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 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 the IB_USER_MAD_ENABLE_PKEY ioctl is 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