Internal and Bypass Modes - 4.0 English

QDMA Subsystem for PCI Express Product Guide (PG302)

Document ID
Release Date
4.0 English

Each queue in QDMA Subsystem for PCIe can be programmed in either of the two H2C Stream modes: internal and bypass. This is done by specifying the mode in the queue context. The H2C Stream Engine knows whether the descriptor being processed is for a queue in internal or bypass mode.

The following figures show the internal mode and bypass mode flows.

Figure 1. H2C Internal Mode Flow
Figure 2. H2C Bypass Mode Flow

For a queue in Internal mode, after the descriptor is fetched from the host it is fed straight to the H2C Stream Engine for processing. In this case, a packet of data cannot span over multiple descriptors. Thus for a queue in internal mode, each descriptor generates exactly one AXI4-Stream packet on the QDMA H2C AXI4-Stream output. If the packet is present in host memory in non-contiguous space, then it has to be defined by more than one descriptor and this requires that the queue be programmed in bypass mode.

In the Bypass mode, after the descriptors are fetched from the host they are sent straight to the user logic using the QDMA bypass output port. The QDMA does not parse these descriptors at all. The user logic can store these descriptors and then send the required information from these descriptors back to QDMA using the QDMA H2C Stream descriptor bypass-in interface. Using this information, the QDMA constructs descriptors which are then fed to the H2C Stream Engine for processing.

When fcrd_en is enabled in the software context, DMA will wait for the user application to provide credits, "Credit return" in the picture above. When fcrd_en is not set, the DMA uses a pointer update, fetches descriptors and sends the descriptor out. The user application should not send in credits. "Credit return" in the above picture does not apply in this case.

The following are the advantages of using the bypass mode:

  • The user logic can have a custom descriptor format. This is possible because QDMA Subsystem for PCIe does not parse descriptors for queues in bypass mode. The user logic parses these descriptors and provides the information required by the QDMA on the H2C Stream bypass-in interface.
  • Immediate data can be passed from the software to the user logic without DMA operation.
  • The user logic can do traffic management by sending the descriptors to the QDMA when it is ready to sink all the data. Descriptors can be cached in local RAM.
  • Perform address translation.

There are some requirements imposed on the user logic when using the bypass mode. Because the bypass mode allows a packet to span multiple descriptors, the user logic needs to indicate to QDMA which descriptor marks the Start-Of-Packet (SOP) and which marks the End-Of-Packet (EOP). At the QDMA H2C Stream bypass-in interface, among other pieces of information, the user logic needs to provide: Address, Length, SOP, and EOP. It is required that once the user logic feeds SOP descriptor information into QDMA, it must eventually feed EOP descriptor information also. Descriptors for these multi-descriptor packets must be fed in sequentially. Other descriptors not belonging to the packet must not be interleaved within the multi-descriptor packet. The user logic must accumulate the descriptors up to the EOP descriptor, before feeding them back to QDMA. Not doing so can result in a hang. The QDMA will generate a TLAST at the QDMA H2C AXI Stream data output once it issues the the last beat for the EOP descriptor. This is guaranteed because the user is required to submit the descriptors for a given packet sequentially.

The H2C stream interface is shared by all the queues, and has the potential for a head of line blocking issue if the user logic does not reserve the space to sink the packet. Quality of service can be severely affected if the packet sizes are large. The Stream engine is designed to saturate PCIe for packet sizes as low as 128B, so Xilinx recommends that you restrict the packet size to be host page size or maximum transfer unit as required by the user application.

A performance control provided in the H2C Stream Engine is the ability to stall requests from being issued to the PCIe RQ/RC if a certain amount of data is outstanding on the PCIe side as seen by the H2C Stream Engine. To use this feature, the SW must program a threshold value in the H2C_REQ_THROT (0xE24) register. After the H2C Stream Engine has more data outstanding to be delivered to the user logic than this threshold, it stops sending further read requests to the PCIe RQ/RC. This feature is disabled by default and can be enabled with the H2C_REQ_THROT (0xE24) register. This feature helps improve the C2H Stream performance, because the H2C Stream Engine can make requests at a much faster rate than the C2H Stream Engine. This can potentially use up the PCIe side resources for H2C traffic which results in C2H traffic suffering. The H2C_REQ_THROT (0xE24) register also allows the SW to separately enable and program the threshold of the maximum number of read requests that can be outstanding in the H2C Stream engine. Thus, this register can be used to individually enable and program the thresholds for the outstanding requests and data in the H2C Stream engine.