Re: [lwip-users] ERR_MEM when sending large data, LWIP stats show no error

[address@hidden]

Adrian Figueroa wrote:
> There is a list of chained DMA transmit descriptors, as they seem to be called. 
> There is always four of them, this is hard coded.

My version of the STM32F4xx HAL driver leaves this as a define to the 
application... But you're limited by DMA-capable RAM, of course.

> For sending, HAL_ETH_TransmitFrame(...) comes next. It checks, how many DMA TX 
> buffers were used for the packet to be sent. This seems stupid, as 
> low_level_output(...) knows how many of them it allocated. It could just pass 
> this number on to HAL_ETH_TransmitFrame(...) but it does not.

Well,

> There is a check, if a DMA descriptor is already in use or not before writing to it. So, 
> I assume that it does not happen that a descriptor is overwritten. However, a DMA 
> descriptor is marked as "in use" only in the HAL_ETH_TransmitFrame(...) 
> function. So, if low_level_output(...) makes use of more than four DMA descriptors, some 
> WILL be overwritten.

If every DMA buffer has room for 1514 bytes and you have netif->mtu set 
to 1500 (I hope), than low_level_output() will never fill more than one 
DMA descriptor!

> I am not sure where to look next.

Me neither, sorry. I can confirm the HAL driver of my F4 fails as well, 
but I haven't yet found the time or the motivation to debug it. Using it 
with this driver is a toy project for me, so there hasn't been a strong 
need for it...

Simon