Re: [lwip-users] Ethernet Driver development guidelines (Blackfin BF536 BF537 integrated EMAC)

[Stephane Lesage]

Bill Auerbach a écrit :

> This was the point I missed, at first, in that discussion.  You don’t 
> need to worry about when the pbuf is freed.  Simply replace the filled 
> one with an empty one (allocate a new one) and when lwIP is done with 
> the original one, it will free it, placing it in the pool for a future 
> allocation in your packet receiver.

Yes that's what I wanted to do.
There's no problem with PBUF_POOL buffers.

The problem appears only when we want to use PBUF_REF types.

> I don't know Blackfin, but if it has buffer descriptors like the PowerPC,
> then this thread answered the question for me for no-copy buffers that the
> Ethernet controller can DMA into.  Works great for me on a PPC.
>
> http://lists.gnu.org/archive/html/lwip-users/2007-12/msg00071.html

Thanks for the link.

My descriptors list does not work like this.
I have to interleave 2 types of descriptors:
- actual Ethernet Frame data
- detailed frame status information on completion/error + Frame Size + 
optional checksum for IP Header and payload (very cool feature, but I'm 
not sure I can use this with LwIP)

My documentation does not give a lot of details about the way the EMAC 
works. It says you must reserve memory for max size and set COUNT=0 
(max) in the DMA descriptors...

Actually after 'deep doc inspection' and experimentation, I discovered
it works normally, the EMAC just sends a FINISH command to the DMA 
controller to jump to next descriptor at the end of a frame. Using small 
buffers is no problem, the DMA can use as many descriptors as necessary 
for a single frame. The last one does not need to have COUNT=0, the EMAC 
will tell the DMA to jump to the next anyway for status and next frame.

So I can use PBUF_POOL exactly the way you do it.

The only problem is that it will consume a whole pbuf for status 
information (8 bytes). Anyway this should be better than 1516 bytes buffers.

I'm now OK with packet buffer memory.

I know you don't use an OS.
But how are you handling packet Output ? Are you waiting actual 
transmission in netif->link_output() ?

Anyone to help me with the blocking/threads/IRQ issues ?

Thanks in Advance.

--
Stephane Lesage
ATEIS International