GTS AXI Multichannel DMA IP for PCI Express* User Guide
A.1.1.3.3. MSI-X/Writeback
The MSI-X and Writeback block updates the host with the current processed queue’s head pointer and interrupt. Apart from a global MSI-X Enable and Writeback Enable for each queue, there is a provision to selectively enable or disable the MSI-X and Writeback on a per-descriptor basis. This feature can be used by applications to throttle the MSI-X/Writeback.
The table below shows the relationship between the global and per-descriptor MSI-X/Writeback Enables.
Global Enable | Per-Descriptor Enable | MSI-X/Writeback Generation |
---|---|---|
1 | 1 | On |
1 | 0 | Off |
0 | 1 | Off |
0 | 0 | Off |
If enabled, a Writeback is sent to the host to update the status (completed descriptor ID) stored in the Q_CONSUMED_HEAD_ADDR location. In addition, for D2H streaming DMA, an additional MWr TLP is issued to the D2H descriptor itself when the IP’s AXI-Stream Subordinate interface has received the first/last data from the user logic. It updates the D2H descriptor packet information fields such as start of a file/packet (SOF), end of a file/packet (EOF), and received payload count (RX_PYLD_CNT).