[RFC,04/18] rust: drm: gem: Add GEM object abstraction

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 7d7828faf89c..7183dfe6473f 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -9,6 +9,7 @@ 
 #include <drm/drm_device.h>
 #include <drm/drm_drv.h>
 #include <drm/drm_file.h>
+#include <drm/drm_gem.h>
 #include <drm/drm_ioctl.h>
 #include <linux/delay.h>
 #include <linux/device.h>
diff --git a/rust/helpers.c b/rust/helpers.c
index 73b2ce607f27..78ec4162b03b 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -18,6 +18,7 @@ 
  * accidentally exposed.
  */
 
+#include <drm/drm_gem.h>
 #include <linux/bug.h>
 #include <linux/build_bug.h>
 #include <linux/device.h>
@@ -374,6 +375,28 @@  void rust_helper_init_completion(struct completion *c)
 }
 EXPORT_SYMBOL_GPL(rust_helper_init_completion);
 
+#ifdef CONFIG_DRM
+
+void rust_helper_drm_gem_object_get(struct drm_gem_object *obj)
+{
+	drm_gem_object_get(obj);
+}
+EXPORT_SYMBOL_GPL(rust_helper_drm_gem_object_get);
+
+void rust_helper_drm_gem_object_put(struct drm_gem_object *obj)
+{
+	drm_gem_object_put(obj);
+}
+EXPORT_SYMBOL_GPL(rust_helper_drm_gem_object_put);
+
+__u64 rust_helper_drm_vma_node_offset_addr(struct drm_vma_offset_node *node)
+{
+	return drm_vma_node_offset_addr(node);
+}
+EXPORT_SYMBOL_GPL(rust_helper_drm_vma_node_offset_addr);
+
+#endif
+
 /*
  * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
  * as the Rust `usize` type, so we can use it in contexts where Rust
diff --git a/rust/kernel/drm/drv.rs b/rust/kernel/drm/drv.rs
index 1dcb651e1417..c138352cb489 100644
--- a/rust/kernel/drm/drv.rs
+++ b/rust/kernel/drm/drv.rs
@@ -126,7 +126,7 @@  pub struct AllocOps {
 }
 
 /// Trait for memory manager implementations. Implemented internally.
-pub trait AllocImpl: Sealed {
+pub trait AllocImpl: Sealed + drm::gem::IntoGEMObject {
     /// The C callback operations for this memory manager.
     const ALLOC_OPS: AllocOps;
 }
@@ -263,7 +263,7 @@  impl<T: Driver> Registration<T> {
             drm,
             registered: false,
             vtable,
-            fops: Default::default(), // TODO: GEM abstraction
+            fops: drm::gem::create_fops(),
             _pin: PhantomPinned,
             _p: PhantomData,
         })
diff --git a/rust/kernel/drm/gem/mod.rs b/rust/kernel/drm/gem/mod.rs
new file mode 100644
index 000000000000..8a7d99613718
--- /dev/null
+++ b/rust/kernel/drm/gem/mod.rs
@@ -0,0 +1,374 @@ 
+// SPDX-License-Identifier: GPL-2.0 OR MIT
+
+//! DRM GEM API
+//!
+//! C header: [`include/linux/drm/drm_gem.h`](../../../../include/linux/drm/drm_gem.h)
+
+use alloc::boxed::Box;
+
+use crate::{
+    bindings,
+    drm::{device, drv, file},
+    error::{to_result, Result},
+    prelude::*,
+};
+use core::{mem, mem::ManuallyDrop, ops::Deref, ops::DerefMut};
+
+/// GEM object functions, which must be implemented by drivers.
+pub trait BaseDriverObject<T: BaseObject>: Sync + Send + Sized {
+    /// Create a new driver data object for a GEM object of a given size.
+    fn new(dev: &device::Device<T::Driver>, size: usize) -> Result<Self>;
+
+    /// Open a new handle to an existing object, associated with a File.
+    fn open(
+        _obj: &<<T as IntoGEMObject>::Driver as drv::Driver>::Object,
+        _file: &file::File<<<T as IntoGEMObject>::Driver as drv::Driver>::File>,
+    ) -> Result {
+        Ok(())
+    }
+
+    /// Close a handle to an existing object, associated with a File.
