Message-Id: <91A174DE-B7A1-4F35-ADAB-39873B17A3D4@collabora.com>
Date: Thu, 4 Sep 2025 10:42:55 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Lyude Paul <lyude@...hat.com>
Cc: dri-devel@...ts.freedesktop.org,
 rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 Danilo Krummrich <dakr@...nel.org>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>,
 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>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>,
 Asahi Lina <lina+kernel@...hilina.net>,
 "open list:DRM DRIVER FOR NVIDIA GPUS [RUST]" <nouveau@...ts.freedesktop.org>
Subject: Re: [PATCH v3 09/14] rust: gem: Introduce DriverObject::Args



> On 29 Aug 2025, at 19:35, Lyude Paul <lyude@...hat.com> wrote:
> 
> This is an associated type that may be used in order to specify a data-type
> to pass to gem objects when construction them, allowing for drivers to more

This doesn’t read very well IMHO.

> easily initialize their private-data for gem objects.

nit: I think that "private data" doesn’t need a hyphen?

> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> 
> ---
> V3:
> * s/BaseDriverObject/DriverObject/
> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> ---
> drivers/gpu/drm/nova/gem.rs |  5 ++-
> rust/kernel/drm/gem/mod.rs  | 75 +++++++++++++++++++++++++++++++++----
> 2 files changed, 71 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/nova/gem.rs b/drivers/gpu/drm/nova/gem.rs
> index 10e3053f1a246..015cb56061a56 100644
> --- a/drivers/gpu/drm/nova/gem.rs
> +++ b/drivers/gpu/drm/nova/gem.rs
> @@ -19,8 +19,9 @@ pub(crate) struct NovaObject {}
> impl gem::DriverObject for NovaObject {
>     type Driver = NovaDriver;
>     type Object = gem::Object<Self>;
> +    type Args = ();
> 
> -    fn new(_dev: &NovaDevice, _size: usize) -> impl PinInit<Self, Error> {
> +    fn new(_dev: &NovaDevice, _size: usize, _args: Self::Args) -> impl PinInit<Self, Error> {
>         try_pin_init!(NovaObject {})
>     }
> }
> @@ -34,7 +35,7 @@ pub(crate) fn new(dev: &NovaDevice, size: usize) -> Result<ARef<gem::Object<Self
>             return Err(EINVAL);
>         }
> 
> -        gem::Object::new(dev, aligned_size)
> +        gem::Object::new(dev, aligned_size, ())
>     }
> 
>     /// Look up a GEM object handle for a `File` and return an `ObjectRef` for it.
> diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
> index f901d4263ee87..fe6ff3762a504 100644
> --- a/rust/kernel/drm/gem/mod.rs
> +++ b/rust/kernel/drm/gem/mod.rs
> @@ -6,13 +6,14 @@
> 
> use crate::{
>     alloc::flags::*,
> -    bindings, drm,
> +    bindings,
>     drm::driver::{AllocImpl, AllocOps},
> +    drm::{self, private::Sealed},
>     error::{to_result, Result},
>     prelude::*,
>     types::{ARef, AlwaysRefCounted, Opaque},
> };
> -use core::{ops::Deref, ptr::NonNull};
> +use core::{marker::PhantomData, ops::Deref, ptr::NonNull};
> 
> /// A type alias for retrieving a [`Driver`]s [`DriverFile`] implementation from its
> /// [`DriverObject`] implementation.
> @@ -21,6 +22,26 @@
> /// [`DriverFile`]: drm::file::DriverFile
> pub type DriverFile<T> = drm::File<<<T as DriverObject>::Driver as drm::Driver>::File>;
> 
> +/// A helper macro for implementing AsRef<OpaqueObject<…>>
> +macro_rules! impl_as_opaque {
> +    ($type:ty where $tparam:ident : $tparam_trait:ident) => {
> +        impl<D, $tparam> core::convert::AsRef<kernel::drm::gem::OpaqueObject<D>> for $type
> +        where
> +            D: kernel::drm::driver::Driver,
> +            Self: kernel::drm::gem::DriverObject<Driver = D>,
> +            Self: kernel::drm::gem::IntoGEMObject,
> +            $tparam: $tparam_trait,
> +        {
> +            fn as_ref(&self) -> &kernel::drm::gem::OpaqueObject<D> {
> +                // SAFETY: This cast is safe via our type invariant.
> +                unsafe { &*((self.as_raw().cast_const()).cast()) }
> +            }
> +        }
> +    };
> +}
> +
> +pub(crate) use impl_as_opaque;
> +
> /// GEM object functions, which must be implemented by drivers.
> pub trait DriverObject: Sync + Send + Sized {
>     /// Parent `Driver` for this object.
