Re: [lwip-users] byte alignment

[address@hidden]

Tyrel Newton wrote:
> > However, if the processor does the final copy (without a DMA 
> > enginge), than it's a bad thing if the data is not aligned. But you 
> > should be able to include a DMA engine in your FPGA, so...
> Xilinx provides a gigabit mac with a built-in DMA (at an additional 
> cost of course), so I definitely have options. I could also definitely 
> write my own DMA, or for that matter, my own non-DMA Ethernet mac that 
> simply accepts and discards a two-byte pad. But all of that is outside 
> the scope (and priority) of my current effort. At the moment, I'm not 
> terribly concerned about Ethernet performance as long as it works and 
> isn't horrendously slow. My investigations into this issue came from 
> re-writing the horrible lwIP driver provided by Xilinx. By re-writing 
> the code in a reasonably intelligent manner, I managed to increase the 
> throughput 4x along with making the system more stable. C-code is 
> easier to change than VHDL . . .
I meant just include a standard RAM-to-RAM DMA controller (at least 
Altera provides something like that for free) and let it copy from your 
real RAM to the MAC's transmit-buffer RAM. For me, that was only a 
matter of 1 hour adding the DMA controller and recompiling the FPGA, the 
code to use it is quite simple and a lot faster than a processor-memcpy.
> > Single PBUF_RAM pbufs or chained pbufs?
> Single PBUF_RAM pbufs. Looking through the TCP code, if the data is 
> being copied into the stack (i.e. via NETCONN_COPY), I'm not even sure 
> how chained pbufs would be created.
Not for the netconn-layer, no. .
> I wouldn't say the system I'm using (at the moment at least) is 
> zero-copy because once I receive the frame from lwIP, I pbuf_ref it, 
> queue it up for transmit, and then eventually copy its payload to the 
> mac's transmit buffer, after which I do a pbuf_free. Although I guess 
> this is still zero-copy from the stack's frame of reference . . . its 
> probably worth distinguishing somewhere between zero-copy macs and 
> zero-copy drivers.
It is zero-copy but without delayed-transmit, and therefore, it's a bit 
out of the scope of that task. However, it would be non-zero-copy and in 
the scope of that task if you were to first copy the data to an aligned 
buffer and then copy that buffer to the MAC.

Simon