[ewg] ofed 1.2 multicast patch documentation

[Or Gerlitz]

Sean,

The ib_sa/rdmacm multicast code in ofed 1.2 comes in the form of one big 
  undocumented patch (merged_sean_rdma_dev_ofed_1_2.patch), is the below 
fine as a change log comment?

Or.

>     Author: Sean Hefty <sean.hefty at intel.com>
>     RDMA/cma: Add multicast communication support
> 
>     Extend rdma_cm to support multicast communication.  Multicast support
>     is added to the existing RDMA_PS_UDP port space, as well as a new
>     RDMA_PS_IPOIB port space.  The latter port space allows joining the
>     multicast groups used by IPoIB, which enables offloading IPoIB traffic
>     to a separate QP.  The port space determines the signature used in the
>     MGID when joining the group.  The newly added RDMA_PS_IPOIB also
>     allows for unicast operations, similar to RDMA_PS_UDP.
> 
>     Supporting the RDMA_PS_IPOIB requires changing how UD QPs are initialized,
>     since we can no longer assume that the qkey is constant.  This requires
>     saving the Q_Key to use when attaching to a device, so that it is
>     available when creating the QP.  The Q_Key information is exported to
>     the user through the existing rdma_init_qp_attr() interface.
> 
>     Multicast support is also exported to userspace through the rdma_ucm.
> 
>     Signed-off-by: Roland Dreier <rolandd at cisco.com>
> 
> 
>     Author: Sean Hefty <sean.hefty at intel.com>
>     IB/sa: Track multicast join/leave requests
> 
>     The IB SA tracks multicast join/leave requests on a per port basis and
>     does not do any reference counting: if two users of the same port join
>     the same group, and one leaves that group, then the SA will remove the
>     port from the group even though there is one user who wants to stay a
>     member left.  Therefore, in order to support multiple users of the
>     same multicast group from the same port, we need to perform reference
>     counting locally.
> 
>     To do this, add an multicast submodule to ib_sa to perform reference
>     counting of multicast join/leave operations.  Modify ib_ipoib (the
>     only in-kernel user of multicast) to use the new interface.
> 
>     Signed-off-by: Roland Dreier <rolandd at cisco.com>
>