gistfile1.txt

    sysctl_tree = SYSCTL_ADD_NODE(&sc->sysctl_ctx,
                                      SYSCTL_STATIC_CHILDREN(_hw_usb_ugsio), OID_AUTO,
                                      unitstr, CTLFLAG_RD, 0, "UGSIO");
    if (sysctl_tree == NULL)
        goto error;

    SYSCTL_ADD_PROC(&sc->sysctl_ctx, SYSCTL_CHILDREN(sysctl_tree),
                    OID_AUTO, "serialnum", CTLTYPE_STRING | CTLFLAG_RW,
                    dev, 0, &ugsio_sysctl_sn, "A", "serial number");