How to define the NULL descriptor in SGDMA

[Altera_Forum]

I use the SGDMA in TSE MAC. 

As its document said, the last descriptor in the descriptor chain must be a NULL descriptor. But I wonder how to define the NULL descriptor. I define it in following way: /***************************************************/  alt_sgdma_descriptor* tx_descriptor = (alt_sgdma_descriptor*)DESCRIPTOR_MEN_BASE;   alt_sgdma_descriptor* tx_descriptor_ = (alt_sgdma_descriptor*)(DESCRIPTOR_MEN_BASE + ALTERA_AVALON_SGDMA_DESCRIPTOR_SIZE);  alt_avalon_sgdma_construct_mem_to_stream_desc(tx_descriptor, tx_descriptor_, read_buffer, 256, 0, 1, 1, 0);   alt_avalon_sgdma_construct_mem_to_stream_desc(tx_descriptor_, (alt_sgdma_descriptor*)NULL, read_buffer, 256, 0, 1, 1, 0); /****************************************************/ where tx_descriptor is the first descriptor, and tx_descriptor_ is the second(also the last descriptor) descriptor,. I define the NULL descriptor use (alt_sgdma_descriptor*)NULL Is that right?

[Altera_Forum]

I also had this question, and I am not sure what is the correct answer. 

  I *think* that a null descriptor is the (last) descriptor with the OWNED_BY_HARDWARE bit set. It's addresses and length are "don't cares".  I think the SG DMA engine will stop upon reaching this descriptor.  The documentation is not quite clear on this.  Regards,  Leon.

[Altera_Forum]

your right leon

[Altera_Forum]

Correction: 

- the null descriptor is the last descriptor with the OWNED_BY_HARDWARE bit _cleared_. (not set as I typed earlier).  Addition: - the null descriptor addresses and length are "don't cares", as the SG DMA engine will not perform a DMA transfer for this last descriptor.  Can someone comment if this is correct or not??

[Altera_Forum]

likewise : That is correct and the alt_avalon_sgdma_construct_* macros takes care of this.  

It always sets the "next" descriptor to NULL.