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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.
+
+  2014 -- a new registration ioctl is now provided which allows additional
+       fields to be provided during registration.
+       Users of this registration call are implicitly setting the use of
+       pkey_index (see below).
+
+Receiving MADs
+
+  MADs are received using read().  The receive side now supports
+  RMPP. The buffer passed to read() must be at least one
+  struct ib_user_mad + 256 bytes. For example:
+
+  If the buffer passed is not large enough to hold the received
+  MAD (RMPP), the errno is set to ENOSPC and the length of the
+  buffer needed is set in mad.length.
+
+  Example for normal MAD (non RMPP) reads:
+	struct ib_user_mad *mad;
+	mad = malloc(sizeof *mad + 256);
+	ret = read(fd, mad, sizeof *mad + 256);
+	if (ret != sizeof mad + 256) {
+		perror("read");
+		free(mad);
+	}
+
+  Example for RMPP reads:
+	struct ib_user_mad *mad;
+	mad = malloc(sizeof *mad + 256);
+	ret = read(fd, mad, sizeof *mad + 256);
+	if (ret == -ENOSPC)) {
+		length = mad.length;
+		free(mad);
+		mad = malloc(sizeof *mad + length);
+		ret = read(fd, mad, sizeof *mad + length);
+	}
+	if (ret < 0) {
+		perror("read");
+		free(mad);
+	}
+
+  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.  The send side does support
+  RMPP so arbitrary length MAD can be sent. For example:
+
+	struct ib_user_mad *mad;
+
+	mad = malloc(sizeof *mad + mad_length);
+
+	/* fill in mad->data */
+
+	mad->hdr.id  = my_agent;	/* req.id from agent registration */
+	mad->hdr.lid = my_dest;		/* in network byte order... */
+	/* etc. */
+
+	ret = write(fd, &mad, sizeof *mad + mad_length);
+	if (ret != sizeof *mad + mad_length)
+		perror("write");
+
+Transaction IDs
+
+  Users of the umad devices can use the lower 32 bits of the
+  transaction ID field (that is, the least significant half of the
+  field in network byte order) in MADs being sent to match
+  request/response pairs.  The upper 32 bits are reserved for use by
+  the kernel and will be overwritten before a MAD is sent.
+
+P_Key Index Handling
+
+  The old ib_umad interface did not allow setting the P_Key index for
+  MADs that are sent and did not provide a way for obtaining the P_Key
+  index of received MADs.  A new layout for struct ib_user_mad_hdr
+  with a pkey_index member has been defined; however, to preserve binary
+  compatibility with older applications, this new layout will not be used
+  unless one of IB_USER_MAD_ENABLE_PKEY or IB_USER_MAD_REGISTER_AGENT2 ioctl's
+  are called before a file descriptor is used for anything else.
+
+  In September 2008, the IB_USER_MAD_ABI_VERSION will be incremented
+  to 6, the new layout of struct ib_user_mad_hdr will be used by
+  default, and the IB_USER_MAD_ENABLE_PKEY ioctl will be removed.
+
+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