linux-kernel - Re: [PATCH v3 1/5] rust: miscdevice: Provide accessor to pull out miscdevice::this

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Message-ID: <20241206071646.GE8882@google.com>
Date: Fri, 6 Dec 2024 07:16:46 +0000
From: Lee Jones <lee@...nel.org>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: linux-kernel@...r.kernel.org, arnd@...db.de, ojeda@...nel.org,
	alex.gaynor@...il.com, boqun.feng@...il.com, gary@...yguo.net,
	bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
	a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu,
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v3 1/5] rust: miscdevice: Provide accessor to pull out
 miscdevice::this_device

On Fri, 06 Dec 2024, Greg KH wrote:

> On Thu, Dec 05, 2024 at 04:25:18PM +0000, Lee Jones wrote:
> > There are situations where a pointer to a `struct device` will become
> > necessary (e.g. for calling into dev_*() functions).  This accessor
> > allows callers to pull this out from the `struct miscdevice`.
> > 
> > Signed-off-by: Lee Jones <lee@...nel.org>
> > ---
> >  rust/kernel/miscdevice.rs | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
> > index 7e2a79b3ae26..55340f316006 100644
> > --- a/rust/kernel/miscdevice.rs
> > +++ b/rust/kernel/miscdevice.rs
> > @@ -10,11 +10,13 @@
> >  
> >  use crate::{
> >      bindings,
> > +    device::Device,
> >      error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
> >      prelude::*,
> >      str::CStr,
> >      types::{ForeignOwnable, Opaque},
> >  };
> > +
> >  use core::{
> >      ffi::{c_int, c_long, c_uint, c_ulong},
> >      marker::PhantomData,
> > @@ -84,6 +86,13 @@ pub fn register(opts: MiscDeviceOptions) -> impl PinInit<Self, Error> {
> >      pub fn as_raw(&self) -> *mut bindings::miscdevice {
> >          self.inner.get()
> >      }
> > +
> > +    /// Returns a pointer to the current Device
> > +    pub fn device(&self) -> &Device {
> > +        // SAFETY: This is only accessible after a successful register() which always
> > +        // initialises this_device with a valid device.
> > +        unsafe { Device::as_ref((*self.as_raw()).this_device) }
> 
> A "raw" pointer that you can do something with without incrementing the
> reference count of it?  Oh wait, no, it's the rust device structure.
> If so, why isn't this calling the get_device() interface instead?  That
> way it's properly incremented and decremented when it "leaves the scope"
> right?
> 
> Or am I missing something here as to why that wouldn't work and this is
> the only way to get access to the 'struct device' of this miscdevice?

Fair point.  I'll speak to Alice.

Another reason why using dev_info() is not possible at the moment.

-- 
Lee Jones [李琼斯]