> On 29 Aug 2025, at 19:35, Lyude Paul <lyude@redhat.com> wrote:
>
> We just added an export() callback that GEM objects can implement, but
> without any way of actually exporting a DmaBuf<T>. So let's add one by
> introducing bindings for drm_gem_prime_export().
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> ---
> rust/kernel/dma_buf.rs | 1 -
> rust/kernel/drm/gem/mod.rs | 24 +++++++++++++++++++++++-
> 2 files changed, 23 insertions(+), 2 deletions(-)
>
> diff --git a/rust/kernel/dma_buf.rs b/rust/kernel/dma_buf.rs
> index a66829afcd129..a2086918efd17 100644
> --- a/rust/kernel/dma_buf.rs
> +++ b/rust/kernel/dma_buf.rs
> @@ -20,7 +20,6 @@ unsafe impl Send for DmaBuf {}
> // SAFETY: `struct dma_buf` is thread-safe
> unsafe impl Sync for DmaBuf {}
>
> -#[expect(unused)]
> impl DmaBuf {
> /// Convert from a `*mut bindings::dma_buf` to a [`DmaBuf`].
> ///
> diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
> index 1ac25fc6d527b..75ffd5541e84c 100644
> --- a/rust/kernel/drm/gem/mod.rs
> +++ b/rust/kernel/drm/gem/mod.rs
> @@ -11,7 +11,7 @@
> bindings, dma_buf,
> drm::driver::{AllocImpl, AllocOps},
> drm::{self, private::Sealed},
> - error::{to_result, Result},
> + error::{from_err_ptr, to_result, Result},
> prelude::*,
> types::{ARef, AlwaysRefCounted, Opaque},
> };
> @@ -225,6 +225,28 @@ fn lookup_handle<D, F, O>(file: &drm::File<F>, handle: u32) -> Result<ARef<Self>
> Ok(unsafe { ARef::from_raw(obj.into()) })
> }
>
> + /// Export a [`DmaBuf`] for this GEM object using the DRM prime helper library.
> + ///
> + /// `flags` should be a set of flags from [`fs::file::flags`](kernel::fs::file::flags).
> + fn prime_export(&self, flags: u32) -> Result<DmaBuf<Self>> {
> + // SAFETY: `as_raw()` always returns a valid pointer to an initialized `drm_gem_object`.
> + let dma_ptr = unsafe { bindings::drm_gem_prime_export(self.as_raw(), flags as i32) };
> +
> + // `drm_gem_prime_export()` returns either an error pointer, or a valid pointer to an
> + // initialized `dma_buf` on success.
> + let dma_ptr = from_err_ptr(dma_ptr)?;
> +
> + // SAFETY:
> + // - We checked that dma_ptr is not an error, so it must point to an initialized dma_buf
> + // - We used drm_gem_prime_export(), so `dma_ptr` will remain valid until a call to
> + // `drm_gem_prime_release()` which we don't call here.
> + let dma_buf = unsafe { dma_buf::DmaBuf::from_raw(dma_ptr) };
> +
> + // INVARIANT: We used drm_gem_prime_export() to create this dma_buf, fulfilling the
> + // invariant that this dma_buf came from a GEM object of type `Self`.
> + Ok(DmaBuf(dma_buf.into(), PhantomData))
> + }
> +
> /// Creates an mmap offset to map the object from userspace.
> fn create_mmap_offset(&self) -> Result<u64> {
> // SAFETY: The arguments are valid per the type invariant.
> --
> 2.50.0
>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>