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Message-Id: <662189D6-44B2-4880-971D-A3D2D748542D@collabora.com>
Date: Thu, 4 Sep 2025 09:11:51 -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 01/14] rust: drm: gem: Simplify use of generics
Hi Lyude,
> On 29 Aug 2025, at 19:35, Lyude Paul <lyude@...hat.com> wrote:
>
> Now that my rust skills have been honed, I noticed that there's a lot of
> generics in our gem bindings that don't actually need to be here. Currently
> the hierarchy of traits in our gem bindings looks like this:
>
> * Drivers implement:
> * BaseDriverObject<T: DriverObject> (has the callbacks)
> * DriverObject (has the drm::Driver type)
> * Crate implements:
> * IntoGEMObject for Object<T> where T: DriverObject
> Handles conversion to/from raw object pointers
> * BaseObject for T where T: IntoGEMObject
> Provides methods common to all gem interfaces
>
> Also of note, this leaves us with two different drm::Driver associated
> types:
> * DriverObject::Driver
> * IntoGEMObject::Driver
>
> I'm not entirely sure of the original intent here unfortunately (if anyone
> is, please let me know!), but my guess is that the idea would be that some
> objects can implement IntoGEMObject using a different ::Driver than
> DriverObject - presumably to enable the usage of gem objects from different
> drivers. A reasonable usecase of course.
>
> However - if I'm not mistaken, I don't think that this is actually how
> things would go in practice. Driver implementations are of course
> implemented by their associated drivers, and generally drivers are not
> linked to each-other when building the kernel. Which is to say that even in
> a situation where we would theoretically deal with gem objects from another
> driver, we still wouldn't have access to its drm::driver::Driver
> implementation. It's more likely we would simply want a variant of gem
> objects in such a situation that have no association with a
> drm::driver::Driver type.
>
> Taking that into consideration, we can assume the following:
> * Anything that implements BaseDriverObject will implement DriverObject
> In other words, all BaseDriverObjects indirectly have an associated
> ::Driver type - so the two traits can be combined into one with no
> generics.
> * Not everything that implements IntoGEMObject will have an associated
> ::Driver, and that's OK.
>
> And with this, we now can do quite a bit of cleanup with the use of
> generics here. As such, this commit:
>
> * Removes the generics on BaseDriverObject
> * Moves DriverObject::Driver into BaseDriverObject
> * Removes DriverObject
> * Removes IntoGEMObject::Driver
> * Add AllocImpl::Driver, which we can use as a binding to figure out the
> correct File type for BaseObject
>
> Leaving us with a simpler trait hierarchy that now looks like this:
>
> * Drivers implement: BaseDriverObject
> * Crate implements:
> * IntoGEMObject for Object<T> where T: DriverObject
> * BaseObject for T where T: IntoGEMObject
>
> Which makes the code a lot easier to understand and build on :).
>
> Signed-off-by: Lyude Paul <lyude@...hat.com>
>
> ---
> V2:
> * Don't refer to Object<T> in callbacks, as this would result in drivers
> getting the wrong gem object type for shmem gem objects once we add
> support for those. Instead, we'll just add a type alias to clean this
> part up.
> V3:
> * Fix nova compilation
> * Also, add an associated driver type to AllocImpl - as we still need the
> current driver accessible from BaseObject so that we can use the driver's
> various associated types, like File
> V4:
> * Add missing Object = Self constraint to type bounds for create_handle,
> lookup_handle. I forgot that if drivers can have private gem objects with
> a different data layout, we can only guarantee gem objects with handles
> are of the same gem object type as the main one in use by the driver.
>
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> ---
> drivers/gpu/drm/nova/gem.rs | 8 ++--
> rust/kernel/drm/driver.rs | 3 ++
> rust/kernel/drm/gem/mod.rs | 77 ++++++++++++++++---------------------
> 3 files changed, 40 insertions(+), 48 deletions(-)
>
> diff --git a/drivers/gpu/drm/nova/gem.rs b/drivers/gpu/drm/nova/gem.rs
> index cd82773dab92c..2760ba4f3450b 100644
> --- a/drivers/gpu/drm/nova/gem.rs
> +++ b/drivers/gpu/drm/nova/gem.rs
> @@ -16,16 +16,14 @@
> #[pin_data]
> pub(crate) struct NovaObject {}
>
> -impl gem::BaseDriverObject<gem::Object<NovaObject>> for NovaObject {
> +impl gem::DriverObject for NovaObject {
> + type Driver = NovaDriver;
> +
> fn new(_dev: &NovaDevice, _size: usize) -> impl PinInit<Self, Error> {
> try_pin_init!(NovaObject {})
> }
> }
>
> -impl gem::DriverObject for NovaObject {
> - type Driver = NovaDriver;
> -}
> -
> impl NovaObject {
> /// Create a new DRM GEM object.
