Embedded Processors and Peripherals

[maxman]

Hi,

What is maximum packet size of DMA Scatter Gather with Packet Mode (as a part of opb_ipif)?

How many buffer descriptors can comprises one packet?

And what is maximum/minimum size of one buffer descriptor?

Do these parameters have any restrictions or not?

Thanks!

Max

[cuiziq]

Hi,

I am now working with the Scatter Gather DMA, and have the same question as yours.

As i know, it is ok to use only one buffer descriptor to form a packet, which you can find in MPMC datasheet for detailed information.

and I have no idea about the maximum size of a buffer descriptor.

Cui

[maxman]

Hi Cui,

Good for you that you can use MPMC. 

I have no such opportunity, bcause we use QNX OS and it's bsp attached to EDK 8.2 and only one way to connect high performance IP is through opb_ipif.  

Where I  found that max buffer discriptor size is 64byte. And if I want make 4kbyte packet I need create 64 BDs. 

And if all quantity of data to be transfered is 2Mbyte then I need create 32768(!!!) BDs.

Max

[joshual]

can't remember it clearly, but there should be some IP core such as opb_central_dma available for you, right?

[maxman]

Hi Joshua,

Yes, opb_central_dma IP exists, but it doesn't support SG packet mode.  So only one way to use SG is through ipif.
But 64 byte size of one buffer descriptor is something abnormal!

In this way it's easier to use Master interface burst transactions for transfering big volume of data. Besides master IP interface doesn't require any software initialization. 

Max