The method to alloc DMA ring buffer

[Tony He]

Hi all,

This is my first question in the kernel newbies mailing list. Forgive
me if there is incorrect behavior.

I'm studying the NIC driver with the book <<Understanding the linux
network internals>>. This book uses 3com 3c59x NIC(3c59x.c) as an
example. The driver is very old and few people discuss it, but it's a
good place to start because it's simpler than many other drivers with
advanced offload features.

In the interrupt handler boomerang_rx(), I see it pre-allocate one new
skb and dma map before calling netif_rx.
boomerang_rx()
{
......
/* Pre-allocate the replacement skb.  If it or its
    * mapping fails then recycle the buffer thats already
    * in place
    */
    newskb = netdev_alloc_skb_ip_align(dev, PKT_BUF_SZ);
    if (!newskb) {
    dev->stats.rx_dropped++;
    goto clear_complete;
    }
    newdma = dma_map_single(vp->gendev, newskb->data,
    PKT_BUF_SZ, DMA_FROM_DEVICE);
    if (dma_mapping_error(vp->gendev, newdma)) {
    dev->stats.rx_dropped++;
    consume_skb(newskb);
    goto clear_complete;
    }

    /* Pass up the skbuff already on the Rx ring. */
    skb = vp->rx_skbuff[entry];
    vp->rx_skbuff[entry] = newskb;
    vp->rx_ring[entry].addr = cpu_to_le32(newdma);
    skb_put(skb, pkt_len);
    dma_unmap_single(vp->gendev, dma, PKT_BUF_SZ, DMA_FROM_DEVICE);
    vp->rx_nocopy++;
    ......
    netif_rx(skb);
    .......
}

However, I see intel e1000 driver optimizes this. Intel doesn't
pre-allocate one at a time.
e1000_clean_rx_irq()
{
......
    /* return some buffers to hardware, one at a time is too slow */
    if (unlikely(cleaned_count >= E1000_RX_BUFFER_WRITE)) {
    adapter->alloc_rx_buf(adapter, rx_ring, cleaned_count);
    cleaned_count = 0;
......
}

I just want to know why this is faster. For all scenarios or some
scenarios? Can someone analyse it rigorously?
My guess is this method reduces the delay when many packets come,
right? Thanks in advance!

Tony