SGDMA debug

nwyfauq would have pointed the next descriptor field to the start of the descriptor chain. The SGDMA uses linked-list descriptors where one descriptor points to the next so to have the DMA wrap back around the end of the chain just needs to point to the start. When the SGDMA consumes descriptors it sets the owned by hardware bit to 0 so if you wrap the end of the descriptor list to the start and want the SGDMA to immediately start consuming those descriptor again you have to set the park control bit as well. This park bit prevents the SGDMA from setting the owned by hardware bit to 0 after it's consumed (i.e. the descriptor remains owned by the SGDMA after the SGDMA is done processing it).

The mSGDMA doesn't use linked-list so there is no direct way to have it recycle descriptors. It has park bits in the descriptor field but the behavior is different. The mSGDMA re-uses the same descriptor if the park bit is enabled and there are no other descriptors buffered. For example if you want to transfer 1MB and keep transfering 1MB from the same location(s) in memory as soon as the 1MB has been transferred the mSGDMA will reuse the same descriptor to transfer the data again. To stop it from re-using the same descriptor you simply write a new descriptor and it will switch over (after the current transfer completes).