+    fn close(
+        _obj: &<<T as IntoGEMObject>::Driver as drv::Driver>::Object,
+        _file: &file::File<<<T as IntoGEMObject>::Driver as drv::Driver>::File>,
+    ) {
+    }
+}
+
+/// Trait that represents a GEM object subtype
+pub trait IntoGEMObject: Sized + crate::private::Sealed {
+    /// Owning driver for this type
+    type Driver: drv::Driver;
+
+    /// Returns a pointer to the raw `drm_gem_object` structure, which must be valid as long as
+    /// this owning object is valid.
+    fn gem_obj(&self) -> *mut bindings::drm_gem_object;
+
+    /// Returns a reference to the raw `drm_gem_object` structure, which must be valid as long as
+    /// this owning object is valid.
+    fn gem_ref(&self) -> &bindings::drm_gem_object {
+        // SAFETY: gem_obj() must be valid per the above requirement.
+        unsafe { &*self.gem_obj() }
+    }
+
+    /// Converts a pointer to a `drm_gem_object` into a pointer to this type.
+    fn from_gem_obj(obj: *mut bindings::drm_gem_object) -> *mut Self;
+}
+
+/// Trait which must be implemented by drivers using base GEM objects.
+pub trait DriverObject: BaseDriverObject<Object<Self>> {
+    /// Parent `Driver` for this object.
+    type Driver: drv::Driver;
+}
+
+unsafe extern "C" fn free_callback<T: DriverObject>(obj: *mut bindings::drm_gem_object) {
+    // SAFETY: All of our objects are Object<T>.
+    let this = crate::container_of!(obj, Object<T>, obj) as *mut Object<T>;
+
+    // SAFETY: The pointer we got has to be valid
+    unsafe { bindings::drm_gem_object_release(obj) };
+
+    // SAFETY: All of our objects are allocated via Box<>, and we're in the
+    // free callback which guarantees this object has zero remaining references,
+    // so we can drop it
+    unsafe { Box::from_raw(this) };
+}
+
+unsafe extern "C" fn open_callback<T: BaseDriverObject<U>, U: BaseObject>(
+    raw_obj: *mut bindings::drm_gem_object,
+    raw_file: *mut bindings::drm_file,
+) -> core::ffi::c_int {
+    // SAFETY: The pointer we got has to be valid.
+    let file = unsafe {
+        file::File::<<<U as IntoGEMObject>::Driver as drv::Driver>::File>::from_raw(raw_file)
+    };
+    let obj =
+        <<<U as IntoGEMObject>::Driver as drv::Driver>::Object as IntoGEMObject>::from_gem_obj(
+            raw_obj,
+        );
+
+    // SAFETY: from_gem_obj() returns a valid pointer as long as the type is
+    // correct and the raw_obj we got is valid.
+    match T::open(unsafe { &*obj }, &file) {
+        Err(e) => e.to_kernel_errno(),
+        Ok(()) => 0,
+    }
+}
+
+unsafe extern "C" fn close_callback<T: BaseDriverObject<U>, U: BaseObject>(
+    raw_obj: *mut bindings::drm_gem_object,
+    raw_file: *mut bindings::drm_file,
+) {
+    // SAFETY: The pointer we got has to be valid.
+    let file = unsafe {
+        file::File::<<<U as IntoGEMObject>::Driver as drv::Driver>::File>::from_raw(raw_file)
+    };
+    let obj =
+        <<<U as IntoGEMObject>::Driver as drv::Driver>::Object as IntoGEMObject>::from_gem_obj(
+            raw_obj,
+        );
+
+    // SAFETY: from_gem_obj() returns a valid pointer as long as the type is
+    // correct and the raw_obj we got is valid.
+    T::close(unsafe { &*obj }, &file);
+}
+
+impl<T: DriverObject> IntoGEMObject for Object<T> {
+    type Driver = T::Driver;
+
+    fn gem_obj(&self) -> *mut bindings::drm_gem_object {
+        &self.obj as *const _ as *mut _
+    }
+
+    fn from_gem_obj(obj: *mut bindings::drm_gem_object) -> *mut Object<T> {
+        crate::container_of!(obj, Object<T>, obj) as *mut Object<T>
+    }
+}
+
+/// Base operations shared by all GEM object classes
+pub trait BaseObject: IntoGEMObject {
+    /// Returns the size of the object in bytes.
+    fn size(&self) -> usize {
+        self.gem_ref().size
+    }
+
+    /// Creates a new reference to the object.
