Re: [PATCH] rust: drm/gem: Remove Object.dev

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Oct 16, 2025 at 04:18:58PM -0400, Lyude Paul wrote:
> I noticed by chance that there's actually already a pointer to this in
> struct drm_gem_object. So, no use in carrying this around!
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> ---
>  rust/kernel/drm/gem/mod.rs | 11 +++--------
>  1 file changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
> index 30c853988b942..28d929edae267 100644
> --- a/rust/kernel/drm/gem/mod.rs
> +++ b/rust/kernel/drm/gem/mod.rs
> @@ -187,12 +187,10 @@ impl<T: IntoGEMObject> BaseObject for T {}
>  /// Invariants
>  ///
>  /// - `self.obj` is a valid instance of a `struct drm_gem_object`.
> -/// - `self.dev` is always a valid pointer to a `struct drm_device`.
>  #[repr(C)]
>  #[pin_data]
>  pub struct Object<T: DriverObject + Send + Sync> {
>      obj: Opaque<bindings::drm_gem_object>,
> -    dev: NonNull<drm::Device<T::Driver>>,
>      #[pin]
>      data: T,
>  }
> @@ -222,9 +220,6 @@ pub fn new(dev: &drm::Device<T::Driver>, size: usize) -> Result<ARef<Self>> {
>              try_pin_init!(Self {
>                  obj: Opaque::new(bindings::drm_gem_object::default()),
>                  data <- T::new(dev, size),
> -                // INVARIANT: The drm subsystem guarantees that the `struct drm_device` will live
> -                // as long as the GEM object lives.
> -                dev: dev.into(),
>              }),
>              GFP_KERNEL,
>          )?;
> @@ -247,9 +242,9 @@ pub fn new(dev: &drm::Device<T::Driver>, size: usize) -> Result<ARef<Self>> {
>  
>      /// Returns the `Device` that owns this GEM object.
>      pub fn dev(&self) -> &drm::Device<T::Driver> {
> -        // SAFETY: The DRM subsystem guarantees that the `struct drm_device` will live as long as
> -        // the GEM object lives, hence the pointer must be valid.
> -        unsafe { self.dev.as_ref() }
> +        // SAFETY: `struct drm_gem_object.dev` is initialized and valid for as long as the GEM
> +        // object lives.
> +        unsafe { drm::Device::from_raw((*self.as_raw()).dev) }

The wording here and in the invariants could be improved to say that
T::Driver is the right choice for the generic parameter.

Alice