linux-kernel - Re: [PATCH 5/5] rust: device: Add a stub abstraction for devices

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Date:   Mon, 13 Mar 2023 11:05:04 -0700
From:   Boqun Feng <boqun.feng@...il.com>
To:     Gary Guo <gary@...yguo.net>
Cc:     Asahi Lina <lina@...hilina.net>, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        Hector Martin <marcan@...can.st>,
        Sven Peter <sven@...npeter.dev>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Neal Gompa <neal@...pa.dev>, rust-for-linux@...r.kernel.org,
        linux-kernel@...r.kernel.org, asahi@...ts.linux.dev
Subject: Re: [PATCH 5/5] rust: device: Add a stub abstraction for devices

On Mon, Mar 13, 2023 at 05:52:02PM +0000, Gary Guo wrote:
[...]
> > diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> > index 9be021e393ca..e57da622d817 100644
> > --- a/rust/kernel/device.rs
> > +++ b/rust/kernel/device.rs
> > @@ -4,7 +4,7 @@
> >  //!
> >  //! C header: [`include/linux/device.h`](../../../../include/linux/device.h)
> >  
> > -use crate::bindings;
> > +use crate::{bindings, str::CStr};
> >  
> >  /// A raw device.
> >  ///
> > @@ -20,4 +20,78 @@ use crate::bindings;
> >  pub unsafe trait RawDevice {
> >      /// Returns the raw `struct device` related to `self`.
> >      fn raw_device(&self) -> *mut bindings::device;
> > +
> > +    /// Returns the name of the device.
> > +    fn name(&self) -> &CStr {
> > +        let ptr = self.raw_device();
> > +
> > +        // SAFETY: `ptr` is valid because `self` keeps it alive.
> > +        let name = unsafe { bindings::dev_name(ptr) };
> > +
> > +        // SAFETY: The name of the device remains valid while it is alive (because the device is
> > +        // never renamed, per the safety requirement of this trait). This is guaranteed to be the
> > +        // case because the reference to `self` outlives the one of the returned `CStr` (enforced
> > +        // by the compiler because of their lifetimes).
> > +        unsafe { CStr::from_char_ptr(name) }
> > +    }
> > +}
> > +
> > +/// A ref-counted device.
> > +///
> > +/// # Invariants
> > +///
> > +/// `ptr` is valid, non-null, and has a non-zero reference count. One of the references is owned by
> > +/// `self`, and will be decremented when `self` is dropped.
> > +pub struct Device {
> > +    pub(crate) ptr: *mut bindings::device,
> > +}
> > +
> 
> Shouldn't this be
> 
> #[repr(transparent)]
> pub struct Device(Opaque<bindings::device>);
> 
> ?

I have the same feeling, for `task_struct`, we have

	#[repr(transparent)]
	pub struct Task(pub(crate) UnsafeCell<bindings::task_struct>);

and
	
	pub struct TaskRef<'a> {
	    task: &'a Task,
	    _not_send: PhantomData<*mut ()>,
	}

I wonder whether we should do the similar for `Device`, meaning `Device`
is just a tranparent representation for `struct device` and another
type (say, `DeviceRef`) is introduced as the reference type, or we can
just use `ARef`.

Regards,
Boqun



> 
> Best,
> Gary