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Message-Id: <D47EACDC-76CE-4D36-9564-210B390C9A82@collabora.com>
Date: Fri, 5 Sep 2025 12:09:42 -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>,
Sumit Semwal <sumit.semwal@...aro.org>,
Christian König <christian.koenig@....com>,
Asahi Lina <lina+kernel@...hilina.net>,
"open list:DRM DRIVER FOR NVIDIA GPUS [RUST]" <nouveau@...ts.freedesktop.org>,
linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
"open list:DMA BUFFER SHARING FRAMEWORK:Keyword:bdma_(?:buf|fence|resv)b <linux-media"@vger.kernel.org (ope>),
"moderated list:DMA BUFFER SHARING FRAMEWORK:Keyword:bdma_(?:buf|fence|resv)b <linaro-mm-sig"@lists.linaro.org (mod>)
Subject: Re: [PATCH v3 13/14] rust: drm: gem: Add export() callback
> On 29 Aug 2025, at 19:35, Lyude Paul <lyude@...hat.com> wrote:
>
> This introduces an optional export() callback for GEM objects, which is
> used to implement the drm_gem_object_funcs->export function.
>
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> ---
> drivers/gpu/drm/nova/gem.rs | 1 +
> rust/kernel/drm/gem/mod.rs | 72 +++++++++++++++++++++++++++++++++++-
> rust/kernel/drm/gem/shmem.rs | 6 ++-
> 3 files changed, 76 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/nova/gem.rs b/drivers/gpu/drm/nova/gem.rs
> index 015cb56061a56..bbce6b0f4e6a4 100644
> --- a/drivers/gpu/drm/nova/gem.rs
> +++ b/drivers/gpu/drm/nova/gem.rs
> @@ -16,6 +16,7 @@
> #[pin_data]
> pub(crate) struct NovaObject {}
>
> +#[vtable]
> impl gem::DriverObject for NovaObject {
> type Driver = NovaDriver;
> type Object = gem::Object<Self>;
> diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
> index f9f9727f14e4a..1ac25fc6d527b 100644
> --- a/rust/kernel/drm/gem/mod.rs
> +++ b/rust/kernel/drm/gem/mod.rs
> @@ -8,7 +8,7 @@
>
> use crate::{
> alloc::flags::*,
> - bindings,
> + bindings, dma_buf,
> drm::driver::{AllocImpl, AllocOps},
> drm::{self, private::Sealed},
> error::{to_result, Result},
> @@ -45,6 +45,7 @@ fn as_ref(&self) -> &kernel::drm::gem::OpaqueObject<D> {
> pub(crate) use impl_as_opaque;
>
> /// GEM object functions, which must be implemented by drivers.
> +#[vtable]
> pub trait DriverObject: Sync + Send + Sized {
> /// Parent `Driver` for this object.
> type Driver: drm::Driver;
> @@ -69,6 +70,11 @@ fn open(_obj: &Self::Object, _file: &DriverFile<Self>) -> Result {
>
> /// Close a handle to an existing object, associated with a File.
> fn close(_obj: &Self::Object, _file: &DriverFile<Self>) {}
> +
> + /// Optional handle for exporting a gem object.
> + fn export(_obj: &Self::Object, _flags: u32) -> Result<DmaBuf<Self::Object>> {
> + unimplemented!()
Shouldn’t this be the vtable-specific build error?
> + }
> }
>
> /// Trait that represents a GEM object subtype
> @@ -138,6 +144,21 @@ extern "C" fn close_callback<T: DriverObject>(
> T::close(obj, file);
> }
>
> +extern "C" fn export_callback<T: DriverObject>(
> + raw_obj: *mut bindings::drm_gem_object,
> + flags: i32,
> +) -> *mut bindings::dma_buf {
> + // SAFETY: `export_callback` is specified in the AllocOps structure for `Object<T>`, ensuring
> + // that `raw_obj` is contained within a `Object<T>`.
