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

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Message-ID: <2024120648-finch-shrubbery-c6f5@gregkh>
Date: Fri, 6 Dec 2024 07:42:50 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: Lee Jones <lee@...nel.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 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?

thanks,

greg k-h