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

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Message-ID: <aPJAOXtv2knFkyDP@google.com>
Date: Fri, 17 Oct 2025 13:10:17 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Lyude Paul <lyude@...hat.com>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, Danilo Krummrich <dakr@...nel.org>, 
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Daniel Almeida <daniel.almeida@...labora.com>, Asahi Lina <lina+kernel@...hilina.net>, 
	Shankari Anand <shankari.ak0208@...il.com>
Subject: Re: [PATCH] rust: drm/gem: Remove Object.dev

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@...hat.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