rust: drm: gem: Add BaseObject::prime_export()

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>

Author: Lyude

rust/kernel/drm/gem/mod.rs

@@ -11,7 +11,7 @@ use crate::{
     bindings, drm,
     drm::driver::{AllocImpl, AllocOps},
     dma_buf,
-    error::{to_result, Result},
+    error::{to_result, from_err_ptr, Result},
     prelude::*,
     sync::aref::{ARef, AlwaysRefCounted},
     types::Opaque,
@@ -219,6 +219,29 @@ pub trait BaseObject: IntoGEMObject {
         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 a `drm_gem_object`.
+        // - `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(unsafe {
+            bindings::drm_gem_prime_export(self.as_raw(), flags as _)
+        })?;
+
+        // 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::as_ref(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.