[PATCH v19 1/4] rust: drm: gem: shmem: Add DmaResvGuard helper

[Lyude Paul <lyude@redhat.com>]

Just a temporary holdover to make locking/unlocking the dma_resv lock much
easier.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Co-authored-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

---
V17:
* Fix format of commit message title
V19:
* Add NotThreadSafe to DmaResvGuard

 rust/kernel/drm/gem/shmem.rs | 39 ++++++++++++++++++++++++++++++++++--
 1 file changed, 37 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs
index 084b798ce795b..da1acc223bad4 100644
--- a/rust/kernel/drm/gem/shmem.rs
+++ b/rust/kernel/drm/gem/shmem.rs
@@ -22,7 +22,10 @@
     error::to_result,
     prelude::*,
     sync::aref::ARef,
-    types::Opaque, //
+    types::{
+        NotThreadSafe,
+        Opaque, //
+    },
 };
 use core::{
     marker::PhantomData,
@@ -30,7 +33,10 @@
         Deref,
         DerefMut, //
     },
-    ptr::NonNull, //
+    ptr::{
+        self,
+        NonNull, //
+    },
 };
 use gem::{
     BaseObjectPrivate,
@@ -244,3 +250,32 @@ impl<T: DriverObject, C: DeviceContext> driver::AllocImpl for Object<T, C> {
         dumb_map_offset: None,
     };
 }
+
+/// Private helper-type for holding the `dma_resv` object for a GEM shmem object.
+///
+/// When this is dropped, the `dma_resv` lock is dropped as well.
+///
+// TODO: This should be replace with a WwMutex equivalent once we have such bindings in the kernel.
+struct DmaResvGuard<'a, T: DriverObject, C: DeviceContext = Registered>(
+    &'a Object<T, C>,
+    NotThreadSafe,
+);
+
+impl<'a, T: DriverObject, C: DeviceContext> DmaResvGuard<'a, T, C> {
+    #[inline(always)]
+    #[expect(unused)]
+    fn new(obj: &'a Object<T, C>) -> Self {
+        // SAFETY: This lock is initialized throughout the lifetime of `object`.
+        unsafe { bindings::dma_resv_lock(obj.raw_dma_resv(), ptr::null_mut()) };
+
+        Self(obj, NotThreadSafe)
+    }
+}
+
+impl<'a, T: DriverObject, C: DeviceContext> Drop for DmaResvGuard<'a, T, C> {
+    #[inline(always)]
+    fn drop(&mut self) {
+        // SAFETY: We are releasing the lock grabbed during the creation of this object.
+        unsafe { bindings::dma_resv_unlock(self.0.raw_dma_resv()) };
+    }
+}
-- 
2.54.0