static int rtl8139_cplus_transmit_one(RTL8139State *s)
{
    if (!rtl8139_transmitter_enabled(s))
    {
        DPRINTF("+++ C+ mode: transmitter disabled\n");
        return 0;
    }

    if (!rtl8139_cp_transmitter_enabled(s))
    {
        DPRINTF("+++ C+ mode: C+ transmitter disabled\n");
        return 0 ;
    }

    PCIDevice *d = PCI_DEVICE(s);
    int descriptor = s->currCPlusTxDesc;

    dma_addr_t cplus_tx_ring_desc = rtl8139_addr64(s->TxAddr[0], s->TxAddr[1]);

    /* Normal priority ring */
    cplus_tx_ring_desc += 16 * descriptor;

    DPRINTF("+++ C+ mode reading TX descriptor %d from host memory at "
        "%08x %08x = 0x"DMA_ADDR_FMT"\n", descriptor, s->TxAddr[1],
        s->TxAddr[0], cplus_tx_ring_desc);

    uint32_t val, txdw0,txdw1,txbufLO,txbufHI;

    pci_dma_read(d, cplus_tx_ring_desc,    (uint8_t *)&val, 4);
    txdw0 = le32_to_cpu(val);
    pci_dma_read(d, cplus_tx_ring_desc+4,  (uint8_t *)&val, 4);
    txdw1 = le32_to_cpu(val);
    pci_dma_read(d, cplus_tx_ring_desc+8,  (uint8_t *)&val, 4);
    txbufLO = le32_to_cpu(val);
    pci_dma_read(d, cplus_tx_ring_desc+12, (uint8_t *)&val, 4);
    txbufHI = le32_to_cpu(val);

    DPRINTF("+++ C+ mode TX descriptor %d %08x %08x %08x %08x\n", descriptor,
        txdw0, txdw1, txbufLO, txbufHI);

/* w0 ownership flag */
#define CP_TX_OWN (1<<31)
/* w0 end of ring flag */
#define CP_TX_EOR (1<<30)
/* first segment of received packet flag */
#define CP_TX_FS (1<<29)
/* last segment of received packet flag */
#define CP_TX_LS (1<<28)
/* large send packet flag */
#define CP_TX_LGSEN (1<<27)
/* large send MSS mask, bits 16...25 */
#define CP_TC_LGSEN_MSS_MASK ((1 << 12) - 1)

/* IP checksum offload flag */
#define CP_TX_IPCS (1<<18)
/* UDP checksum offload flag */
#define CP_TX_UDPCS (1<<17)
/* TCP checksum offload flag */
#define CP_TX_TCPCS (1<<16)

/* w0 bits 0...15 : buffer size */
#define CP_TX_BUFFER_SIZE (1<<16)
#define CP_TX_BUFFER_SIZE_MASK (CP_TX_BUFFER_SIZE - 1)
/* w1 add tag flag */
#define CP_TX_TAGC (1<<17)
/* w1 bits 0...15 : VLAN tag (big endian) */
#define CP_TX_VLAN_TAG_MASK ((1<<16) - 1)
/* w2 low  32bit of Rx buffer ptr */
/* w3 high 32bit of Rx buffer ptr */

/* set after transmission */
/* FIFO underrun flag */
#define CP_TX_STATUS_UNF (1<<25)
/* transmit error summary flag, valid if set any of three below */
#define CP_TX_STATUS_TES (1<<23)
/* out-of-window collision flag */
#define CP_TX_STATUS_OWC (1<<22)
/* link failure flag */
#define CP_TX_STATUS_LNKF (1<<21)
/* excessive collisions flag */
#define CP_TX_STATUS_EXC (1<<20)

    if (!(txdw0 & CP_TX_OWN))
    {
        DPRINTF("C+ Tx mode : descriptor %d is owned by host\n", descriptor);
        return 0 ;
    }

