Re: [lwip-users] STM32 RBUS (Receive Buffer Unavailable) bit set after debugger break

[Zachary Smith]

I am using STM32F217 and I'm sure the Ethernet MAC is the same.
You are likely correct in thinking the problem is the RBUS interrupt 
flag.  When you hit the breakpoint the Ethernet DMA probably fills up 
all the RX descriptor buffers and asserts that interrupt flag and then 
is not able to receive anything else until you take care of the 
interrupt and then resume reception.

You have to enable the RBU interrupt and clear it if it happens.  Also, 
if I remember correctly, if this interrupt happens it can happen that 
you don't get a normal Receive interrupt like you normally would so you 
are not triggered to read from the buffers.

enable the RBU interrupt:
ETH_DMAITConfig(ETH_DMA_IT_AIS | ETH_DMA_IT_RBU, ENABLE);

ISR: clear the flag:
if(ETH_GetDMAITStatus(ETH_DMA_IT_RBU) != RESET)
    ETH_DMAClearITPendingBit(ETH_DMA_IT_RBU | ETH_DMA_IT_AIS);

Then reception will be in the suspended state until you resume reception 
by writing to the DMARPDR (receive poll demand) register.  The manual 
says the DMA will start checking for available descriptors again when 
you write to this register with any value:
ETH->DMARPDR = 0;

In my case, if the RBU interrupt happens I clear the flag and then send 
a msg to my TCP task to do an Ethernet input.  My Ethernet driver always 
does the resume reception command after reading from a receive 
descriptor.  That way the rx descriptors are read and then reception 
resumed.

Hope that helps.



On 9/12/2012 4:35 PM, Mark Lakata wrote:
> I've got my STM32F407 port for FreeRTOS 7.2 and lwIP1.40 running now, 
> with ping and HTTP working well (I started with the ST's demo which 
> was based on FreeRTOS6.1.0 and lwIP1.3.2, and merged it into the 
> latest code).
>
> However, if I stop the program momentarily with the debugger, or hit a 
> breakpoint, the lwIP stack slows down a lot.  The ping time goes from 
> <1ms to 300-500 ms typically, sometimes > 2000 ms. It is easy to 
> reproduce, including the original ST demo code:
>   1. run demo code
>   2. count to 5
>   3. do a ping from another computer
>   4. hit 'break' in the debugger from IAR EWARM
>   5. hit 'run' right away
>   6. do another ping
>
>
> I think it is related to this problem, discusses in an earlier thread: 
> http://lists.nongnu.org/archive/html/lwip-users/2012-05/msg00111.html
>
> It seems that once you stop the CPU, the DMA engine gets tied up in 
> knots and doesn't recover, until a reset. It is related to the RBUS 
> interrupt flag (Receive Buffer unavailable) in the ETH_DMASR status 
> register. I see this bit go high forever once the problem occurs. It 
> does not self clear with the existing code.
>
> I'm sure this can be fixed with a software workaround, but I haven't 
> found it or figured it out. I'm just starting out on lwIP and the 
> STM32F4x7 (first week).
>
> Any ideas?
>
> thanks,
> Mark Lakata
>
>
>
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