ibv_get_cq_event man page on YellowDog

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IBV_GET_CQ_EVENT(3)	Libibverbs Programmer's Manual	   IBV_GET_CQ_EVENT(3)

NAME
       ibv_get_cq_event,  ibv_ack_cq_events  -	get and acknowledge completion
       queue (CQ) events

SYNOPSIS
       #include <infiniband/verbs.h>

       int ibv_get_cq_event(struct ibv_comp_channel *channel,
			    struct ibv_cq **cq, void **cq_context);
       void ibv_ack_cq_events(struct ibv_cq *cq, unsigned int nevents);

DESCRIPTION
       ibv_get_cq_event() waits for the next completion event in  the  comple‐
       tion  event  channel channel.  The argument cq is used to return the CQ
       that caused the event and cq_context is used to return  the  CQ's  con‐
       text.

       ibv_ack_cq_events() acknowledges nevents events on the CQ cq.

RETURN VALUE
       ibv_get_cq_event() returns 0 on success, and -1 on error.

       ibv_ack_cq_events() returns no value.

NOTES
       All  completion events that ibv_get_cq_event() returns must be acknowl‐
       edged using ibv_ack_cq_events().	 To avoid races, destroying a CQ  will
       wait  for  all  completion events to be acknowledged; this guarantees a
       one-to-one correspondence between acks and successful gets.

       Calling ibv_ack_cq_events() may be relatively expensive	in  the	 data‐
       path,  since it must take a mutex.  Therefore it may be better to amor‐
       tize this cost by keeping a count  of  the  number  of  events  needing
       acknowledgement	and  acking  several  completion events in one call to
       ibv_ack_cq_events().

EXAMPLES
       The following code example demonstrates one possible way to  work  with
       completion events. It performs the following steps:

       Stage I: Preparation
       1. Creates a CQ
       2. Requests for notification upon a new (first) completion event

       Stage II: Completion Handling Routine
       3. Wait for the completion event and ack it
       4. Request for notification upon the next completion event
       5. Empty the CQ

       Note  that an extra event may be triggered without having a correspond‐
       ing completion entry in the CQ.	This occurs if a completion  entry  is
       added  to  the CQ between Step 4 and Step 5, and the CQ is then emptied
       (polled) in Step 5.

       cq = ibv_create_cq(ctx, 1, ev_ctx, channel, 0);
       if (!cq) {
	       fprintf(stderr, "Failed to create CQ\n");
	       return 1;
       }

       /* Request notification before any completion can be created */
       if (ibv_req_notify_cq(cq, 0)) {
	       fprintf(stderr, "Couldn't request CQ notification\n");
	       return 1;
       }

       .
       .
       .

       /* Wait for the completion event */
       if (ibv_get_cq_event(channel, &ev_cq, &ev_ctx)) {
	       fprintf(stderr, "Failed to get cq_event\n");
	       return 1;
       }

       /* Ack the event */
       ibv_ack_cq_events(ev_cq, 1);

       /* Request notification upon the next completion event */
       if (ibv_req_notify_cq(cq, 0)) {
	       fprintf(stderr, "Couldn't request CQ notification\n");
	       return 1;
       }

       /* Empty the CQ: poll all of the completions from the CQ (if any exist) */
       do {
	       ne = ibv_poll_cq(cq, 1, &wc);
	       if (ne < 0) {
		       fprintf(stderr, "Failed to poll completions from the CQ\n");
		       return 1;
	       }

	       if (wc.status != IBV_WC_SUCCESS) {
		       fprintf(stderr, "Completion with status 0x%x was found\n", wc.status);
		       return 1;
	       }
       } while (ne);

       The following code example demonstrates one possible way to  work  with
       completion  events  in  non-blocking  mode.   It performs the following
       steps:

       1. Set the completion event channel to be non-blocked
       2. Poll the channel until there it has a completion event
       3. Get the completion event and ack it

       /* change the blocking mode of the completion channel */
       flags = fcntl(channel->fd, F_GETFL);
       rc = fcntl(channel->fd, F_SETFL, flags | O_NONBLOCK);
       if (rc < 0) {
	    fprintf(stderr, "Failed to change file descriptor of completion event channel\n");
	    return 1;
       }

       /*
	* poll the channel until it has an event and sleep ms_timeout
	* milliseconds between any iteration
	*/
       my_pollfd.fd	 = channel->fd;
       my_pollfd.events	 = POLLIN;
       my_pollfd.revents = 0;

       do {
	    rc = poll(&my_pollfd, 1, ms_timeout);
       } while (rc == 0);
       if (rc < 0) {
	    fprintf(stderr, "poll failed\n");
	    return 1;
       }
       ev_cq = cq;

       /* Wait for the completion event */
       if (ibv_get_cq_event(channel, &ev_cq, &ev_ctx)) {
	       fprintf(stderr, "Failed to get cq_event\n");
	       return 1;
       }

       /* Ack the event */
       ibv_ack_cq_events(ev_cq, 1);

SEE ALSO
       ibv_create_comp_channel(3),   ibv_create_cq(3),	 ibv_req_notify_cq(3),
       ibv_poll_cq(3)

AUTHORS
       Dotan Barak
	      <dotanb@mellanox.co.il>

libibverbs			  2006-10-31		   IBV_GET_CQ_EVENT(3)
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