    DPRINTF("+++ C+ Tx mode : transmitting from descriptor %d\n", descriptor);

    if (txdw0 & CP_TX_FS)
    {
        DPRINTF("+++ C+ Tx mode : descriptor %d is first segment "
            "descriptor\n", descriptor);

        /* reset internal buffer offset */
        s->cplus_txbuffer_offset = 0;
    }

    int txsize = txdw0 & CP_TX_BUFFER_SIZE_MASK;
    dma_addr_t tx_addr = rtl8139_addr64(txbufLO, txbufHI);

    /* make sure we have enough space to assemble the packet */
    if (!s->cplus_txbuffer)
    {
        s->cplus_txbuffer_len = CP_TX_BUFFER_SIZE;
        s->cplus_txbuffer = g_malloc(s->cplus_txbuffer_len);
        s->cplus_txbuffer_offset = 0;

        DPRINTF("+++ C+ mode transmission buffer allocated space %d\n",
            s->cplus_txbuffer_len);
    }

    if (s->cplus_txbuffer_offset + txsize >= s->cplus_txbuffer_len)
    {
        /* The spec didn't tell the maximum size, stick to CP_TX_BUFFER_SIZE */
        txsize = s->cplus_txbuffer_len - s->cplus_txbuffer_offset;
        DPRINTF("+++ C+ mode transmission buffer overrun, truncated descriptor"
                "length to %d\n", txsize);
    }

    /* append more data to the packet */

    DPRINTF("+++ C+ mode transmit reading %d bytes from host memory at "
            DMA_ADDR_FMT" to offset %d\n", txsize, tx_addr,
            s->cplus_txbuffer_offset);

    pci_dma_read(d, tx_addr,
                 s->cplus_txbuffer + s->cplus_txbuffer_offset, txsize);
    s->cplus_txbuffer_offset += txsize;

    /* seek to next Rx descriptor */
    if (txdw0 & CP_TX_EOR)
    {
        s->currCPlusTxDesc = 0;
    }
    else
    {
        ++s->currCPlusTxDesc;
        if (s->currCPlusTxDesc >= 64)
            s->currCPlusTxDesc = 0;
    }

    /* transfer ownership to target */
    txdw0 &= ~CP_TX_OWN;

    /* reset error indicator bits */
    txdw0 &= ~CP_TX_STATUS_UNF;
    txdw0 &= ~CP_TX_STATUS_TES;
    txdw0 &= ~CP_TX_STATUS_OWC;
    txdw0 &= ~CP_TX_STATUS_LNKF;
    txdw0 &= ~CP_TX_STATUS_EXC;

    /* update ring data */
    val = cpu_to_le32(txdw0);
    pci_dma_write(d, cplus_tx_ring_desc, (uint8_t *)&val, 4);

    /* Now decide if descriptor being processed is holding the last segment of packet */
    if (txdw0 & CP_TX_LS)
    {
        uint8_t dot1q_buffer_space[VLAN_HLEN];
        uint16_t *dot1q_buffer;

        DPRINTF("+++ C+ Tx mode : descriptor %d is last segment descriptor\n",
            descriptor);

        /* can transfer fully assembled packet */

        uint8_t *saved_buffer  = s->cplus_txbuffer;
        int      saved_size    = s->cplus_txbuffer_offset;
        int      saved_buffer_len = s->cplus_txbuffer_len;

        /* create vlan tag */
        if (txdw1 & CP_TX_TAGC) {
            /* the vlan tag is in BE byte order in the descriptor
             * BE + le_to_cpu() + ~swap()~ = cpu */
            DPRINTF("+++ C+ Tx mode : inserting vlan tag with ""tci: %u\n",
                bswap16(txdw1 & CP_TX_VLAN_TAG_MASK));

            dot1q_buffer = (uint16_t *) dot1q_buffer_space;
            dot1q_buffer[0] = cpu_to_be16(ETH_P_VLAN);
            /* BE + le_to_cpu() + ~cpu_to_le()~ = BE */
            dot1q_buffer[1] = cpu_to_le16(txdw1 & CP_TX_VLAN_TAG_MASK);
        } else {
            dot1q_buffer = NULL;
        }

