Question about Network buffers allocation described in "Porting FreeRTOS+TCP to a Different Microcontroller" guide

Libraries
1

Hi,

in order to include FreeRTOS+TCP in my project, I followed the guide “Porting FreeRTOS+TCP to a Different Microcontroller” as no complete examples were available for my platform.
I’m using FreeRTOS 10.2.1.

The example reported on the porting guide contains the following example for vNetworkInterfaceAllocateRAMToBuffers(…) function:

for( x = 0; x < ipconfigNUM_NETWORK_BUFFERS; x++ )
{
…
/* The following line is also required, but will not be required in future versions. */
( ( uint32_t * ) &ucBuffers[ x ][ 0 ] ) = ( uint32_t ) &( pxNetworkBuffers[ x ] );
}

The Network Buffer Descriptors contain a reference to the pucEthernetBuffer while this operation generate a cross-reference from the pucEthernetBuffer back to its Network Buffer Descriptor.
I verified that this is leveraged by the UDP Zero Copy flow.
It looks a temporary shortcut to get the descriptor reference from the data buffer itself. Can you clarify what “future versions” means ?
If it won’t be removed in the future, is it possible to make it more explicit ?

Thanks.

2

You understood it right: there is a mutual reference between Network buffer descriptor and the actual data buffer.

The data buffer starts 10 bytes before pucEthernetBuffer:

4-byte pointer to descriptor
6 bytes spare
--------------------
14-byte Ethernet header pointed to by **pucEthernetBuffer**
20-byte IP-header
N-byte protocol (TCP/UDP)

There are 2 macro’s defining the 10-byte header:

#define ipconfigPACKET_FILLER_SIZE   ( 2 )
#define ipBUFFER_PADDING             ( 8 + ipconfigPACKET_FILLER_SIZE )

The 2 bytes (of ipconfigPACKET_FILLER_SIZE) are needed to get a correct alignment of the IP-header. The 8 bytes serve to get a good alignment for DMA.

All EMACs that I have seen until now, understand the 2-byte offset.

The routine that you point at is used by BufferAllocation_1.c only. BufferAllocation_1.2 uses pvPortMalloc() to allocate each buffer.

/* The following line is also required, but will not be required in future versions. */

This comment is outdated and has disappeared from all drivers. Drivers are still responsible to set the pointer. It is not recommended to change the 2 macros here above.

Here you find the latest Network Interfaces for FreeRTOS+TCP.

If you are writing a new Network Interface, you can always ask questions here in this post.

Good examples of network interfaces are e.g. STM32Fxx and DriverSAM. Note that they use a PHY-driver (Common/phyHandling.c) that recognises most 100 Mbps Ethernet PHY’s.