Re: [PATCH v4] net: add Faraday FTMAC100 10/100 Ethernet driver

From: Po-Yu Chuang
Date: Thu Feb 24 2011 - 02:28:21 EST

Hi David,

On Tue, Feb 1, 2011 at 12:35 PM, David Miller <davem@xxxxxxxxxxxxx> wrote:
> From: Po-Yu Chuang <ratbert.chuang@xxxxxxxxx>
> Date: Tue, 1 Feb 2011 11:56:16 +0800
>> If I simply allocate a page for each rx ring entry, I still need to allocate
>> an skb and copy at least packet header in first page to skb->data. Then
>> add the page of rest of payload to skb by skb_fill_page_desc().
> You should attach the pages, the use __pskb_pull_tail() to bring in the
> headers to the linear skb->data area.
> See drivers/net/niu.c:niu_process_rx_pkt().

I tried two ways to implement zero-copy.
One is to preallocate skb big enough for any rx packet and use the skb
as rx buffer.
The other is use page as rx buffer, use skb_fill_page_desc() to add a
data page to
skb and then pull only header to skb by __pskb_pull_tail() as you suggested.

Two implementations are slower than the original memcpy version.
(benchmarked with iperf)

I guess the problem is because a HW restriction that the rx buffer must be
64 bits aligned. Since I cannot make rx buffer starts at offset 2 bytes, the
IP header, TCP header and data are not 4 bytes aligned. The performance
drops drastically.

Therefore, I will submit later a v6 which is still using memcpy().

best regards,
Po-Yu Chuang
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