> + let obj = unsafe { T::Object::from_raw(raw_obj) };
> +
> + match T::export(obj, flags as u32) {
> + // DRM takes a hold of the reference
> + Ok(buf) => buf.into_raw(),
> + Err(e) => e.to_ptr(),
> + }
> +}
> +
> impl<T: DriverObject> IntoGEMObject for Object<T> {
> fn as_raw(&self) -> *mut bindings::drm_gem_object {
> self.obj.get()
> @@ -248,7 +269,11 @@ impl<T: DriverObject> Object<T> {
> open: Some(open_callback::<T>),
> close: Some(close_callback::<T>),
> print_info: None,
> - export: None,
> + export: if T::HAS_EXPORT {
> + Some(export_callback::<T>)
> + } else {
> + None
> + },
> pin: None,
> unpin: None,
> get_sg_table: None,
> @@ -375,6 +400,49 @@ fn as_raw(&self) -> *mut bindings::drm_gem_object {
>
> impl<D: drm::Driver> Sealed for OpaqueObject<D> {}
>
> +/// A [`dma_buf::DmaBuf`] which has been exported from a GEM object.
> +///
> +/// The [`dma_buf::DmaBuf`] will be released when this type is dropped.
> +///
> +/// # Invariants
> +///
> +/// - `self.0` points to a valid initialized [`dma_buf::DmaBuf`] for the lifetime of this object.
> +/// - The GEM object from which this [`dma_buf::DmaBuf`] was exported from is guaranteed to be of
> +/// type `T`.
> +pub struct DmaBuf<T: IntoGEMObject>(NonNull<dma_buf::DmaBuf>, PhantomData<T>);
> +
> +impl<T: IntoGEMObject> Deref for DmaBuf<T> {
> + type Target = dma_buf::DmaBuf;
> +
> + #[inline]
> + fn deref(&self) -> &Self::Target {
> + // SAFETY: This pointer is guaranteed to be valid by our type invariants.
> + unsafe { self.0.as_ref() }
> + }
> +}
> +
> +impl<T: IntoGEMObject> Drop for DmaBuf<T> {
> + #[inline]
> + fn drop(&mut self) {
> + // SAFETY:
> + // - `dma_buf::DmaBuf` is guaranteed to have an identical layout to `struct dma_buf`
> + // by its type invariants.
> + // - We hold the last reference to this `DmaBuf`, making it safe to destroy.
How can we be sure of this?
> + unsafe { bindings::drm_gem_dmabuf_release(self.0.cast().as_ptr()) }
> + }
> +}
> +
> +impl<T: IntoGEMObject> DmaBuf<T> {
> + /// Leak the reference for this [`DmaBuf`] and return a raw pointer to it.
> + #[inline]
> + pub(crate) fn into_raw(self) -> *mut bindings::dma_buf {
Then this should perhaps be called leak()? At least if we’re following the std nomenclature.
> + let dma_ptr = self.as_raw();
> +
> + core::mem::forget(self);
> + dma_ptr
> + }
> +}
> +
> pub(super) const fn create_fops() -> bindings::file_operations {
> // SAFETY: As by the type invariant, it is safe to initialize `bindings::file_operations`
> // zeroed.
> diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
> index 1437cda27a22c..b3a70e6001842 100644
> --- a/rust/kernel/drm/gem/shmem.rs
> +++ b/rust/kernel/drm/gem/shmem.rs
> @@ -66,7 +66,11 @@ impl<T: DriverObject> Object<T> {
> open: Some(super::open_callback::<T>),
> close: Some(super::close_callback::<T>),
> print_info: Some(bindings::drm_gem_shmem_object_print_info),
> - export: None,
> + export: if T::HAS_EXPORT {
> + Some(super::export_callback::<T>)
> + } else {
> + None
> + },
> pin: Some(bindings::drm_gem_shmem_object_pin),
> unpin: Some(bindings::drm_gem_shmem_object_unpin),
> get_sg_table: Some(bindings::drm_gem_shmem_object_get_sg_table),
> --
> 2.50.0
>
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