> pub(crate) fn new(dev: &NovaDevice, size: usize) -> Result<ARef<gem::Object<Self>>> {
> diff --git a/rust/kernel/drm/driver.rs b/rust/kernel/drm/driver.rs
> index fe7e8d06961aa..dae0f4d1bbe3c 100644
> --- a/rust/kernel/drm/driver.rs
> +++ b/rust/kernel/drm/driver.rs
> @@ -86,6 +86,9 @@ pub struct AllocOps {
>
> /// Trait for memory manager implementations. Implemented internally.
> pub trait AllocImpl: super::private::Sealed + drm::gem::IntoGEMObject {
> + /// The [`Driver`] implementation for this [`AllocImpl`].
> + type Driver: drm::Driver;
> +
> /// The C callback operations for this memory manager.
> const ALLOC_OPS: AllocOps;
> }
> diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
> index b71821cfb5eaa..31c5799d995c5 100644
> --- a/rust/kernel/drm/gem/mod.rs
> +++ b/rust/kernel/drm/gem/mod.rs
> @@ -15,31 +15,31 @@
> use core::{mem, ops::Deref, ptr::NonNull};
>
> /// GEM object functions, which must be implemented by drivers.
> -pub trait BaseDriverObject<T: BaseObject>: Sync + Send + Sized {
> +pub trait DriverObject: Sync + Send + Sized {
> + /// Parent `Driver` for this object.
> + type Driver: drm::Driver;
> +
> /// Create a new driver data object for a GEM object of a given size.
> - fn new(dev: &drm::Device<T::Driver>, size: usize) -> impl PinInit<Self, Error>;
> + fn new(dev: &drm::Device<Self::Driver>, size: usize) -> impl PinInit<Self, Error>;
>
> /// Open a new handle to an existing object, associated with a File.
> fn open(
> - _obj: &<<T as IntoGEMObject>::Driver as drm::Driver>::Object,
> - _file: &drm::File<<<T as IntoGEMObject>::Driver as drm::Driver>::File>,
> + _obj: &<Self::Driver as drm::Driver>::Object,
> + _file: &drm::File<<Self::Driver as drm::Driver>::File>,
> ) -> Result {
> Ok(())
> }
>
> /// Close a handle to an existing object, associated with a File.
> fn close(
> - _obj: &<<T as IntoGEMObject>::Driver as drm::Driver>::Object,
> - _file: &drm::File<<<T as IntoGEMObject>::Driver as drm::Driver>::File>,
> + _obj: &<Self::Driver as drm::Driver>::Object,
> + _file: &drm::File<<Self::Driver as drm::Driver>::File>,
> ) {
> }
> }
>
> /// Trait that represents a GEM object subtype
> pub trait IntoGEMObject: Sized + super::private::Sealed + AlwaysRefCounted {
> - /// Owning driver for this type
> - type Driver: drm::Driver;
> -
> /// 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;
> @@ -74,25 +74,15 @@ unsafe fn dec_ref(obj: NonNull<Self>) {
> }
> }
>
> -/// Trait which must be implemented by drivers using base GEM objects.
> -pub trait DriverObject: BaseDriverObject<Object<Self>> {
> - /// Parent `Driver` for this object.
> - type Driver: drm::Driver;
> -}
> -
> -extern "C" fn open_callback<T: BaseDriverObject<U>, U: BaseObject>(
> +extern "C" fn open_callback<T: DriverObject>(
> raw_obj: *mut bindings::drm_gem_object,
> raw_file: *mut bindings::drm_file,
> ) -> core::ffi::c_int {
> // SAFETY: `open_callback` is only ever called with a valid pointer to a `struct drm_file`.
> - let file = unsafe {
> - drm::File::<<<U as IntoGEMObject>::Driver as drm::Driver>::File>::from_raw(raw_file)
> - };
> - // SAFETY: `open_callback` is specified in the AllocOps structure for `Object<T>`, ensuring that
> - // `raw_obj` is indeed contained within a `Object<T>`.
