??: [PATCH] fs_enet: fix freescale FCC ethernet dp buffer alignment

?? yangyong at neusoft.com
Mon Jun 20 14:01:19 EST 2011

Hi David Laight, Thank you so much~~
1. Hardware based on FPGA, it dose not support buffer chaining.
2. How dose the dma map support buffer chaining function?
3. On the align issue, the hardware designer start to fix it, maybe it's a
better way.  

???: netdev-owner at vger.kernel.org [mailto:netdev-owner at vger.kernel.org] ??
David Laight
????: 2011?6?17? 18:16
???: yangyong at neusoft.com; Holger Brunck; linuxppc-dev at lists.ozlabs.org
??: netdev at vger.kernel.org; Clive Stubbings; Vitaly Bordug
??: RE: [PATCH] fs_enet: fix freescale FCC ethernet dp buffer alignment

> Hello,
> Motioned to the memory aligned, now there is such requirement:
> When the driver send an packet to hardware, the skb's address passed
> stack do a dma map into hardware, the skb's dma address must 
> be 64-byte aligned.

Does the hardware support buffer chaining?
In which case you only need to copy the data upto the first
64 byte boundary into another buffer.

Actually, given that you are likely to have to fixup every
fragment of the frame being transmitted, if might be worth
allocating a fixed transmnit buffer area and copying the
frames into it prior to sending.
Certainly you need to allow for transmits made up of a
significant number of small buffers linked together.

Really you should beat up the hardware designers!

Copying the data to even a 4 byte boundary is almost
always a misaligned copy. Typically this only applies
to the receive dma - when writing a 2 byte pad before
the frame data would be much better.


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