Move data from NIOS to hardware and then back to NIOS

That's correct, to do this with the SGDMA on the ACDS you would need to control a pair of SGDMAs for doing MM-->ST and ST-->MM. With the mSGDMA if you wedged your block between the read and write masters then your control would be just triggering a normal DMA transfer (assuming for every input there is one output from your block).

The pipeline bridge isn't necessary, it's just included to add some additional pipelining to the design. If you are running the included software from the mSGDMA design make sure you change the settings near the top of the main.c file to represent your own system memory address base and span. I did a minor update to that software to fix a bug which would cause the source and destination buffers to overlap so make sure you have the latest main.c file.

You probably don't need to modify many of the settings. The length width parameter just dictates the maximum number of bytes you can transfer in a single transfer. For example if you chose 20 bits that means you can transfer slightly less than 1MB of data in a single descriptor. The only reason why it's a parameter and not hardcoded to be 32-bits is that when a timing critical path shows up in one of the masters it can typically be solved by just reducing that length register width to something more sensible (being able to transfer ~4GB in one shot doesn't make sense in SOPC Builder which only has a 4GB space per master anyway).

Since you are dealing with MM to ST the symbol size is 8-bits. So if your component has an 8-bit input/output you can still setup the mSGDMA for a wider data path. This will help increase your memory throughput since multiple symbols can be fetched every clock cycle. I would use the C code as a guideline, most of it has nothing to do with the mSGDMA so it may end up leading to confusion. If you want to see a simplier application check out this design example which is configured for MM --> ST which performs frame buffering to an LCD. The only difference is you would use MM --> MM and setup the descriptors slightly differently.

http://www.alterawiki.com/wiki/modular_sgdma