IDT PCI Express 89HPES32NT24xG2, User Manual, Page: 730 / 744

IDT Usage Models
PES32NT24xG2 User Manual 26 - 20 January 30, 2013
Notes
Immediate Descriptor Usage
Goal
Describe the use of a DMA “immediate data transfer descriptor” in combination with NT doorbells as a
mechanism to notify a target device of a completion of a DMA data transfer to the device (i.e., via interrupt
generation).
Assumptions
– The DMA is configured to transfer data across the NTB, from a memory location in partition A to
a memory location in partition B.
– The DMA is located in the upstream port of partition A.
– The user wishes that upon completion of the DMA data transfer, an interrupt be generated by the
switch to notify both partitions.
– The DMA function in partition A is configured to generate an MSI upon completion of a DMA
transfer operation.
– The NT function in the upstream port of partition A is configured to map its configuration space to
BAR 0. This gives an agent in partition A, including the DMA, access to the memory-mapped NT
doorbell registers.
– Outbound doorbell bit[0] in partition A has been preconfigured to set inbound doorbell bit[0] in
partition B.
Description
The DMA descriptor list is set up by software. At the end of the list is an immediate data transfer
descriptor (see section Immediate Data Transfer Descriptor on page 15-13). The immediate data transfer
descriptor is configured such that:
– The address in the descriptor is that of the memory-mapped NT outbound doorbell register (e.g.,
OUTDBELLSET).
– The data in the descriptor sets outbound doorbell bit[0] in the OUTDBELLSET register.
The DMA operation is started by software (e.g., by setting the RUN bit in the DMACxSTS register).
Upon completion of the DMA transfer, the following occurs:
– The DMA issues an MSI to the root complex in partition A.
– Inbound doorbell bit[0] is set in the NT function of partition B. This triggers the NT function to
generate an interrupt to the root complex in partition B.
At this point, the root complex in both partitions have been notified of the DMA data transfer completion
and can proceed appropriately. It is possible to use a similar mechanism with the NT messaging registers,
instead of the NT doorbell registers. The NT messaging mechanism allows a message exchange in addi-
tion to the generation of the interrupt in partition B.
Failover
Active / Passive Failover Configuration
Goal
Describe an active/passive failover configuration, where the switch detects a failover condition (using a
failover signal trigger) and automatically reconfigures switch partitions to swap the active/passive ports.