+    fn reference(&self) -> ObjectRef<Self> {
+        // SAFETY: Having a reference to an Object implies holding a GEM reference
+        unsafe {
+            bindings::drm_gem_object_get(self.gem_obj());
+        }
+        ObjectRef {
+            ptr: self as *const _,
+        }
+    }
+
+    /// Creates a new handle for the object associated with a given `File`
+    /// (or returns an existing one).
+    fn create_handle(
+        &self,
+        file: &file::File<<<Self as IntoGEMObject>::Driver as drv::Driver>::File>,
+    ) -> Result<u32> {
+        let mut handle: u32 = 0;
+        // SAFETY: The arguments are all valid per the type invariants.
+        to_result(unsafe {
+            bindings::drm_gem_handle_create(file.raw() as *mut _, self.gem_obj(), &mut handle)
+        })?;
+        Ok(handle)
+    }
+
+    /// Looks up an object by its handle for a given `File`.
+    fn lookup_handle(
+        file: &file::File<<<Self as IntoGEMObject>::Driver as drv::Driver>::File>,
+        handle: u32,
+    ) -> Result<ObjectRef<Self>> {
+        // SAFETY: The arguments are all valid per the type invariants.
+        let ptr = unsafe { bindings::drm_gem_object_lookup(file.raw() as *mut _, handle) };
+
+        if ptr.is_null() {
+            Err(ENOENT)
+        } else {
+            Ok(ObjectRef {
+                ptr: ptr as *const _,
+            })
+        }
+    }
+
+    /// 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.
+        to_result(unsafe {
+            // TODO: is this threadsafe?
+            bindings::drm_gem_create_mmap_offset(self.gem_obj())
+        })?;
+        Ok(unsafe {
+            bindings::drm_vma_node_offset_addr(&self.gem_ref().vma_node as *const _ as *mut _)
+        })
+    }
+}
+
+impl<T: IntoGEMObject> BaseObject for T {}
+
+/// A base GEM object.
+#[repr(C)]
+pub struct Object<T: DriverObject> {
+    obj: bindings::drm_gem_object,
+    // The DRM core ensures the Device exists as long as its objects exist, so we don't need to
+    // manage the reference count here.
+    dev: ManuallyDrop<device::Device<T::Driver>>,
+    inner: T,
+}
+
+impl<T: DriverObject> Object<T> {
+    /// The size of this object's structure.
+    pub const SIZE: usize = mem::size_of::<Self>();
+
+    const OBJECT_FUNCS: bindings::drm_gem_object_funcs = bindings::drm_gem_object_funcs {
+        free: Some(free_callback::<T>),
+        open: Some(open_callback::<T, Object<T>>),
+        close: Some(close_callback::<T, Object<T>>),
+        print_info: None,
+        export: None,
+        pin: None,
+        unpin: None,
+        get_sg_table: None,
+        vmap: None,
+        vunmap: None,
+        mmap: None,
+        vm_ops: core::ptr::null_mut(),
+    };
+
+    /// Create a new GEM object.
+    pub fn new(dev: &device::Device<T::Driver>, size: usize) -> Result<UniqueObjectRef<Self>> {
+        let mut obj: Box<Self> = Box::try_new(Self {
+            // SAFETY: This struct is expected to be zero-initialized
+            obj: unsafe { mem::zeroed() },
+            // SAFETY: The drm subsystem guarantees that the drm_device will live as long as
+            // the GEM object lives, so we can conjure a reference out of thin air.
+            dev: ManuallyDrop::new(unsafe { device::Device::from_raw(dev.ptr) }),
+            inner: T::new(dev, size)?,
+        })?;
+
+        obj.obj.funcs = &Self::OBJECT_FUNCS;
+        to_result(unsafe {
+            bindings::drm_gem_object_init(dev.raw() as *mut _, &mut obj.obj, size)
+        })?;
+
+        let obj_ref = UniqueObjectRef {
+            ptr: Box::leak(obj),
+        };
+
+        Ok(obj_ref)
+    }
+
+    /// Returns the `Device` that owns this GEM object.
