4.7. DMA Descriptor Controller Registers
The DMA Descriptor Controller manages Read and Write DMA operations. The DMA Descriptor Controller is available for use with Endpoint variations. The Descriptor Controller supports up to 128 descriptors each for Read and Write Data Movers. Read and Write are from the perspective of the FPGA. A read is from PCIe* address space to the FPGA Avalon® -MM address space. A write is to PCIe* address space from the FPGA Avalon® -MM space.
You program the Descriptor Controller internal registers with the location and size of the descriptor table residing in the PCIe* address space. The DMA Descriptor Controller instructs the Read Data Mover to copy the table to its own internal FIFO. When the DMA Descriptor Controller is instantiated as a separate component, it drives table entries on the RdDmaRxData_i[159:0] and WrDmaRxData_i[159:0] buses. When the DMA Descriptor Controller is embedded inside the Avalon-MM DMA bridge, it drives this information on internal buses. .
The Read Data Mover transfers data from the PCIe address space to Avalon-MM address space. It issues memory read TLPs on the PCIe link. It writes the data returned to a location in the Avalon® -MM address space. The source address is the address for the data in the PCIe address space. The destination address is in the Avalon-MM address space.
The Write Data Mover reads data from the Avalon-MM address space and writes to the PCIe address space. It issues memory write TLPs on the PCIe link. The source address is in the Avalon-MM address space. The destination address is in the PCIe address space.
The DMA Descriptor Controller records the completion status for read and write descriptors in separate status tables. Each table has 128 consecutive DWORD entries that correspond to the 128 descriptors. The actual descriptors are stored immediately after the status entries at offset 0x200 from the values programmed into the RC Read Descriptor Base and RC Write Descriptor Base registers. The status and descriptor table must be located on a 32-byte boundary in Root Complex memory.
The Descriptor Controller writes a 1 to the done bit of the status DWORD to indicate successful completion. The Descriptor Controller also sends an MSI interrupt for the final descriptor. After receiving this MSI, host software can poll the done bit to determine status. The status table precedes the descriptor table in memory. The Descriptor Controller does not write the done bit nor send an MSI as each descriptor completes. It only writes the done bit or sends an MSI for the descriptor whose ID is stored in the RD_DMA_LAST PTR or WR_DMA_LAST_PTR registers. The Descriptor Controller supports out-of-order completions. Consequently, it is possible for the done bit to be set before all descriptors have completed.
- Program the RD_DMA_LAST_PTR = 63.
- Program the RD_DMA_LAST_PTR = 127.
- Poll the status DWORD for read descriptor 63.
- Poll the status DWORD for read descriptor 127.
Many commercial system Root Ports return out-of-order Read Completions based on optimized accesses to host memory channels. Consequently, the done status stored for descriptor <n> does not necessarily mean that descriptors <n-1> and <n -2> have also completed. You must request the completion status for every descriptor by writing the descriptor ID for every descriptor to RD_DMA_LAST_PTR or WR_DMA_LAST_PTR.