[ofa-general] RDS flow control

[Roland Dreier]

 > You can use ib_modify_qp() to set the QP state to IB_QPS_SQD (drain),
 > modify the IB_QP_RNR_RETRY parameter, and modify the QP back to
 > IB_QPS_RTS. It seems to me that modify QP could allow a RTS to RTS
 > transition and set the IB_QP_RNR_RETRY count but the qp_state_table[]
 > doesn't seem to indicate that is valid.

The IB spec doesn't allow changing RNR retry on RTS to RTS transitions.
Probably because synchronizing the change with in-flight send requests
(that might be doing RNR handling at that moment) is too much of a mess.

 - R.