        /* reset the card space to protect from recursive call */
        s->cplus_txbuffer = NULL;
        s->cplus_txbuffer_offset = 0;
        s->cplus_txbuffer_len = 0;

        if (txdw0 & (CP_TX_IPCS | CP_TX_UDPCS | CP_TX_TCPCS | CP_TX_LGSEN))
        {
            DPRINTF("+++ C+ mode offloaded task checksum\n");

            /* Large enough for Ethernet and IP headers? */
            if (saved_size < ETH_HLEN + sizeof(struct ip_header)) {
                goto skip_offload;
            }

            /* ip packet header */
            struct ip_header *ip = NULL;
            int hlen = 0;
            uint8_t  ip_protocol = 0;
            uint16_t ip_data_len = 0;

            uint8_t *eth_payload_data = NULL;
            size_t   eth_payload_len  = 0;

            int proto = be16_to_cpu(*(uint16_t *)(saved_buffer + 12));
            if (proto != ETH_P_IP)
            {
                goto skip_offload;
            }

            DPRINTF("+++ C+ mode has IP packet\n");

            /* Note on memory alignment: eth_payload_data is 16-bit aligned
             * since saved_buffer is allocated with g_malloc() and ETH_HLEN is
             * even.  32-bit accesses must use ldl/stl wrappers to avoid
             * unaligned accesses.
             */
            eth_payload_data = saved_buffer + ETH_HLEN;
            eth_payload_len  = saved_size   - ETH_HLEN;

            ip = (struct ip_header*)eth_payload_data;

            if (IP_HEADER_VERSION(ip) != IP_HEADER_VERSION_4) {
                DPRINTF("+++ C+ mode packet has bad IP version %d "
                    "expected %d\n", IP_HEADER_VERSION(ip),
                    IP_HEADER_VERSION_4);
                goto skip_offload;
            }

            hlen = IP_HDR_GET_LEN(ip);
            if (hlen < sizeof(struct ip_header) || hlen > eth_payload_len) {
                goto skip_offload;
            }

            ip_protocol = ip->ip_p;

            ip_data_len = be16_to_cpu(ip->ip_len);
            if (ip_data_len < hlen || ip_data_len > eth_payload_len) {
                goto skip_offload;
            }
            ip_data_len -= hlen;

            if (txdw0 & CP_TX_IPCS)
            {
                DPRINTF("+++ C+ mode need IP checksum\n");

                ip->ip_sum = 0;
                ip->ip_sum = ip_checksum(ip, hlen);
                DPRINTF("+++ C+ mode IP header len=%d checksum=%04x\n",
                    hlen, ip->ip_sum);
            }

            if ((txdw0 & CP_TX_LGSEN) && ip_protocol == IP_PROTO_TCP)
            {
                /* Large enough for the TCP header? */
                if (ip_data_len < sizeof(tcp_header)) {
                    goto skip_offload;
                }

                int large_send_mss = (txdw0 >> 16) & CP_TC_LGSEN_MSS_MASK;

                DPRINTF("+++ C+ mode offloaded task TSO MTU=%d IP data %d "
                    "frame data %d specified MSS=%d\n", ETH_MTU,
                    ip_data_len, saved_size - ETH_HLEN, large_send_mss);

                int tcp_send_offset = 0;
                int send_count = 0;

                /* maximum IP header length is 60 bytes */
                uint8_t saved_ip_header[60];

                /* save IP header template; data area is used in tcp checksum calculation */
                memcpy(saved_ip_header, eth_payload_data, hlen);

                /* a placeholder for checksum calculation routine in tcp case */
                uint8_t *data_to_checksum     = eth_payload_data + hlen - 12;
                //                    size_t   data_to_checksum_len = eth_payload_len  - hlen + 12;

