uverbs_get_const
err = uverbs_get_const(&cmd_out_len, attrs,
ret = uverbs_get_const(&object_id, attrs, UVERBS_ATTR_INFO_OBJECT_ID);
ret = uverbs_get_const(&port_num, attrs,
rc = uverbs_get_const(&cmd, attrs, UVERBS_ATTR_WRITE_CMD);
ret = uverbs_get_const(&advice, attrs, UVERBS_ATTR_ADVISE_MR_ADVICE);