Re: [PATCH v3 1/5] rust: miscdevice: Provide accessor to pull out miscdevice::this_device

[Greg KH <gregkh@linuxfoundation.org>]

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@kernel.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