                /* pointer to TCP header */
                tcp_header *p_tcp_hdr = (tcp_header*)(eth_payload_data + hlen);

                int tcp_hlen = TCP_HEADER_DATA_OFFSET(p_tcp_hdr);

                /* Invalid TCP data offset? */
                if (tcp_hlen < sizeof(tcp_header) || tcp_hlen > ip_data_len) {
                    goto skip_offload;
                }

                /* ETH_MTU = ip header len + tcp header len + payload */
                int tcp_data_len = ip_data_len - tcp_hlen;
                int tcp_chunk_size = ETH_MTU - hlen - tcp_hlen;

                DPRINTF("+++ C+ mode TSO IP data len %d TCP hlen %d TCP "
                    "data len %d TCP chunk size %d\n", ip_data_len,
                    tcp_hlen, tcp_data_len, tcp_chunk_size);

                /* note the cycle below overwrites IP header data,
                   but restores it from saved_ip_header before sending packet */

                int is_last_frame = 0;

                for (tcp_send_offset = 0; tcp_send_offset < tcp_data_len; tcp_send_offset += tcp_chunk_size)
                {
                    uint16_t chunk_size = tcp_chunk_size;

                    /* check if this is the last frame */
                    if (tcp_send_offset + tcp_chunk_size >= tcp_data_len)
                    {
                        is_last_frame = 1;
                        chunk_size = tcp_data_len - tcp_send_offset;
                    }

                    DPRINTF("+++ C+ mode TSO TCP seqno %08x\n",
                            ldl_be_p(&p_tcp_hdr->th_seq));

                    /* add 4 TCP pseudoheader fields */
                    /* copy IP source and destination fields */
                    memcpy(data_to_checksum, saved_ip_header + 12, 8);

                    DPRINTF("+++ C+ mode TSO calculating TCP checksum for "
                        "packet with %d bytes data\n", tcp_hlen +
                        chunk_size);

                    if (tcp_send_offset)
                    {
                        memcpy((uint8_t*)p_tcp_hdr + tcp_hlen, (uint8_t*)p_tcp_hdr + tcp_hlen + tcp_send_offset, chunk_size);
                    }

                    /* keep PUSH and FIN flags only for the last frame */
                    if (!is_last_frame)
                    {
                        TCP_HEADER_CLEAR_FLAGS(p_tcp_hdr, TH_PUSH | TH_FIN);
                    }

                    /* recalculate TCP checksum */
                    ip_pseudo_header *p_tcpip_hdr = (ip_pseudo_header *)data_to_checksum;
                    p_tcpip_hdr->zeros      = 0;
                    p_tcpip_hdr->ip_proto   = IP_PROTO_TCP;
                    p_tcpip_hdr->ip_payload = cpu_to_be16(tcp_hlen + chunk_size);

                    p_tcp_hdr->th_sum = 0;

                    int tcp_checksum = ip_checksum(data_to_checksum, tcp_hlen + chunk_size + 12);
                    DPRINTF("+++ C+ mode TSO TCP checksum %04x\n",
                        tcp_checksum);

                    p_tcp_hdr->th_sum = tcp_checksum;

                    /* restore IP header */
                    memcpy(eth_payload_data, saved_ip_header, hlen);

                    /* set IP data length and recalculate IP checksum */
                    ip->ip_len = cpu_to_be16(hlen + tcp_hlen + chunk_size);

                    /* increment IP id for subsequent frames */
                    ip->ip_id = cpu_to_be16(tcp_send_offset/tcp_chunk_size + be16_to_cpu(ip->ip_id));

                    ip->ip_sum = 0;
                    ip->ip_sum = ip_checksum(eth_payload_data, hlen);
                    DPRINTF("+++ C+ mode TSO IP header len=%d "
                        "checksum=%04x\n", hlen, ip->ip_sum);

                    int tso_send_size = ETH_HLEN + hlen + tcp_hlen + chunk_size;
                    DPRINTF("+++ C+ mode TSO transferring packet size "
                        "%d\n", tso_send_size);
                    rtl8139_transfer_frame(s, saved_buffer, tso_send_size,
                        0, (uint8_t *) dot1q_buffer);

