What is the difference betwen DMA and MSGDMA? Which is better to use?

I assume you are talking about the DMA that shows up in Qsys. The DMA is capable of moving data from memory to memory and only buffers a single descriptor. What that means is that after the DMA moves the data then to get the DMA to move more data you have to send it a new descriptor. So there is a lot of stop and go that happens with non-SG DMAs. The mSGDMA which stands for modular scatter gather DMA is capable of buffering multiple descriptors and can handle ST-->MM, MM-->ST, and MM-->MM transfers. So the biggest difference is that a host doesn't have to wait for the mSGDMA to finish a transfer before telling it what to do next, it can buffer multiple descriptors and move onto the next transfer when the current transfer completes. In terms of hardware resources the mSGDMA is larger but it's also around 10 years newer so it supports many more features than the old DMA. Maybe if you explained what you want a DMA for I can recommend which one to use.

I have a big image (~50 mb) on Linux and I want to send it to SDRAM, process it in hardware and send it back to Linux.

Can your hardware handle the data come in as a stream? If so you might not need to temporarily copy it to SDRAM and just have the DMA pull it out and stream it to your hardware using Avalon-ST. Then you can have another DMA pull the data using Avalon-ST and write it back to memory.

No, unfortunately data can not come as a stream. It must be put to SDRAM

The old DMA is quite limited especially when it comes to bursting so if throughput is important you may want to go with the mSGDMA for that alone. If you leave out the descriptor fetching engine then they both have similar frontends where a host just writes to a few registers and kicks a go bit to get data movements started. The burst issue with the old DMA that I don't think ever got fixed was once you enabled bursting you were limited to transferring only a single burst of data. I bring this up because you posted this thread in the SoC section so I'm assuming you are DMA'ing data from the HPS memory space which as an AXI NoC which is turned for bursts and not single beat transactions. Where does the big image originate? In other words is it already in main memory or is it stored in a flash file system? Also are you really talking about HPS systems or is this perhaps a Nios II system?

DMA vs MSGDMA | Altera Community

14 Replies

Altera_Forum
Honored Contributor
9 years ago
Hi Tomek,
I'm doing very similar thing as You do. My recommendation is definitely the mSGDMA. The source DMA is configured as "Memory-mapped to Streaming", the Sink DMA is configured as "Streaming to Memory Mapped". In between You implement your processing functions. I use standard ST-Packets to mark Image start and stop. I use also reserved memory which I define in device tree file and use for DMA transfers. This is because DMA needs physical address which is hard to get in Linux without implementing kernel modules. If you have reserved memory You can mmap this and access it from HPS ARM processor via virtual adress and give the physical adress in DMA descriptors. Enable "burst mode" to have higher performance. I'm not using IRQ based response channel in DMA, I'm counting symbols which are coming into sink DMA and read them out using Memory mapped PIO component. I also have compiled/ported some of the NIOS code of the software layer for creating the DMA descriptors for MSGDMA and schedule sync transfers.
Altera_Forum
Honored Contributor
9 years ago
Important question... I based my design on the DE1_SOC_Linux_FB demonstration. Should I generate and compile new preloader when i added two msgdmas and SDRAM controller?
Altera_Forum
Honored Contributor
9 years ago
As a general rule of thumb any time the Qsys system changes I would get into a habit of regenerating the preloader. I don't think it'll make a difference here since the preloader doesn't know anything about FPGA DMAs but you don't want to face a hardware + software mismatch getting into a habit of using the same preloader across multiple generated Qsys systems.
Altera_Forum
Honored Contributor
9 years ago
Hi Alex,
Can you show me how you connected signals in qsys? How to reserve memory? And how to start dma from c code?

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DMA vs MSGDMA

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