> @@ -29,8 +50,15 @@ pub trait DriverObject: Sync + Send + Sized {
>     /// The GEM object type that will be passed to various callbacks.
>     type Object: AllocImpl;
> 
> +    /// The data type to use for passing arguments to [`BaseDriverObject::new`].
> +    type Args;
> +
>     /// Create a new driver data object for a GEM object of a given size.
> -    fn new(dev: &drm::Device<Self::Driver>, size: usize) -> impl PinInit<Self, Error>;
> +    fn new(
> +        dev: &drm::Device<Self::Driver>,
> +        size: usize,
> +        args: Self::Args,
> +    ) -> impl PinInit<Self, Error>;
> 
>     /// Open a new handle to an existing object, associated with a File.
>     fn open(_obj: &Self::Object, _file: &DriverFile<Self>) -> Result {
> @@ -42,7 +70,7 @@ fn close(_obj: &Self::Object, _file: &DriverFile<Self>) {}
> }
> 
> /// Trait that represents a GEM object subtype
> -pub trait IntoGEMObject: Sized + super::private::Sealed + AlwaysRefCounted {
> +pub trait IntoGEMObject: Sized + Sealed + AlwaysRefCounted {
>     /// Returns a reference to the raw `drm_gem_object` structure, which must be valid as long as
>     /// this owning object is valid.
>     fn as_raw(&self) -> *mut bindings::drm_gem_object;
> @@ -233,11 +261,11 @@ impl<T: DriverObject> Object<T> {
>     };
> 
>     /// Create a new GEM object.
> -    pub fn new(dev: &drm::Device<T::Driver>, size: usize) -> Result<ARef<Self>> {
> +    pub fn new(dev: &drm::Device<T::Driver>, size: usize, args: T::Args) -> Result<ARef<Self>> {
>         let obj: Pin<KBox<Self>> = KBox::pin_init(
>             try_pin_init!(Self {
>                 obj: Opaque::new(bindings::drm_gem_object::default()),
> -                data <- T::new(dev, size),
> +                data <- T::new(dev, size, args),
>                 // INVARIANT: The drm subsystem guarantees that the `struct drm_device` will live
>                 // as long as the GEM object lives.
>                 dev: dev.into(),
> @@ -289,7 +317,7 @@ extern "C" fn free_callback(obj: *mut bindings::drm_gem_object) {
>     }
> }
> 
> -impl<T: DriverObject> super::private::Sealed for Object<T> {}
> +impl<T: DriverObject> Sealed for Object<T> {}
> 
> impl<T: DriverObject> Deref for Object<T> {
>     type Target = T;
> @@ -313,6 +341,39 @@ impl<T: DriverObject> AllocImpl for Object<T> {
>     };
> }
> 
> +impl_as_opaque!(Object<T> where T: DriverObject);
> +
> +/// A GEM object whose private-data layout is not known.
> +///
> +/// Not all GEM objects are created equal, and subsequently drivers may occasionally need to deal
> +/// with situations where they are working with a GEM object but have no knowledge of its
> +/// private-data layout.
> +///
> +/// It may be used just like a normal [`Object`], with the exception that it cannot access
> +/// driver-private data.
> +///
> +/// # Invariant
> +///
> +/// Via `#[repr(transparent)]`, this type is guaranteed to have an identical data layout to
> +/// `struct drm_gem_object`.
> +#[repr(transparent)]
> +pub struct OpaqueObject<T: drm::Driver>(Opaque<bindings::drm_gem_object>, PhantomData<T>);

I’m not sure whether this belongs in this patch.

I agree with the motivation, but where exactly is this used for now? I don't
see it being passed in a callback, for example. The only way to get one would
be through as_ref() IIUC, but who would call this and why?

> +
> +impl<T: drm::Driver> IntoGEMObject for OpaqueObject<T> {
> +    unsafe fn from_raw<'a>(self_ptr: *mut bindings::drm_gem_object) -> &'a Self {
> +        // SAFETY:
> +        // - This cast is safe via our type invariant.
> +        // - `self_ptr` is guaranteed to be a valid pointer to a gem object by our safety contract.
> +        unsafe { &*self_ptr.cast::<Self>().cast_const() }
> +    }
> +
> +    fn as_raw(&self) -> *mut bindings::drm_gem_object {
> +        self.0.get()
> +    }
> +}
> +
> +impl<D: drm::Driver> Sealed for OpaqueObject<D> {}
> +
> pub(super) const fn create_fops() -> bindings::file_operations {
>     // SAFETY: As by the type invariant, it is safe to initialize `bindings::file_operations`
>     // zeroed.
> -- 
> 2.50.0
> 

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