Due to the discrete nature of the HIP08 timer unit, a requester might
finish the timeout period sooner, in elapsed real time, than its responder
does, even when both sides share the identical RNR timeout length included
in the RNR Nak packet and the responder indeed starts the timing prior to
the requester. Furthermore, if a 'providential' resend packet arrived
before the responder's timeout period expired, the responder is certainly
entitled to drop the packet silently in the light of IB protocol.
To address this problem, our team made good use of certain hardware facts:
1) The timing resolution regards the transmission arrangements is 1
microsecond, e.g. if cq_period field is set to 3, it would be
interpreted as 3 microsecond by hardware
2) A QPC field shall inform the hardware how many timing unit (ticks)
constitutes a full microsecond, which, by default, is 1000
3) It takes 14ns for the processor to handle a packet in the buffer, so
the RNR timeout length of 10ns would ensure our processing mechanism is
disabled during the entire timeout period and the packet won't be
dropped silently
To achieve (3), we permanently set the QPC field mentioned in (2) to zero
which nominally indicates every time tick is equivalent to a microsecond
in wall-clock time; now, a RNR timeout period at face value of 10 would
only last 10 ticks, which is 10ns in wall-clock time.
It's worth noting that we adapt the driver by magnifying certain
configuration parameters(cq_period, eq_period and ack_timeout)by 1000
given the user assumes the configuring timing unit to be microseconds.
Also, this particular improvisation is only deployed on HIP08 since other
hardware has already solved this issue.
Fixes: cfc85f3e4b ("RDMA/hns: Add profile support for hip08 driver")
Link: https://lore.kernel.org/r/20211209140655.49493-1-liangwenpeng@huawei.com
Signed-off-by: Yangyang Li <liyangyang20@huawei.com>
Signed-off-by: Wenpeng Liang <liangwenpeng@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
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