                    /* add transferred count to TCP sequence number */
                    stl_be_p(&p_tcp_hdr->th_seq,
                             chunk_size + ldl_be_p(&p_tcp_hdr->th_seq));
                    ++send_count;
                }

                /* Stop sending this frame */
                saved_size = 0;
            }
            else if (txdw0 & (CP_TX_TCPCS|CP_TX_UDPCS))
            {
                DPRINTF("+++ C+ mode need TCP or UDP checksum\n");

                /* maximum IP header length is 60 bytes */
                uint8_t saved_ip_header[60];
                memcpy(saved_ip_header, eth_payload_data, hlen);

                uint8_t *data_to_checksum     = eth_payload_data + hlen - 12;
                //                    size_t   data_to_checksum_len = eth_payload_len  - hlen + 12;

                /* add 4 TCP pseudoheader fields */
                /* copy IP source and destination fields */
                memcpy(data_to_checksum, saved_ip_header + 12, 8);

                if ((txdw0 & CP_TX_TCPCS) && ip_protocol == IP_PROTO_TCP)
                {
                    DPRINTF("+++ C+ mode calculating TCP checksum for "
                        "packet with %d bytes data\n", ip_data_len);

                    ip_pseudo_header *p_tcpip_hdr = (ip_pseudo_header *)data_to_checksum;
                    p_tcpip_hdr->zeros      = 0;
                    p_tcpip_hdr->ip_proto   = IP_PROTO_TCP;
                    p_tcpip_hdr->ip_payload = cpu_to_be16(ip_data_len);

                    tcp_header* p_tcp_hdr = (tcp_header *) (data_to_checksum+12);

                    p_tcp_hdr->th_sum = 0;

                    int tcp_checksum = ip_checksum(data_to_checksum, ip_data_len + 12);
                    DPRINTF("+++ C+ mode TCP checksum %04x\n",
                        tcp_checksum);

                    p_tcp_hdr->th_sum = tcp_checksum;
                }
                else if ((txdw0 & CP_TX_UDPCS) && ip_protocol == IP_PROTO_UDP)
                {
                    DPRINTF("+++ C+ mode calculating UDP checksum for "
                        "packet with %d bytes data\n", ip_data_len);

                    ip_pseudo_header *p_udpip_hdr = (ip_pseudo_header *)data_to_checksum;
                    p_udpip_hdr->zeros      = 0;
                    p_udpip_hdr->ip_proto   = IP_PROTO_UDP;
                    p_udpip_hdr->ip_payload = cpu_to_be16(ip_data_len);

                    udp_header *p_udp_hdr = (udp_header *) (data_to_checksum+12);

                    p_udp_hdr->uh_sum = 0;

                    int udp_checksum = ip_checksum(data_to_checksum, ip_data_len + 12);
                    DPRINTF("+++ C+ mode UDP checksum %04x\n",
                        udp_checksum);

                    p_udp_hdr->uh_sum = udp_checksum;
                }

                /* restore IP header */
                memcpy(eth_payload_data, saved_ip_header, hlen);
            }
        }

skip_offload:
        /* update tally counter */
        ++s->tally_counters.TxOk;

        DPRINTF("+++ C+ mode transmitting %d bytes packet\n", saved_size);

        rtl8139_transfer_frame(s, saved_buffer, saved_size, 1,
            (uint8_t *) dot1q_buffer);

        /* restore card space if there was no recursion and reset offset */
        if (!s->cplus_txbuffer)
        {
            s->cplus_txbuffer        = saved_buffer;
            s->cplus_txbuffer_len    = saved_buffer_len;
            s->cplus_txbuffer_offset = 0;
        }
        else
        {
            g_free(saved_buffer);
        }
    }
    else
    {
        DPRINTF("+++ C+ mode transmission continue to next descriptor\n");
    }

    return 1;
}