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Re: [lwip-users] Delay fixes dropped packets

From: address@hidden
Subject: Re: [lwip-users] Delay fixes dropped packets
Date: Sun, 09 Aug 2009 10:17:31 +0200
User-agent: Thunderbird (Macintosh/20090605)

From your description, it seems like memory beloging to a packet is being changed after being delivered to your netif->linkoutput function. Does your MAC support DMA? If so, make sure the pbufs are freed after receiving a transmit interrupt for the packet. (pbuf_free(p) then must *not* be called directly in the linkoutput function! Doing so leads to memory being freed before it is actually transmitted. Thus the next call to pbuf_alloc will get the same pbuf and may alter the data while the MAC sends it using its DMA engine.


JM wrote:
I'm using lwIP on a Stellaris micro, and saw lots of dropped packets. Upon further investigation, it turns out these packets were failing TCP checksum. I noticed that if I enabled debugging in the correct areas, and enough of it, the issue went away. Long story short, after lots of trial and error I discovered that if I disable debugging and add a delay of 300us at the very end, before the return, in ip_output_if(), it fixes the problem. If I decrease the delay, the problem starts appearing and becomes worse as I continue to decrease the delay. The driver for the onboard ethernet controller is from the Luminary driver library. I don't imagine that a very high percentage of users here use the same chip I'm using, but generally speaking, any idea what I can look for; what would cause this? I'm mainly recieving data. It's streaming audio data so once I initiate it, the stack only has to ACK.


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