]> git.proxmox.com Git - mirror_ubuntu-bionic-kernel.git/blame - Documentation/connector/connector.txt
Documentation: xillybus: fix spelling mistake
[mirror_ubuntu-bionic-kernel.git] / Documentation / connector / connector.txt
CommitLineData
7672d0b5
EP
1/*****************************************/
2Kernel Connector.
3/*****************************************/
4
5Kernel connector - new netlink based userspace <-> kernel space easy
6to use communication module.
7
41144ca3
MF
8The Connector driver makes it easy to connect various agents using a
9netlink based network. One must register a callback and an identifier.
10When the driver receives a special netlink message with the appropriate
11identifier, the appropriate callback will be called.
7672d0b5
EP
12
13From the userspace point of view it's quite straightforward:
14
15 socket();
16 bind();
17 send();
18 recv();
19
41144ca3
MF
20But if kernelspace wants to use the full power of such connections, the
21driver writer must create special sockets, must know about struct sk_buff
22handling, etc... The Connector driver allows any kernelspace agents to use
23netlink based networking for inter-process communication in a significantly
7672d0b5
EP
24easier way:
25
7069331d 26int cn_add_callback(struct cb_id *id, char *name, void (*callback) (struct cn_msg *, struct netlink_skb_parms *));
34470e0b
DF
27void cn_netlink_send_multi(struct cn_msg *msg, u16 len, u32 portid, u32 __group, int gfp_mask);
28void cn_netlink_send(struct cn_msg *msg, u32 portid, u32 __group, int gfp_mask);
7672d0b5
EP
29
30struct cb_id
31{
32 __u32 idx;
33 __u32 val;
34};
35
41144ca3
MF
36idx and val are unique identifiers which must be registered in the
37connector.h header for in-kernel usage. void (*callback) (void *) is a
38callback function which will be called when a message with above idx.val
39is received by the connector core. The argument for that function must
7672d0b5
EP
40be dereferenced to struct cn_msg *.
41
42struct cn_msg
43{
41144ca3 44 struct cb_id id;
7672d0b5
EP
45
46 __u32 seq;
47 __u32 ack;
48
49 __u32 len; /* Length of the following data */
50 __u8 data[0];
51};
52
53/*****************************************/
54Connector interfaces.
55/*****************************************/
56
7069331d 57int cn_add_callback(struct cb_id *id, char *name, void (*callback) (struct cn_msg *, struct netlink_skb_parms *));
7672d0b5 58
41144ca3 59 Registers new callback with connector core.
7672d0b5 60
41144ca3
MF
61 struct cb_id *id - unique connector's user identifier.
62 It must be registered in connector.h for legal in-kernel users.
63 char *name - connector's callback symbolic name.
7069331d
PR
64 void (*callback) (struct cn..) - connector's callback.
65 cn_msg and the sender's credentials
7672d0b5 66
41144ca3 67
7672d0b5
EP
68void cn_del_callback(struct cb_id *id);
69
41144ca3
MF
70 Unregisters new callback with connector core.
71
72 struct cb_id *id - unique connector's user identifier.
7672d0b5 73
7672d0b5 74
34470e0b
DF
75int cn_netlink_send_multi(struct cn_msg *msg, u16 len, u32 portid, u32 __groups, int gfp_mask);
76int cn_netlink_send(struct cn_msg *msg, u32 portid, u32 __groups, int gfp_mask);
7672d0b5 77
41144ca3
MF
78 Sends message to the specified groups. It can be safely called from
79 softirq context, but may silently fail under strong memory pressure.
80 If there are no listeners for given group -ESRCH can be returned.
7672d0b5 81
41144ca3 82 struct cn_msg * - message header(with attached data).
34470e0b
DF
83 u16 len - for *_multi multiple cn_msg messages can be sent
84 u32 port - destination port.
85 If non-zero the message will be sent to the
86 given port, which should be set to the
87 original sender.
41144ca3 88 u32 __group - destination group.
34470e0b 89 If port and __group is zero, then appropriate group will
7672d0b5
EP
90 be searched through all registered connector users,
91 and message will be delivered to the group which was
92 created for user with the same ID as in msg.
93 If __group is not zero, then message will be delivered
94 to the specified group.
