How to define the NULL descriptor in SGDMA

Question

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 · Answer

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 · Answer

your right leon

altera_forum · Answer

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 · Answer

likewise : That is correct and the alt_avalon_sgdma_construct_* macros takes care of this.   It always sets the "next" descriptor to NULL.

Forum Discussion