RDMA/uverbs: Check for NULL driver methods for every write call

Add annotations to the uverbs_api structure indicating which driver
methods are called by the implementation. If the required method
is NULL the write API will be not be callable.

This effectively duplicates the cmd_mask system, however it does it by
expressing invariants required by the core code, not by delegating
decision making to the driver. This is another step toward eliminating
cmd_mask.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>

include/rdma/uverbs_ioctl.h

@@ -350,6 +350,7 @@ enum uapi_definition_kind {
 
 enum uapi_definition_scope {
 	UAPI_SCOPE_OBJECT = 1,
+	UAPI_SCOPE_METHOD = 2,
 };
 
 struct uapi_definition {
@@ -422,6 +423,21 @@ struct uapi_definition {
 				sizeof(void *)),                               \
 	}
 
+/*
+ * Method is only supported if the function pointer named ibdev_fn in struct
+ * ib_device is not NULL.
+ */
+#define UAPI_DEF_METHOD_NEEDS_FN(ibdev_fn)                                     \
+	{                                                                      \
+		.kind = UAPI_DEF_IS_SUPPORTED_DEV_FN,                          \
+		.scope = UAPI_SCOPE_METHOD,                                    \
+		.needs_fn_offset =                                             \
+			offsetof(struct ib_device, ibdev_fn) +                 \
+			BUILD_BUG_ON_ZERO(                                     \
+				sizeof(((struct ib_device *)0)->ibdev_fn) !=   \
+				sizeof(void *)),                               \
+	}
+
 /* Call a function to determine if the entire object is supported or not */
 #define UAPI_DEF_IS_OBJ_SUPPORTED(_func)                                       \
 	{                                                                      \