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

[Po-Yu Chuang]

Dear MichaÅ,

2011/1/25 MichaÅ MirosÅaw <mirqus@xxxxxxxxx>:
> W dniu 24 stycznia 2011 09:26 uÅytkownik Po-Yu Chuang
> <ratbert.chuang@xxxxxxxxx> napisaÅ:
>> 2011/1/21 MichaÅ MirosÅaw <mirqus@xxxxxxxxx>:
>>> Did you test this? This looks like it will result in double free after
>>> packet RX, as you are giving the same page (referenced once) to two
>>> distinct RX descriptors, that may be assigned different packets.
>>
>> Yes, this is tested.
>>
>>> Since your not implementing any RX offloads, you might just allocate
>>> fresh skb's with alloc_skb() and store skb pointer in rxdes3. Since
>>
>> rxdes3 does not store virtual address of an skb.
>> It stores the address of the buffer allocated while open() and freed
>> only when stop().
>> The data in that buffer will be memcpy()ed to an skb allocated in
>> ftmac100_rx_packet().
>> No double free happens.
>
> Ah, I blindly assumed that you're just appending the buffers to the
> skb (using skb_fill_page_desc() and friends). Since you have to mark
> descriptors for the device anyway, it might be faster to allocate new
> skbs and map those as rx buffers (changing the descriptor's buffer
> address after every RX) instead of keeping static buffer and copying
> every time. (For small packets it wastes lot of memory, though - so
> the right choice depends on the expected workload.)

The reason I chose to use memcpy rx buffer to skb is that I didn't know
how to deal with multi-segment packets if I preallocated skb for each
rx descriptor. This function seems to be what I need.
Let me rework this. Thanks.

best regards,
Po-Yu Chuang
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