addressing on-chip memory(cyclone IV) from PC via PCIe

Hi Dave,

Sorry I have no practical experience with ready-made DMA engines. The one I’m using is self-made. Recently I got aware of northwest logic (http://nwlogic.com/)’s dma backend core (http://nwlogic.com/docs/dma_back-end_core.pdf) but I intend to stay with my current solution.

Writing DMA engines is not *that* hard, at least not for DMA receive (device→main memory) engines. There are only occasional descriptor table reads which can happen with single non-interleaved DMA reads and an easy timeout logic, but all other accesses are uncomplicated DMA writes. Only the other direction, DMA transmit (main memory→device) requires significant thinking about tags, completion reordering and request-selective handling of the mysterious completion timeout mechanism. On the driver side, there is not much difference between the two data transfer directions.

The advantage of writing your own DMA engine is that you can make it fit your application and driver operation best. For example, one design might supply the data in a FIFO while another supplies it in a memory-like structure, and there are quite some different needs for the descriptor structure and the data blocks in terms of size and regarding stream/block orientation. And, by designing your own core, you learn a lot about PCI and PCIe.

The downside: If you can’t wrap your head around the transaction ordering stuff in the FPGA or in the driver or in between (interrupts, transaction table locking and updates), you will have a hard time debugging all the issues out of your design. Some mechanisms require early correct integration, and if not done so, you might have to rewrite your design completely once you understood the issue. Finally, I’d recommend to make the DMA engine design together with the driver development by one person – there is so much that looks logical in a DMA engine design which cannot be supported efficiently by the driver, and vice versa, that there should be a short iteration loop between those two tasks to implement.

– Matthias