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+USERSPACE MAD ACCESS
+
+Device files
+
+ Each port of each InfiniBand device has a "umad" device and an
+ "issm" device attached. For example, a two-port HCA will have two
+ umad devices and two issm devices, while a switch will have one
+ device of each type (for switch port 0).
+
+Creating MAD agents
+
+ A MAD agent can be created by filling in a struct ib_user_mad_reg_req
+ and then calling the IB_USER_MAD_REGISTER_AGENT ioctl on a file
+ descriptor for the appropriate device file. If the registration
+ request succeeds, a 32-bit id will be returned in the structure.
+ For example:
+
+ struct ib_user_mad_reg_req req = { /* ... */ };
+ ret = ioctl(fd, IB_USER_MAD_REGISTER_AGENT, (char *) &req);
+ if (!ret)
+ my_agent = req.id;
+ else
+ perror("agent register");
+
+ Agents can be unregistered with the IB_USER_MAD_UNREGISTER_AGENT
+ ioctl. Also, all agents registered through a file descriptor will
+ be unregistered when the descriptor is closed.
+
+Receiving MADs
+
+ MADs are received using read(). The buffer passed to read() must be
+ large enough to hold at least one struct ib_user_mad. For example:
+
+ struct ib_user_mad mad;
+ ret = read(fd, &mad, sizeof mad);
+ if (ret != sizeof mad)
+ perror("read");
+
+ In addition to the actual MAD contents, the other struct ib_user_mad
+ fields will be filled in with information on the received MAD. For
+ example, the remote LID will be in mad.lid.
+
+ If a send times out, a receive will be generated with mad.status set
+ to ETIMEDOUT. Otherwise when a MAD has been successfully received,
+ mad.status will be 0.
+
+ poll()/select() may be used to wait until a MAD can be read.
+
+Sending MADs
+
+ MADs are sent using write(). The agent ID for sending should be
+ filled into the id field of the MAD, the destination LID should be
+ filled into the lid field, and so on. For example:
+
+ struct ib_user_mad mad;
+
+ /* fill in mad.data */
+
+ mad.id = my_agent; /* req.id from agent registration */
+ mad.lid = my_dest; /* in network byte order... */
+ /* etc. */
+
+ ret = write(fd, &mad, sizeof mad);
+ if (ret != sizeof mad)
+ perror("write");
+
+Setting IsSM Capability Bit
+
+ To set the IsSM capability bit for a port, simply open the
+ corresponding issm device file. If the IsSM bit is already set,
+ then the open call will block until the bit is cleared (or return
+ immediately with errno set to EAGAIN if the O_NONBLOCK flag is
+ passed to open()). The IsSM bit will be cleared when the issm file
+ is closed. No read, write or other operations can be performed on
+ the issm file.
+
+/dev files
+
+ To create the appropriate character device files automatically with
+ udev, a rule like
+
+ KERNEL="umad*", NAME="infiniband/%k"
+ KERNEL="issm*", NAME="infiniband/%k"
+
+ can be used. This will create device nodes named
+
+ /dev/infiniband/umad0
+ /dev/infiniband/issm0
+
+ for the first port, and so on. The InfiniBand device and port
+ associated with these devices can be determined from the files
+
+ /sys/class/infiniband_mad/umad0/ibdev
+ /sys/class/infiniband_mad/umad0/port
+
+ and
+
+ /sys/class/infiniband_mad/issm0/ibdev
+ /sys/class/infiniband_mad/issm0/port