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Re: [lwip-devel] Re: [task #7896] Support zero-copy drivers

From: Micael
Subject: Re: [lwip-devel] Re: [task #7896] Support zero-copy drivers
Date: Sun, 23 May 2010 07:29:07 +0200
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On Saturday 22 May 2010 23.44.14 Mike Kleshov wrote:
> I just had another look. It gets more confusing. I have designs based
> on MCF5223x as well as MCF5225x. The manual for MCF5223x says that
> only the bits [15..5] of the length field of the transmit buffer
> descriptor are used by the DMA, while bits [4..0] are ignored. 

While I haven't used the 5223x in any design (yet) this sounds like a typo.
Normally the buffer ring start address pointer need to be aligned, but not  
the lengths field in the the actual transmit descriptors. Or maybe an errata 
never fixed.

The FEC is very similar in the devices I have been using so far (mainly 5282 
and 52259), but of course there may be minor changes that I have missed.
The FEC on the first coldfire with ethernet MAC, the 5272, was a different 
beast though.
On the 5282 I did DMA zero recevive but left out on zero copy transmit for 
another stack (interniche), since it was much trickier, as the buffer 
management of (that version of) interniche did not lend itself to this 

And while I did not do tx, I have no recollection of the size being rounded to 
any particular boundary and this was on the 5282, which I think is the second 
Coldfire V2 device with emedded FEC.

> As soon as I have a chance, I'll test this in hardware. I'll also
> submit a service request to Freescale.

Keep us updated! 

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