TX_CIRC_BUF_SIZE
int count = CIRC_CNT_TO_END(head, tail, TX_CIRC_BUF_SIZE);
smp_store_release(&bg->tx_circ_buf.tail, (tail + written) & (TX_CIRC_BUF_SIZE - 1));
if (CIRC_SPACE(head, tail, TX_CIRC_BUF_SIZE) >= 1) {
(head + 1) & (TX_CIRC_BUF_SIZE - 1));
bg->tx_circ_buf.buf = devm_kmalloc(&bg->sd->dev, TX_CIRC_BUF_SIZE, GFP_KERNEL);
bg->gb_hd = gb_hd_create(&gb_hdlc_driver, &bg->sd->dev, TX_CIRC_BUF_SIZE, GB_MAX_CPORTS);