rust: drm: gem: Introduce shmem::SGTable

Currently we expose the ability to retrieve an SGTable for an shmem gem
object using gem::shmem::Object::<T>::sg_table(). However, this only gives
us a borrowed reference. This being said - retrieving an SGTable is a
fallible operation, and as such it's reasonable that a driver may want to
hold onto an SGTable for longer then a reference would allow in order to
avoid having to deal with fallibility every time they want to access the
SGTable. One such driver with this usecase is the Asahi driver.

So to support this, let's introduce shmem::SGTable - which both holds a
pointer to the SGTable and a reference to its respective GEM object in
order to keep the GEM object alive for as long as the shmem::SGTable. The
type can be used identically to a normal SGTable.

Signed-off-by: Lyude Paul <lyude@redhat.com>

Author: Lyude

rust/kernel/drm/gem/shmem.rs

@@ -173,6 +173,25 @@ impl<T: DriverObject> Object<T> {
         // pointer to a scatterlist
         Ok(unsafe { scatterlist::SGTable::from_raw(sgt) })
     }
+
+    /// Creates (if necessary) and returns an owned reference to a scatter-gather table of DMA pages
+    /// for this object.
+    ///
+    /// This is the same as [`sg_table`](Self::sg_table), except that it instead returns an
+    /// [`shmem::SGTable`] which holds a reference to the associated gem object, instead of a
+    /// reference to an [`scatterlist::SGTable`].
+    ///
+    /// This will pin the object in memory.
+    ///
+    /// [`shmem::SGTable`]: SGTable
+    pub fn owned_sg_table(&self) -> Result<SGTable<T>> {
+        Ok(SGTable {
+            sgt: self.sg_table()?.into(),
+            // INVARIANT: We take an owned refcount to `self` here, ensuring that `sgt` remains
+            // valid for as long as this `SGTable`.
+            _owner: self.into(),
+        })
+    }
 }
 
 impl<T: DriverObject> Deref for Object<T> {
@@ -223,3 +242,34 @@ impl<T: DriverObject> driver::AllocImpl for Object<T> {
         dumb_map_offset: None,
     };
 }
+
+/// An owned reference to a scatter-gather table of DMA address spans for a GEM shmem object.
+///
+/// This object holds an owned reference to the underlying GEM shmem object, ensuring that the
+/// [`scatterlist::SGTable`] referenced by this type remains valid for the lifetime of this object.
+///
+/// # Invariants
+///
+/// - `sgt` is kept alive by `_owner`, ensuring it remains valid for as long as `Self`.
+/// - `sgt` corresponds to the owned object in `_owner`.
+/// - This object is only exposed in situations where we know the underlying `SGTable` will not be
+///   modified for the lifetime of this object. Thus, it is safe to send/access this type across
+///   threads.
+pub struct SGTable<T: DriverObject> {
+    sgt: NonNull<scatterlist::SGTable>,
+    _owner: ARef<Object<T>>,
+}
+
+// SAFETY: This object is thread-safe via our type invariants.
+unsafe impl<T: DriverObject> Send for SGTable<T> {}
+// SAFETY: This object is thread-safe via our type invariants.
+unsafe impl<T: DriverObject> Sync for SGTable<T> {}
+
+impl<T: DriverObject> Deref for SGTable<T> {
+    type Target = scatterlist::SGTable;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: Creating an immutable reference to this is safe via our type invariants.
+        unsafe { self.sgt.as_ref() }
+    }
+}