Re: [lwip-users] Understanding pbuf's

[Bill Auerbach]

Martin,

 

>Thanks for your reply. I am currently also using a generic queue to store pointers to pbufs.

>But I still don't understand how this correlates to "pre-allocation".

 

>Maybe, I misunderstand the meaning but I do not allocate any pbufs. All I have is a queue

>of pbuf pointers. In the ISR, all I can do is then update the reference to the newly

>received frame's payload. 

 

If you do not allocate pbufs, what do the pbuf pointers point to that are in your queue?

 

I do the same – in the DMA RX (which interrupts when the packet data is ready to be DMAd) I DMA into the pbuf payload – but that pbuf was preallocated and in the array of pbuf pointers. That payload pointer is set by pbuf_alloc.  I don’t have to allocate in the RX ISR – I only need to kick off the DMA and update to the next index in the array.

 

Bill

 

 

From: address@hidden [mailto:address@hidden On Behalf Of Martin Osterloh
Sent: Tuesday, April 17, 2012 12:41 PM
To: Mailing list for lwIP users
Subject: Re: [lwip-users] Understanding pbuf's

 

Bill,

 

Thanks for your reply. I am currently also using a generic queue to store pointers to pbufs.

But I still don't understand how this correlates to "pre-allocation".

 

Maybe, I misunderstand the meaning but I do not allocate any pbufs. All I have is a queue of pbuf pointers. In the ISR, all I can do is then update the reference to the newly received frame's payload. 

 

What am I missing something?

 

Thanks again,

-Martin

On Tue, Apr 17, 2012 at 10:46 AM, Bill Auerbach <address@hidden> wrote:

>I was also wondering: To pre-allocate pbufs, do I have to that in pbuf_init() (which is currently empty) or does LwiP take care of that internally? 

 

I create an array of pbuf pointers in ethernetif_init.  The ISR adds a RX packet to a pbuf next  in line in the array using a circular queue using the array index in and out slot.  Then my low_level_input (which is polled in my case) removes the new RX pbuf(s) and uses pbuf_alloc to replace it/them.  I then return the new RX pbuf.  lwIP will free this packet when it’s done.  The size of the array depends on how much memory you want to tie up in pbufs as they will always be allocated for keeping the array full.

 

 

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