>
> On Tue Feb 3, 2026 at 3:46 PM GMT, Matthew Maurer wrote:
> > This adds support for creating a DebugFS directory which is aware that
> > it is bound to a device. As a result, callbacks under that directory
> > have access to a bound device which gives them efficient access to other
> > Devres, ability to use dev_err! and friends, etc.
> >
> > Signed-off-by: Matthew Maurer <mmaurer@...gle.com>
> > ---
> >  rust/kernel/debugfs.rs | 40 ++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 40 insertions(+)
> >
> > diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
> > index d7b8014a6474698235203f2b7d8fec96f2bb43f8..ac614d693fa73929d095b669e9ba61958bec609e 100644
> > --- a/rust/kernel/debugfs.rs
> > +++ b/rust/kernel/debugfs.rs
> > @@ -11,6 +11,11 @@
> >  #[cfg(CONFIG_DEBUG_FS)]
> >  use crate::sync::Arc;
> >  use crate::{
> > +    device::{
> > +        Bound,
> > +        Device, //
> > +    },
> > +    devres::Devres,
> >      fmt,
> >      prelude::*,
> >      str::CStr,
> > @@ -722,3 +727,38 @@ fn new(name: &CStr) -> ScopedDir<'data, 'static> {
> >          }
> >      }
> >  }
> > +
> > +impl<'a, T: 'a + Send> Devres<Scope<T>> {
> > +    /// Creates a new scope, which is a directory at the root of the debugfs filesystem,
> > +    /// associated with some data `T`, enclosed in a [`Devres`] for the provided device.
> > +    ///
> > +    /// The `init` closure is called to populate the directory with files and subdirectories. These
> > +    /// files can reference the data stored in the scope. Because it is stored inside a `Devres`,
> > +    /// the init method is granted access to a `&Device<Bound>`.
> > +    ///
> > +    /// This can be used for cheaply accessing device-protected data inside DebugFS methods or
> > +    /// accessing device-specific methods (e.g. [`dev_err!`]).
> > +    ///
> > +    /// The entire directory tree created within the scope will be removed when the returned
> > +    /// `Scope` handle is dropped.
> > +    pub fn dir<E: 'a, F>(
> > +        dev: &'a Device<Bound>,
> > +        data: impl PinInit<T, E> + 'a,
> > +        name: &'a CStr,
> > +        init: F,
> > +    ) -> impl PinInit<Self, Error> + 'a
> > +    where
> > +        F: for<'data, 'dir> FnOnce(&'data T, &'data Device<Bound>, &'dir ScopedDir<'data, 'dir>)
> > +            + 'a,
> > +        Error: From<E>,
> > +    {
> > +        Devres::new(
> > +            dev,
> > +            Scope::new(data, |data| {
> > +                let scoped = ScopedDir::new(name);
> > +                init(data, dev, &scoped);
> > +                scoped.into_entry()
> > +            }),
> > +        )
> > +    }
> > +}
>
> I think it is a big strange to have this on `Devres` (in patch v6 it has `Devres::dir` doesn't make
> too much sense). I would suggest that we domsomething like
>
>     impl<'a, T: 'a + Send> Scope<T> {
>         pub fn devres_dir(
>             ...
>         ) -> impl PinInit<Devres<Self>, Error> + 'a;
>     }
>
> To me `Devres` is just a generic container type, just like `Arc` and `ARef`, so
> the assoc functions should be defined on the concrete type.
>
> Also: is there a reason that this needs a special API, and by
>
>     Devres::new(device, Scope::dir(data, c"name", |data| {
>         // use data and device
>     });
>
> ?

Yes - that won't work, because the function being provided to
`Scope::dir` is `for<'data, 'dir> FnOnce(&'data T, &'dir
ScopedDir<'data, 'dir>)` - this means that *intentionally*, if you
capture any non-static-lifetime variable from outside the closure, you
won't be able to use it with the methods on `ScopedDir`, because the
`'data` lifetime bound should stop you. In the general case, we
wouldn't want a reference with the same lifetime as `device` in that
example to be usable inside the debugfs callbacks. The device of a
Devres wrapped scope is a special case because we know that it will
outlive it.

>
> Best,
> Gary
>
>