FreeBSD Architecture Handbook
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# 12.4 Asynchronous Events

If an asynchronous event callback has been set up then the callback function should be defined.

    static void
    ahc_async(void *callback_arg, u_int32_t code, struct cam_path *path, void *arg)

- callback_arg - the value supplied when registering the callback

- code - identifies the type of event

- path - identifies the devices to which the event applies

- arg - event-specific argument

Implementation for a single type of event, AC_LOST_DEVICE, looks like:

        struct xxx_softc *softc;
        struct cam_sim *sim;
        int targ;
        struct ccb_trans_settings neg;

        sim = (struct cam_sim *)callback_arg;
        softc = (struct xxx_softc *)cam_sim_softc(sim);
        switch (code) {
        case AC_LOST_DEVICE:
            targ = xpt_path_target_id(path);
            if(targ <= OUR_MAX_SUPPORTED_TARGET) {
                clean_negotiations(softc, targ);
                /* send indication to CAM */
                neg.bus_width = 8;
                neg.sync_period = neg.sync_offset = 0;
                neg.valid = (CCB_TRANS_BUS_WIDTH_VALID
                    | CCB_TRANS_SYNC_RATE_VALID | CCB_TRANS_SYNC_OFFSET_VALID);
                xpt_async(AC_TRANSFER_NEG, path, &neg);
            }
            break;
        default:
            break;
        }

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