41144ca3 95 int gfp_mask - GFP mask.
7672d0b5 96
41144ca3 97 Note: When registering new callback user, connector core assigns
a33f3224 98 netlink group to the user which is equal to its id.idx.
7672d0b5
EP
99
100/*****************************************/
101Protocol description.
102/*****************************************/
103
41144ca3
MF
104The current framework offers a transport layer with fixed headers. The
105recommended protocol which uses such a header is as following:
7672d0b5
EP
106
107msg->seq and msg->ack are used to determine message genealogy. When
41144ca3
MF
108someone sends a message, they use a locally unique sequence and random
109acknowledge number. The sequence number may be copied into
7672d0b5
EP
110nlmsghdr->nlmsg_seq too.
111
41144ca3 112The sequence number is incremented with each message sent.
7672d0b5 113
41144ca3
MF
114If you expect a reply to the message, then the sequence number in the
115received message MUST be the same as in the original message, and the
116acknowledge number MUST be the same + 1.
7672d0b5 117
41144ca3
MF
118If we receive a message and its sequence number is not equal to one we
119are expecting, then it is a new message. If we receive a message and
120its sequence number is the same as one we are expecting, but its
8a0427d1 121acknowledge is not equal to the sequence number in the original
41144ca3 122message + 1, then it is a new message.
7672d0b5 123
41144ca3 124Obviously, the protocol header contains the above id.
7672d0b5 125
41144ca3 126The connector allows event notification in the following form: kernel
7672d0b5 127driver or userspace process can ask connector to notify it when
41144ca3
MF
128selected ids will be turned on or off (registered or unregistered its
129callback). It is done by sending a special command to the connector
130driver (it also registers itself with id={-1, -1}).
7672d0b5 131
41144ca3
MF
132As example of this usage can be found in the cn_test.c module which
133uses the connector to request notification and to send messages.
7672d0b5
EP
134
135/*****************************************/
136Reliability.
137/*****************************************/
138
41144ca3 139Netlink itself is not a reliable protocol. That means that messages can
7672d0b5 140be lost due to memory pressure or process' receiving queue overflowed,
41144ca3
MF
141so caller is warned that it must be prepared. That is why the struct
142cn_msg [main connector's message header] contains u32 seq and u32 ack
143fields.
eb0d6041
EP
144
145/*****************************************/
146Userspace usage.
147/*****************************************/
41144ca3 148
eb0d6041 1492.6.14 has a new netlink socket implementation, which by default does not
41144ca3
MF
150allow people to send data to netlink groups other than 1.
151So, if you wish to use a netlink socket (for example using connector)
152with a different group number, the userspace application must subscribe to
153that group first. It can be achieved by the following pseudocode:
eb0d6041
EP
154
155s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
156
157l_local.nl_family = AF_NETLINK;
158l_local.nl_groups = 12345;
159l_local.nl_pid = 0;
160
161if (bind(s, (struct sockaddr *)&l_local, sizeof(struct sockaddr_nl)) == -1) {
162 perror("bind");
163 close(s);
164 return -1;
165}
166
167{
168 int on = l_local.nl_groups;
169 setsockopt(s, 270, 1, &on, sizeof(on));
170}
171
172Where 270 above is SOL_NETLINK, and 1 is a NETLINK_ADD_MEMBERSHIP socket
41144ca3
MF
173option. To drop a multicast subscription, one should call the above socket
174option with the NETLINK_DROP_MEMBERSHIP parameter which is defined as 0.
eb0d6041
EP
175
1762.6.14 netlink code only allows to select a group which is less or equal to
177the maximum group number, which is used at netlink_kernel_create() time.
178In case of connector it is CN_NETLINK_USERS + 0xf, so if you want to use
179group number 12345, you must increment CN_NETLINK_USERS to that number.
180Additional 0xf numbers are allocated to be used by non-in-kernel users.
181
182Due to this limitation, group 0xffffffff does not work now, so one can
183not use add/remove connector's group notifications, but as far as I know,
184only cn_test.c test module used it.
185
186Some work in netlink area is still being done, so things can be changed in
1872.6.15 timeframe, if it will happen, documentation will be updated for that
188kernel.