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


From: Amena El Homsi
Subject: Re: [lwip-users] Packet queuing to an ARP table entry
Date: Wed, 15 Nov 2017 15:05:39 +0200



On Wed, Nov 15, 2017 at 1:22 PM, Simon Goldschmidt <address@hidden> wrote:
Amena El Homsi wrote:
> What I know is PBUF_ROM doesn't mean the actual ROM (stable for very long) but it
> means that the data is immutable and it is located in memory which is not managed
> by the pbuf system. Am I wrong?

Yes, you are wrong. ROM means stable for very long, not const :-)
Actually I had this understanding from this link http://www.nongnu.org/lwip/2_0_x/group__pbuf.html 
 and from the Dunkels book " Design and Implementation of the lwIP TCP/IP Stack".
Thanks for your clarifying, but is there a serious problem in using PBUF_ROM as I did? 

ROM really means stable for long. E.g. when sending via tcp_write with the nocopy flag,
ROM means the data is stable until all retransmissions are done. This can be up to some
minutes or hours, depending on the connection. TCP has the sent callback to tell the
application when it is done, but ARP does not have this.

As such, when queueing for ARP, we can prevent copying, but you'd never know when the
frame has actually been sent.

In this respect, I would be OK with the chang in ARP output, but that won't help you much
as your usage of PBUF_ROM seems wrong.

> so I have to add some instructions inside the LwIP code, right?

You can always change lwIP code, but in my opinion, it's always better to work together
to get things "mainlined". We'd have to change the queue handling in etharp_query()
to change this.

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..

Simon

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--

Amena El-Homsi
Computer & Communication Engineer
Dipl. Eng,  M.S.

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