linux-kernel - Re: [PATCH v4 6/9] rust: drm: gem: shmem: Add DRM shmem helper abstraction

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Message-ID: <aPJE8EC5OmlmSrYt@google.com>
Date: Fri, 17 Oct 2025 13:30:24 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Lyude Paul <lyude@...hat.com>
Cc: dri-devel@...ts.freedesktop.org, rust-for-linux@...r.kernel.org, 
	Daniel Almeida <daniel.almeida@...labora.com>, Asahi Lina <lina@...hilina.net>, 
	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>, 
	Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Asahi Lina <lina+kernel@...hilina.net>, 
	Viresh Kumar <viresh.kumar@...aro.org>, Shankari Anand <shankari.ak0208@...il.com>, 
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 6/9] rust: drm: gem: shmem: Add DRM shmem helper abstraction

On Thu, Oct 16, 2025 at 05:08:19PM -0400, Lyude Paul wrote:
> From: Asahi Lina <lina@...hilina.net>
> 
> The DRM shmem helper includes common code useful for drivers which
> allocate GEM objects as anonymous shmem. Add a Rust abstraction for
> this. Drivers can choose the raw GEM implementation or the shmem layer,
> depending on their needs.
> 
> Signed-off-by: Asahi Lina <lina@...hilina.net>
> Signed-off-by: Daniel Almeida <daniel.almeida@...labora.com>
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> +impl<T: DriverObject> gem::IntoGEMObject for Object<T> {
> +    fn as_raw(&self) -> *mut bindings::drm_gem_object {
> +        // SAFETY: Our immutable reference is proof that this is are to dereference
> +        unsafe { &raw mut (*self.obj.get()).base }

Typo.

Also, in principle, this should refer to a type invariant saying that
`obj` is a valid shmem object.

> +    }
> +
> +    unsafe fn from_raw<'a>(obj: *mut bindings::drm_gem_object) -> &'a Object<T> {
> +        // SAFETY: The safety contract of from_gem_obj() guarantees that `obj` is contained within
> +        // `Self`
> +        unsafe {
> +            let obj: *mut Opaque<_> =
> +                container_of!(obj, bindings::drm_gem_shmem_object, base).cast();

Please use Opaque::cast_from() instead of the unrestricted cast()
operation.

There are several instances of this throughout the file.

Alice