Re: [lwip-users] Packet queuing to an ARP table entry

[Amena El Homsi]

On Wed, Nov 15, 2017 at 3:15 PM, Simon Goldschmidt <address@hidden> wrote:

Amena El Homsi wrote:
> Actually I had this understanding from this link http://www.nongnu.org/lwip/2_0_x/group__pbuf.html

That page says "PBUF_ROM: pbuf data is stored in ROM". Could you tell us from which line on that page you have your understanding from?

 "PBUF_ROM: no buffer memory is allocated for the pbuf, even for protocol headers. Additional headers must be prepended by allocating another pbuf and chain in to the front of the ROM pbuf. It is assumed that the memory used is really similar to ROM in that it is immutable and will not be changed. Memory which is dynamic should generally not be attached to PBUF_ROM pbufs. Use PBUF_REF instead."

"Similar to ROM" doesn't  mean it is ROM, they are similar in that it is immutable.

Another link http://www.ecoscentric.com/ecospro/doc/html/ref/lwip-basics-pbufs.html says:

"Note that data pointed at by a PBUF_ROM pbuf does not literally have to point at read-only memory. All it means is that the data must not change, even if control has returned to the application."

>  and from the Dunkels book " Design and Implementation of the lwIP TCP/IP Stack".

That one is totally outdated. The stack at that time didn't handle zero copy RX or TX correctly.

> Thanks for your clarifying, but is there a serious problem in using PBUF_ROM as I did? 

I tried to explain that before. Your ROM is not ROM. Data must be kept unchanged until it is actually sent. How do you ensure this??

Our System will take care of this, Data will be kept unchanged until it is actually sent. Headers are copied from the Pbuf_RAM to the frame memory and the whole frame will stay in frame memory until it is actually sent.

>> Regarding your original problem: why don't you put (some of) the memp pools into the
>> frame memory?
> I don't think this will be easy because we have our own memory manager system and the frame memory will be used by the HW MAC and SW MAC, however pools are managed by PBUF manager.
> It seems that I am facing a serious problem now, your suggestions are really appreciated..

As I don't know your exact hardware/software setup, how could I suggest anything more here?

Simon

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Amena El-Homsi

Computer & Communication Engineer

Dipl. Eng,  M.S.