+    pub fn dev(&self) -> &device::Device<T::Driver> {
+        &self.dev
+    }
+}
+
+impl<T: DriverObject> crate::private::Sealed for Object<T> {}
+
+impl<T: DriverObject> Deref for Object<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.inner
+    }
+}
+
+impl<T: DriverObject> DerefMut for Object<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.inner
+    }
+}
+
+impl<T: DriverObject> drv::AllocImpl for Object<T> {
+    const ALLOC_OPS: drv::AllocOps = drv::AllocOps {
+        gem_create_object: None,
+        prime_handle_to_fd: Some(bindings::drm_gem_prime_handle_to_fd),
+        prime_fd_to_handle: Some(bindings::drm_gem_prime_fd_to_handle),
+        gem_prime_import: None,
+        gem_prime_import_sg_table: None,
+        gem_prime_mmap: Some(bindings::drm_gem_prime_mmap),
+        dumb_create: None,
+        dumb_map_offset: None,
+        dumb_destroy: None,
+    };
+}
+
+/// A reference-counted shared reference to a base GEM object.
+pub struct ObjectRef<T: IntoGEMObject> {
+    // Invariant: the pointer is valid and initialized, and this ObjectRef owns a reference to it.
+    ptr: *const T,
+}
+
+/// SAFETY: GEM object references are safe to share between threads.
+unsafe impl<T: IntoGEMObject> Send for ObjectRef<T> {}
+unsafe impl<T: IntoGEMObject> Sync for ObjectRef<T> {}
+
+impl<T: IntoGEMObject> Clone for ObjectRef<T> {
+    fn clone(&self) -> Self {
+        self.reference()
+    }
+}
+
+impl<T: IntoGEMObject> Drop for ObjectRef<T> {
+    fn drop(&mut self) {
+        // SAFETY: Having an ObjectRef implies holding a GEM reference.
+        // The free callback will take care of deallocation.
+        unsafe {
+            bindings::drm_gem_object_put((*self.ptr).gem_obj());
+        }
+    }
+}
+
+impl<T: IntoGEMObject> Deref for ObjectRef<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: The pointer is valid per the invariant
+        unsafe { &*self.ptr }
+    }
+}
+
+/// A unique reference to a base GEM object.
+pub struct UniqueObjectRef<T: IntoGEMObject> {
+    // Invariant: the pointer is valid and initialized, and this ObjectRef owns the only reference
+    // to it.
+    ptr: *mut T,
+}
+
+impl<T: IntoGEMObject> UniqueObjectRef<T> {
+    /// Downgrade this reference to a shared reference.
+    pub fn into_ref(self) -> ObjectRef<T> {
+        let ptr = self.ptr as *const _;
+        core::mem::forget(self);
+
+        ObjectRef { ptr }
+    }
+}
+
+impl<T: IntoGEMObject> Drop for UniqueObjectRef<T> {
+    fn drop(&mut self) {
+        // SAFETY: Having a UniqueObjectRef implies holding a GEM
+        // reference. The free callback will take care of deallocation.
+        unsafe {
+            bindings::drm_gem_object_put((*self.ptr).gem_obj());
+        }
+    }
+}
+
+impl<T: IntoGEMObject> Deref for UniqueObjectRef<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: The pointer is valid per the invariant
+        unsafe { &*self.ptr }
+    }
+}
+
+impl<T: IntoGEMObject> DerefMut for UniqueObjectRef<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        // SAFETY: The pointer is valid per the invariant
+        unsafe { &mut *self.ptr }
+    }
+}
+
+pub(super) fn create_fops() -> bindings::file_operations {
+    bindings::file_operations {
+        owner: core::ptr::null_mut(),
+        open: Some(bindings::drm_open),
+        release: Some(bindings::drm_release),
+        unlocked_ioctl: Some(bindings::drm_ioctl),
+        #[cfg(CONFIG_COMPAT)]
+        compat_ioctl: Some(bindings::drm_compat_ioctl),
+        #[cfg(not(CONFIG_COMPAT))]
+        compat_ioctl: None,
+        poll: Some(bindings::drm_poll),
+        read: Some(bindings::drm_read),
+        llseek: Some(bindings::noop_llseek),
+        mmap: Some(bindings::drm_gem_mmap),
+        ..Default::default()
+    }
+}
diff --git a/rust/kernel/drm/mod.rs b/rust/kernel/drm/mod.rs
index a767942d0b52..c44760a1332f 100644
--- a/rust/kernel/drm/mod.rs
+++ b/rust/kernel/drm/mod.rs
@@ -5,4 +5,5 @@ 
 pub mod device;
 pub mod drv;
 pub mod file;
+pub mod gem;
 pub mod ioctl;