[PATCH v3 14/14] rust: drm: gem: Add BaseObject::prime_export()

[Lyude Paul]

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@xxxxxxxxxx>
---
 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