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

Lee Jones
2024-12-06T07:16:52Z

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@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?

Fair point. I'll speak to Alice.

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

--
Lee Jones [李琼斯]