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 completion event channel channel.  Fills
       the arguments cq with the CQ that got the event and cq_context with the CQ's context.

       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 acknowledged 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 datapath, since  it  must  take  a  mutex.
       Therefore  it  may  be  better  to  amortize 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 corresponding 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(ev_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;
               }

               /* there may be an extra event with no completion in the CQ */
               if (ne == 0)
                       continue;

               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
              <dotanba@gmail.com>