Update Linux to v5.4.2
Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/Documentation/driver-api/connector.rst b/Documentation/driver-api/connector.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+================
+Kernel Connector
+================
+
+Kernel connector - new netlink based userspace <-> kernel space easy
+to use communication module.
+
+The Connector driver makes it easy to connect various agents using a
+netlink based network. One must register a callback and an identifier.
+When the driver receives a special netlink message with the appropriate
+identifier, the appropriate callback will be called.
+
+From the userspace point of view it's quite straightforward:
+
+ - socket();
+ - bind();
+ - send();
+ - recv();
+
+But if kernelspace wants to use the full power of such connections, the
+driver writer must create special sockets, must know about struct sk_buff
+handling, etc... The Connector driver allows any kernelspace agents to use
+netlink based networking for inter-process communication in a significantly
+easier way::
+
+ int cn_add_callback(struct cb_id *id, char *name, void (*callback) (struct cn_msg *, struct netlink_skb_parms *));
+ void cn_netlink_send_multi(struct cn_msg *msg, u16 len, u32 portid, u32 __group, int gfp_mask);
+ void cn_netlink_send(struct cn_msg *msg, u32 portid, u32 __group, int gfp_mask);
+
+ struct cb_id
+ {
+ __u32 idx;
+ __u32 val;
+ };
+
+idx and val are unique identifiers which must be registered in the
+connector.h header for in-kernel usage. `void (*callback) (void *)` is a
+callback function which will be called when a message with above idx.val
+is received by the connector core. The argument for that function must
+be dereferenced to `struct cn_msg *`::
+
+ struct cn_msg
+ {
+ struct cb_id id;
+
+ __u32 seq;
+ __u32 ack;
+
+ __u32 len; /* Length of the following data */
+ __u8 data[0];
+ };
+
+Connector interfaces
+====================
+
+ .. kernel-doc:: include/linux/connector.h
+
+ Note:
+ When registering new callback user, connector core assigns
+ netlink group to the user which is equal to its id.idx.
+
+Protocol description
+====================
+
+The current framework offers a transport layer with fixed headers. The
+recommended protocol which uses such a header is as following:
+
+msg->seq and msg->ack are used to determine message genealogy. When
+someone sends a message, they use a locally unique sequence and random
+acknowledge number. The sequence number may be copied into
+nlmsghdr->nlmsg_seq too.
+
+The sequence number is incremented with each message sent.
+
+If you expect a reply to the message, then the sequence number in the
+received message MUST be the same as in the original message, and the
+acknowledge number MUST be the same + 1.
+
+If we receive a message and its sequence number is not equal to one we
+are expecting, then it is a new message. If we receive a message and
+its sequence number is the same as one we are expecting, but its
+acknowledge is not equal to the sequence number in the original
+message + 1, then it is a new message.
+
+Obviously, the protocol header contains the above id.
+
+The connector allows event notification in the following form: kernel
+driver or userspace process can ask connector to notify it when
+selected ids will be turned on or off (registered or unregistered its
+callback). It is done by sending a special command to the connector
+driver (it also registers itself with id={-1, -1}).
+
+As example of this usage can be found in the cn_test.c module which
+uses the connector to request notification and to send messages.
+
+Reliability
+===========
+
+Netlink itself is not a reliable protocol. That means that messages can
+be lost due to memory pressure or process' receiving queue overflowed,
+so caller is warned that it must be prepared. That is why the struct
+cn_msg [main connector's message header] contains u32 seq and u32 ack
+fields.
+
+Userspace usage
+===============
+
+2.6.14 has a new netlink socket implementation, which by default does not
+allow people to send data to netlink groups other than 1.
+So, if you wish to use a netlink socket (for example using connector)
+with a different group number, the userspace application must subscribe to
+that group first. It can be achieved by the following pseudocode::
+
+ s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
+
+ l_local.nl_family = AF_NETLINK;
+ l_local.nl_groups = 12345;
+ l_local.nl_pid = 0;
+
+ if (bind(s, (struct sockaddr *)&l_local, sizeof(struct sockaddr_nl)) == -1) {
+ perror("bind");
+ close(s);
+ return -1;
+ }
+
+ {
+ int on = l_local.nl_groups;
+ setsockopt(s, 270, 1, &on, sizeof(on));
+ }
+
+Where 270 above is SOL_NETLINK, and 1 is a NETLINK_ADD_MEMBERSHIP socket
+option. To drop a multicast subscription, one should call the above socket
+option with the NETLINK_DROP_MEMBERSHIP parameter which is defined as 0.
+
+2.6.14 netlink code only allows to select a group which is less or equal to
+the maximum group number, which is used at netlink_kernel_create() time.
+In case of connector it is CN_NETLINK_USERS + 0xf, so if you want to use
+group number 12345, you must increment CN_NETLINK_USERS to that number.
+Additional 0xf numbers are allocated to be used by non-in-kernel users.
+
+Due to this limitation, group 0xffffffff does not work now, so one can
+not use add/remove connector's group notifications, but as far as I know,
+only cn_test.c test module used it.
+
+Some work in netlink area is still being done, so things can be changed in
+2.6.15 timeframe, if it will happen, documentation will be updated for that
+kernel.
+
+Code samples
+============
+
+Sample code for a connector test module and user space can be found
+in samples/connector/. To build this code, enable CONFIG_CONNECTOR
+and CONFIG_SAMPLES.