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FreeRTOS_TCP - zero copy - ipBUFFER_PADDING

Posted by joehinkle on June 19, 2016

My question is not about ipconfigPACKETFILLERSIZE, it's about the other 8 bytes.

FreeRTOS_IP.h defines those additional 8 bytes as:

uint32_t pointer;	// word-aligned
uchar_8 filler[6];

Specifically the filler[6]; Why 6? Why not 2?

If filler was filler[2] - the buffer would still be 32bit aligned.

I'm using 50 buffers, so that's an extra 200 bytes wasted is there is not a good reason for them.

******* ALSO ******

The use of the define ...ipconfigPACKETFILLERSIZE

Besides being used in association with network buffers, it is also used in FreeRTOSIPPrivate.h

uint8t fillPacket[ ipconfigPACKETFILLER_SIZE ];

It's part of the socket definition.

Question ... Why have a socket element size be defined by a network buffer define?

I searched for fillPacket and it's never used .... I suggest removing it or changing the define that specifies its size.


FreeRTOS_TCP - zero copy - ipBUFFER_PADDING

Posted by rtel on June 19, 2016

I think this is to ensure members of the packet structure that appear later on become aligned in a way that is more efficient on less capable architectures. It is also required by some DMA peripherals on some hardware (being somewhat non-specific) which require more than 4-byte alignment.

There is a comment that may help in FreeRTOS_IP.h, but I think the comment is actually out of date, but does show that at some point the definition was only 2. I have marked the comment as invalid, so it will get updated too. As it appears now:

~~~ /* Space left at the beginning of a network buffer storage area to store a pointer back to the network buffer. Should be a multiple of 8 to ensure 8 byte alignment is maintained on architectures that require it.

In order to get a 32-bit alignment of network packets, an offset of 2 bytes would be desirable, as defined by ipconfigPACKETFILLERSIZE. So the malloc'd buffer will have the following contents: uint32t pointer; // word-aligned uchar8 filler[6]; << ETH-header >> // half-word-aligned uchar8 dest[6]; // start of pucEthernetBuffer uchar8 dest[6]; uchar16t type; << IP-header >> // word-aligned uint8t ucVersionHeaderLength; etc */ ~~~

I have also queried the your other question about fillPacket (which doesn't comply with the naming standard) so will get back to you on that, but as it is not used anywhere, I think it is in place to ensure the alignment of the buffer that follows it in the structure.

FreeRTOS_TCP - zero copy - ipBUFFER_PADDING

Posted by joehinkle on June 19, 2016

How I read the comment --- the 2 is actually referring to the two bytes associated with ipconfigPACKETFILLERSIZE ... Those 2 bytes will position the buffer such that the 32 bit entries in the IP header are 32bit aligned.

The filler is only there because THOSE 2 bytes are ipconfigPACKETFILLERSIZE + "pointer" + 4 extra bytes -- which are the ones I am questioning.

As for my second question and your reply about alignment within a structure ... then the premise is faulty since the alignment has nothing to do with network bufferers.

When ipconfigPACKETFILLERSIZE is set to 0 ... all the suggested alignment disappears.

I don't think we've yet answered my two questions --- sorry.

FreeRTOS_TCP - zero copy - ipBUFFER_PADDING

Posted by joehinkle on June 19, 2016

To be more specific ... I writing a zero copy driver for an NXP K64 (I wrote a K60 for a different stack several years ago).

The EMAC provides a config setting to ignore the first two bytes of the buffer --- there by aligning the IP address fields. So payload is actually 1518 but allocated buffer is 1520.

You might want to clarify on the web site when using the zero copy methed -- that the actual address that is placed inside the DMA descriptor (at least in the Kinetis K series) is:

Excerpt from web site on zero copy Receive ~~~ Copy the pointer to the newly allocated Ethernet frame to a temporary variable.

pucTemp = pxDescriptor->pucEthernetBuffer;

/* This example assumes that the DMADescriptor_t type has a member
called pucEthernetBuffer that points to the Ethernet buffer containing
the received data, and a member called xDataLength that holds the length
of the received data.  Update the newly allocated network buffer descriptor
to point to the Ethernet buffer that contains the received data. */
pxDescriptor->pucEthernetBuffer = pxDMARxDescriptor->pucEthernetBuffer;
pxDescriptor->xDataLength = pxDMARxDescriptor->xDataLength;

/* Update the Ethernet Rx DMA descriptor to point to the newly allocated
Ethernet buffer. */

pxDMARxDescriptor->puxEthernetBuffer = pucTemp;


Last line SHOULD be ....

pxDMARxDescriptor->puxEthernetBuffer = pucTemp - 2;

The minus 2 is to set the pointer where the EMAC will ignore the first 2 bytes.

So ... with the 2 extra bytes (ipconfigPACKET_FILLER_SIZE) and the 4 byte pointer that needs to be placed at the front ... that requires NO Extra filler to be involved... so in reality ... the filler in the header is just the size required for ipconfigPACKET_FILLER_SIZE.

FreeRTOS_TCP - zero copy - ipBUFFER_PADDING

Posted by edwards3 on June 19, 2016

What about the multiple of 8 requirement? Maybe that should be configurable too?

FreeRTOS_TCP - zero copy - ipBUFFER_PADDING

Posted by joehinkle on June 19, 2016

How do you get from CODE back to normal posting.

In the last of my links above .. code style got set and I can't get out!

FreeRTOS_TCP - zero copy - ipBUFFER_PADDING

Posted by joehinkle on June 20, 2016

I have in my old header file that the xmit and rcv buffers should be 8 aligned but I can no longer find where it states that in the manual.

Can you point to that requirement?


FreeRTOS_TCP - zero copy - ipBUFFER_PADDING

Posted by rtel on June 20, 2016

It's only a requirement for some DMA hardware, not all.

FreeRTOS_TCP - zero copy - ipBUFFER_PADDING

Posted by joehinkle on June 20, 2016

Referring to my question on the extra 4 bytes of filler in the network buffer ...

I found this post suggesting that the buffer MUST be 8 bytes aligned.


Looking at the code for pxUDPPayloadBuffertoNetworkBuffer, it is passed a pointer to a working buffer and then adjusts it for payload and ipBUFFERPADDING to get a pointer to the start of the buffer created by the NIC driver using BufferManagement1.c

It then performs a test on the pointer -- cast to dword --- to test if the last 2 bits are zero. If so, then the buffer is considered valid.

Here's the line of code:

if( ( ( ( uint32_t ) pucBuffer ) & ( sizeof( pucBuffer ) - 1 ) ) == 0 )

If at the time the original bug was found (see link above) -- IF the test was done on sizeof( pucBuffer ) and not on ( sizeof( pucBuffer ) - 1 ) ) as it is today .. then 8 byte alignment would have been required back then.

Can anyone check to see if the test has changed because to me -- the current code tests for 4 byte alignment --- which means the extra 4 byte of filler is not required.

Sorry if it sounds like I'm beating a dead horse, but I think the extra 4 byte filler is left over from previous code and is no longer required.

If no one replies, I will test when I get my driver and stack up and running.

FreeRTOS_TCP - zero copy - ipBUFFER_PADDING

Posted by joehinkle on June 20, 2016

Never mind -- I found my question/concern.

I've been searching the MAC spec for the word align. Never found anything related the align 8.

I'm working on the Driver, what to do with the MAC DMA descriptors, and it says the address must be divisable by 8. You get what you search for!!

So, I'm putting the 4 byte filler back in.

I saw in the FreeRtos_IP.h --- that it metioned that those bytes might be used as flags in the future.

Are they used as flags today or can the NIC Driver use them if needed?

FreeRTOS_TCP - zero copy - ipBUFFER_PADDING

Posted by heinbali01 on June 21, 2016

Hi Joe,

I'm sorry that I kept silent for so long, but it's summer holiday time here :-)

The reason to have the pointer located before the network buffer is the zero-copy option for UDP. A UDP payload buffer can be translated to a network buffer and vice versa with these complementary functions:

~~~~ NetworkBufferDescriptort *pxUDPPayloadBuffertoNetworkBuffer( void *pvBuffer ) uint8t *pcNetworkBuffertoUDPPayloadBuffer( NetworkBufferDescriptor_t *pxNetworkBuffer ) ~~~~

You are right that for some platforms, the extra 4 bytes in front of the network buffer are not essential. And so this declaration:

~~~~ #define ipBUFFERPADDING ( 8u + ipconfigPACKETFILLER_SIZE ) ~~~~

may sometimes be replaced with:

~~~~ #define ipBUFFERPADDING ( 4u + ipconfigPACKETFILLER_SIZE ) ~~~~

I don't mind allowing ipBUFFER_PADDING to get overridden by a new item called ipconfigBUFFER_PADDING from FreeRTOSIPConfig.h:


if( ipconfigBUFFER_PADDING != 0 )
#define ipBUFFER_PADDING    ( 8u + ipconfigPACKET_FILLER_SIZE )


The following code was introduced just to avoid an exception:

~~~~ /* Here a pointer was placed to the network descriptor, As a pointer is dereferenced, make sure it is well aligned */ if( ( ( ( uint32_t ) pucBuffer ) & ( sizeof( pucBuffer ) - 1 ) ) == 0 ) ~~~~

We were in doubt about either using the live test or use a configASSERT(). But as pxUDPPayloadBuffer_to_NetworkBuffer may also be called from user space, we decided to have the above test.

As Richard wrote, the "more than 4-byte" alignment is required by some DMA's. The extra 2 bytes ipconfigPACKET_FILLER_SIZE were indeed introduced to get a proper 32-byte alignment for all 32-bit fields in the IP- and protocol headers. Many EMAC's have an option that says: "skip the first 2 bytes" (as you mentioned).

FreeRTOS+TCP is still being developed further: it will soon have the possibility to use IPv6 headers, and also allow for multiple interfaces (EMAC's). We're still looking for more users who want to test the new (still experimental) version.

If you want to use IPv6, the extra 4 bytes will also get a purpose: store the IP-type in case of a IPv4 packet.


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