[PATCH v4 3/3] rust: Add dma_buf stub bindings

[Lyude Paul <lyude@redhat.com>]

In order to implement the gem export callback, we need a type to represent
struct dma_buf. So - this commit introduces a set of stub bindings for
dma_buf. These bindings provide a ref-counted DmaBuf object, but don't
currently implement any functionality for using the DmaBuf.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>

---
V3:
* Rename as_ref() to from_raw()
V4:
* Add missing period to rustdoc at top of file

 rust/kernel/dma_buf.rs | 40 ++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs     |  1 +
 2 files changed, 41 insertions(+)
 create mode 100644 rust/kernel/dma_buf.rs

diff --git a/rust/kernel/dma_buf.rs b/rust/kernel/dma_buf.rs
new file mode 100644
index 0000000000000..50be3e4dd4098
--- /dev/null
+++ b/rust/kernel/dma_buf.rs
@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! DMA buffer API.
+//!
+//! C header: [`include/linux/dma-buf.h`](srctree/include/linux/dma-buf.h)
+
+use bindings;
+use kernel::types::*;
+
+/// A DMA buffer object.
+///
+/// # Invariants
+///
+/// The data layout of this type is equivalent to that of `struct dma_buf`.
+#[repr(transparent)]
+pub struct DmaBuf(Opaque<bindings::dma_buf>);
+
+// SAFETY: `struct dma_buf` is thread-safe
+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`].
+    ///
+    /// # Safety
+    ///
+    /// The caller guarantees that `self_ptr` points to a valid initialized `struct dma_buf` for the
+    /// duration of the lifetime of `'a`, and promises to not violate rust's data aliasing rules
+    /// using the reference provided by this function.
+    pub(crate) unsafe fn from_raw<'a>(self_ptr: *mut bindings::dma_buf) -> &'a Self {
+        // SAFETY: Our data layout is equivalent to `dma_buf` .
+        unsafe { &*self_ptr.cast() }
+    }
+
+    pub(crate) fn as_raw(&self) -> *mut bindings::dma_buf {
+        self.0.get()
+    }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index fcffc3988a903..59242d83efe21 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -81,6 +81,7 @@
 pub mod device_id;
 pub mod devres;
 pub mod dma;
+pub mod dma_buf;
 pub mod driver;
 #[cfg(CONFIG_DRM = "y")]
 pub mod drm;
-- 
2.51.0