> - let obj = unsafe {
> - <<<U as IntoGEMObject>::Driver as drm::Driver>::Object as IntoGEMObject>::from_raw(raw_obj)
> - };
> + let file = unsafe { drm::File::<<T::Driver as drm::Driver>::File>::from_raw(raw_file) };
> + // SAFETY: `open_callback` is specified in the AllocOps structure for `DriverObject<T>`,
> + // ensuring that `raw_obj` is contained within a `DriverObject<T>`
> + let obj = unsafe { <<T::Driver as drm::Driver>::Object as IntoGEMObject>::from_raw(raw_obj) };
>
> match T::open(obj, file) {
> Err(e) => e.to_errno(),
> @@ -100,26 +90,21 @@ extern "C" fn open_callback<T: BaseDriverObject<U>, U: BaseObject>(
> }
> }
>
> -extern "C" fn close_callback<T: BaseDriverObject<U>, U: BaseObject>(
> +extern "C" fn close_callback<T: DriverObject>(
> raw_obj: *mut bindings::drm_gem_object,
> raw_file: *mut bindings::drm_file,
> ) {
> // SAFETY: `open_callback` is only ever called with a valid pointer to a `struct drm_file`.
> - let file = unsafe {
> - drm::File::<<<U as IntoGEMObject>::Driver as drm::Driver>::File>::from_raw(raw_file)
> - };
> + let file = unsafe { drm::File::<<T::Driver as drm::Driver>::File>::from_raw(raw_file) };
> +
> // SAFETY: `close_callback` is specified in the AllocOps structure for `Object<T>`, ensuring
> // that `raw_obj` is indeed contained within a `Object<T>`.
> - let obj = unsafe {
> - <<<U as IntoGEMObject>::Driver as drm::Driver>::Object as IntoGEMObject>::from_raw(raw_obj)
> - };
> + let obj = unsafe { <<T::Driver as drm::Driver>::Object as IntoGEMObject>::from_raw(raw_obj) };
>
> T::close(obj, file);
> }
>
> impl<T: DriverObject> IntoGEMObject for Object<T> {
> - type Driver = T::Driver;
> -
> fn as_raw(&self) -> *mut bindings::drm_gem_object {
> self.obj.get()
> }
> @@ -141,10 +126,12 @@ fn size(&self) -> usize {
>
> /// Creates a new handle for the object associated with a given `File`
> /// (or returns an existing one).
> - fn create_handle(
> - &self,
> - file: &drm::File<<<Self as IntoGEMObject>::Driver as drm::Driver>::File>,
> - ) -> Result<u32> {
> + fn create_handle<D, F>(&self, file: &drm::File<F>) -> Result<u32>
> + where
> + Self: AllocImpl<Driver = D>,
> + D: drm::Driver<Object = Self, File = F>,
> + F: drm::file::DriverFile,
Shouldn’t this be F: drm::file::DriverFile<Driver =D>?
As you said in the commit message, I don’t see where exactly we would have
two competing drm::Driver types when calling this function, but we should
perhaps enforce this bound anyways.
> + {
> let mut handle: u32 = 0;
> // SAFETY: The arguments are all valid per the type invariants.
> to_result(unsafe {
> @@ -154,10 +141,12 @@ fn create_handle(
> }
>
> /// Looks up an object by its handle for a given `File`.
> - fn lookup_handle(
> - file: &drm::File<<<Self as IntoGEMObject>::Driver as drm::Driver>::File>,
> - handle: u32,
> - ) -> Result<ARef<Self>> {
> + fn lookup_handle<D, F>(file: &drm::File<F>, handle: u32) -> Result<ARef<Self>>
> + where
> + Self: AllocImpl<Driver = D>,
> + D: drm::Driver<Object = Self, File = F>,
> + F: drm::file::DriverFile,
Same here?
> + {
> // SAFETY: The arguments are all valid per the type invariants.
> let ptr = unsafe { bindings::drm_gem_object_lookup(file.as_raw().cast(), handle) };
> if ptr.is_null() {
> @@ -212,8 +201,8 @@ impl<T: DriverObject> Object<T> {
>
> const OBJECT_FUNCS: bindings::drm_gem_object_funcs = bindings::drm_gem_object_funcs {
> free: Some(Self::free_callback),
> - open: Some(open_callback::<T, Object<T>>),
> - close: Some(close_callback::<T, Object<T>>),
> + open: Some(open_callback::<T>),
> + close: Some(close_callback::<T>),
> print_info: None,
> export: None,
> pin: None,
> @@ -296,6 +285,8 @@ fn deref(&self) -> &Self::Target {
> }
>
> impl<T: DriverObject> AllocImpl for Object<T> {
> + type Driver = T::Driver;
> +
> const ALLOC_OPS: AllocOps = AllocOps {
> gem_create_object: None,
> prime_handle_to_fd: None,
> --
> 2.50.0
>
With the DriverFile comment sorted out:
Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>
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