Does OpenMPI always use SEND/RECV protocol between heterogeneous processors with different endianness?

I tried btl_openib_flags to be 2 , 4 and 6 respectively to allowe RDMA, but the bandwidth between the two heterogeneous nodes is slow, same as the bandwidth when btl_openib_flags to be 1. Seems to me SEND/RECV is always used no matter btl_openib_flags is. Can I force OpenMPI to use RDMA between x86 and PPC? I only transfer MPI_BYTE, so we do not need the support for endianness.

Mi Yan