Linux - copie skb-> données à plusieurs descripteurs - Stack Overflow

i am studying 8139too.c driver. for the transmit, the driver calls skb_copy_and_csum_dev() to copy the entire socket buffer into a descriptor ring whose buffer is big enough for the entire socket buffer. if the descriptor ring buffer is smaller than skb->data, what is the correct way to break break skb->data up and copy skb->data into multiple descriptors? (assuming scatter/gather is not being used)

thank you very much.

In the function **rtl8139_start_xmit() ** of 8139 driver, it first check whether the length of skb->data is larger than TX_BUF_SIZE, which is the MAX_ETH_FRAME_SIZE. If it is larger than TX_BUF_SIZE, the driver drops the packet.

if (likely(len < TX_BUF_SIZE)) {
    if (len < ETH_ZLEN)
        memset(tp->tx_buf[entry], 0, ETH_ZLEN);
    skb_copy_and_csum_dev(skb, tp->tx_buf[entry]);
    dev_kfree_skb(skb);
} else {
    dev_kfree_skb(skb);
    tp->stats.tx_dropped++;
    return 0;
}

Generally, if the packet you try to send is larger than MAX_ETH_FRAME_SIZE, the IP layer of the protocol stack will fragment the packet, like you said "break XXX up". But when packet goes down to driver, it won't be broken up any more.

More infomation:
IP fragmentation on wikipedia

i am studying 8139too.c driver. for the transmit, the driver calls skb_copy_and_csum_dev() to copy the entire socket buffer into a descriptor ring whose buffer is big enough for the entire socket buffer. if the descriptor ring buffer is smaller than skb->data, what is the correct way to break break skb->data up and copy skb->data into multiple descriptors? (assuming scatter/gather is not being used)

thank you very much.

In the function **rtl8139_start_xmit() ** of 8139 driver, it first check whether the length of skb->data is larger than TX_BUF_SIZE, which is the MAX_ETH_FRAME_SIZE. If it is larger than TX_BUF_SIZE, the driver drops the packet.

if (likely(len < TX_BUF_SIZE)) {
    if (len < ETH_ZLEN)
        memset(tp->tx_buf[entry], 0, ETH_ZLEN);
    skb_copy_and_csum_dev(skb, tp->tx_buf[entry]);
    dev_kfree_skb(skb);
} else {
    dev_kfree_skb(skb);
    tp->stats.tx_dropped++;
    return 0;
}

Generally, if the packet you try to send is larger than MAX_ETH_FRAME_SIZE, the IP layer of the protocol stack will fragment the packet, like you said "break XXX up". But when packet goes down to driver, it won't be broken up any more.

More infomation:
IP fragmentation on wikipedia

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