media: v4l2-mem2mem: Add a kref to the v4l2_m2m_dev structure

Adding a reference count to the v4l2_m2m_dev structure allow safely
sharing it across multiple hardware nodes. This can be used to prevent
running jobs concurrently on m2m cores that have some internal resource
sharing.

Signed-off-by: Ming Qian <ming.qian@oss.nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>
[hverkuil: fix typos in v4l2_m2m_put documentation]

include/media/v4l2-mem2mem.h

@@ -547,6 +547,27 @@ v4l2_m2m_register_media_controller(struct v4l2_m2m_dev *m2m_dev,
  */
 void v4l2_m2m_release(struct v4l2_m2m_dev *m2m_dev);
 
+/**
+ * v4l2_m2m_get() - take a reference to the m2m_dev structure
+ *
+ * @m2m_dev: opaque pointer to the internal data to handle M2M context
+ *
+ * This is used to share the M2M device across multiple devices. This
+ * can be used to avoid scheduling two hardware nodes concurrently.
+ */
+void v4l2_m2m_get(struct v4l2_m2m_dev *m2m_dev);
+
+/**
+ * v4l2_m2m_put() - remove a reference to the m2m_dev structure
+ *
+ * @m2m_dev: opaque pointer to the internal data to handle M2M context
+ *
+ * Once the M2M device has no more references, v4l2_m2m_release() will be
+ * called automatically. Users of this method should never call
+ * v4l2_m2m_release() directly. See v4l2_m2m_get() for more details.
+ */
+void v4l2_m2m_put(struct v4l2_m2m_dev *m2m_dev);
+
 /**
  * v4l2_m2m_ctx_init() - allocate and initialize a m2m context
  *