use descriptor chain in SGDMA

IIRC the alt_avalon_sgdma_construct_stream_to_mem_desc function ensures that the data cache is flushed correctly.

Your settings look fine. The different memory regions (text, data, heap, stack, bss) are defined in the BSP settings but I was just asking this question in case you were using a table defined as a global variable, or a buffer allocated with malloc(). As you are using a pointers with directly the correct address as defined in system.h it should be fine.

Could you have a look at your BSP settings nevertheless and check that you don't have any memory region configured to your on-chip memory right now? maybe your software is overwriting the descriptors.