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

[address@hidden]

Mike Kleshov wrote:
> I am wondering how zero-copy transmission could be implemented with Coldfire
> v2 Ethernet controller. The limitations of its DMA is that transmit buffers
> must be 32-bit aligned and each buffer's size must be a multiple of 16 bytes.
> The latter limitation is strange since in the very same microcontroller manual
> they suggest to allocate one buffer for IP header, another for TCP header and
> so on, but those headers' sizes are not multiples of 16 bytes...
>    
That looks strange, indeed. I've had a quick look at the the PDF 
provided at www.freescale.com and I couldn't find a note about the size 
having to be a multiple of 16 bytes. Instead, I found that the 
TX-buffers have to be 4-byte-aligned while the RX-buffers have to be 
16-byte aligned. This brings us to the fact that there is at least a 
different alignment requirement for TX- or RX-buffers than for the rest 
of the stack.
> Anyway, with those limitations I cannot see how zero-copy transmission could
> work without imposing severe restrictions on how the application supplies data
> to lwip. Basically, it would not be practical.
> Am I right in this?
>    
Zero-copy from the application to the wire would indeed not be practical 
unless you would write the application to generate data into a given 
pbuf queue and in your port make sure the pbuf queue is allocated in a 
way that fits your MAC. However, zero-copy from lwIP's internal TCP 
buffers to the wire could still be worth to try.

Simon