RE: [PATCH v2 0/3] new ib_drain_qp() API

["Steve Wise" <swise-7bPotxP6k4+P2YhJcF5u+vpXobYPEAuW@public.gmane.org>]

> >>> Hello Steve,
> >>>
> >>> How about creating three functions - one that drains the receive queue,
> >>> one that drains the send queue and a third function that drains both ?
> >>> The latter function then can call the two former functions. And since
> >>> only one of these three functions will have a user in your patch series
> >>> (the function that drains the RQ), how about only introducing only that
> >>> function now and to wait with introducing the two other functions until
> >>> these have a user ?
> >>
> >> That sounds reasonable.  Simpler too perhaps.  We'll see if anyone else
> >> has an opinion.
> >
> > Another option is for ib_drain_qp() to just skip queues with IB_POLL_DIRECT CQ processing.
> 
> I'd rather not skip silently anything. ib_drain_qp() semantics is that
> it drains all the pending posts on the queue-pair (i.e. both send and
> receive queues).
> 
> We can split to send and receive, but the fact that srp uses direct
> polling CQ is not sufficient for that, most if not all will benefit from
> both.
>

If we split it into ib_drain_rq(), ib_drain_sq(), and ib_drain_qp(), then srp would only call ib_drain_rq().  Others can call
ib_drain_qp() if they want both SQ and RQ drained.
 
> I'd suggest to look at the CQ context and act accordingly, something
> like:
> 
> 	if (cq->poll_ctx == IB_POLL_DIRECT) {
> 		while (!wait_for_completion_timeout(&sdrain.done,
> 				mescs_to_jiffies(100))
> 			ib_process_cq_direct(cq, 1024)
> 	} else {
> 		wait_for_completion(&sdrain.done);
> 	}
> 
> Thoughts?

I don't like the forced 100ms block.  You could call ib_process_cq_direct() first and check the return code, then block if needed.
But is there utility in providing drain for IB_POLL_DIRECT users?  

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