Re: [PATCH v2 1/4] rust: device: implement Device::parent()

[Danilo Krummrich <dakr@kernel.org>]

On Fri, Mar 21, 2025 at 07:40:57AM -0700, Boqun Feng wrote:
> On Thu, Mar 20, 2025 at 11:27:43PM +0100, Danilo Krummrich wrote:
> > Device::parent() returns a reference to the device' parent device, if
> > any.
> > 
> > Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> > Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> > ---
> >  rust/kernel/device.rs | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> > 
> > diff --git a/rust/kernel/device.rs b/rust/kernel/device.rs
> > index 21b343a1dc4d..f6bdc2646028 100644
> > --- a/rust/kernel/device.rs
> > +++ b/rust/kernel/device.rs
> > @@ -65,6 +65,21 @@ pub(crate) fn as_raw(&self) -> *mut bindings::device {
> >          self.0.get()
> >      }
> >  
> > +    /// Returns a reference to the parent device, if any.
> > +    pub fn parent<'a>(&self) -> Option<&'a Self> {
> > +        // SAFETY:
> > +        // - By the type invariant `self.as_raw()` is always valid.
> > +        // - The parent device is only ever set at device creation.
> > +        let parent = unsafe { (*self.as_raw()).parent };
> > +
> > +        if parent.is_null() {
> > +            None
> > +        } else {
> > +            // SAFETY: Since `parent` is not NULL, it must be a valid pointer to a `struct device`.
> > +            Some(unsafe { Self::as_ref(parent) })
> 
> The safety comment also needs to explain why the parent device won't be
> gone, I assume a struct device holds a refcount of its parent?

Correct, this is taken generically in device_add().

> Therefore
> the borrow checker would ensure the parent exists as long as the Device
> is borrowed.

Yes.