[PATCH v2 11/12] rust: drm: gem: Add export() callback

[Lyude Paul]

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 at redhat.com>
---
 drivers/gpu/drm/nova/gem.rs  |  1 +
 rust/kernel/drm/gem/mod.rs   | 73 +++++++++++++++++++++++++++++++++++-
 rust/kernel/drm/gem/shmem.rs |  6 ++-
 3 files changed, 77 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/nova/gem.rs b/drivers/gpu/drm/nova/gem.rs
index f2f23320110dd..6cc7a65a50fae 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::BaseDriverObject 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 6d87e75690d2f..ef697f323e52c 100644
--- a/rust/kernel/drm/gem/mod.rs
+++ b/rust/kernel/drm/gem/mod.rs
@@ -10,7 +10,8 @@
     alloc::flags::*,
     bindings, drm::{self, private::Sealed},
     drm::driver::{AllocImpl, AllocOps},
-    error::{to_result, Result},
+    dma_buf,
+    error::{to_result, from_err_ptr, Result},
     prelude::*,
     types::{ARef, AlwaysRefCounted, Opaque},
 };
@@ -44,6 +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 BaseDriverObject: Sync + Send + Sized {
     /// Parent `Driver` for this object.
     type Driver: drm::Driver;
@@ -68,6 +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!()
+    }
 }
 
 /// Trait that represents a GEM object subtype
@@ -137,6 +144,21 @@ extern "C" fn close_callback<T: BaseDriverObject>(
     T::close(obj, file);
 }
 
+extern "C" fn export_callback<T: BaseDriverObject>(
+    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::as_ref(raw_obj) };
+
+    match T::export(obj, flags as _) {
+        // DRM takes a hold of the reference
+        Ok(buf) => buf.into_raw(),
+        Err(e) => e.to_ptr(),
+    }
+}
+
 impl<T: BaseDriverObject> IntoGEMObject for Object<T> {
     fn as_raw(&self) -> *mut bindings::drm_gem_object {
         self.obj.get()
@@ -247,7 +269,11 @@ impl<T: BaseDriverObject> 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,
@@ -376,6 +402,49 @@ 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.
+        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 {
+        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 bff038df93334..799f15c38cc36 100644
--- a/rust/kernel/drm/gem/shmem.rs
+++ b/rust/kernel/drm/gem/shmem.rs
@@ -77,7 +77,11 @@ impl<T: BaseDriverObject> 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),
-- 
2.49.0