QDMA Performance Optimization - 3.4 English

AMD provides multiple example designs for you to experiment. All example designs can be downloaded from GitHub. Performance example design can be selected from the CED example design.

Following are the QDMA register settings recommended by AMD for better performance. Performance numbers vary depending on systems and OS used.

• QDMA_C2H_INT_TIMER_TICK (0xB0C) set to 50. Corresponding to 100 ns (1 tick = 4 ns for 250 MHz user clock)
• C2H trigger mode set to User + Timer, with counter set to 64 and timer to match round trip latency. Global register for timer should have a value of 30 for 3 μs
• TX/RX API burst size = 64, ring depth = 2048. The driver should update TX/RX PIDX in batches of 64
• PCIe MPS = 256 bytes, MRRS = 4K bytes, 10-bit Tag Enabled, Relaxed Ordering Enabled
• QDMA_C2H_WRB_COAL_CFG (0xB50) bits[31:26] set to 63.This is maximum buffer size for WRB.
• The driver updates the completion CIDX in batches of 64 to reduce number of MMIO writes before updating the C2H PIDX
• The driver should update the H2C PIDX in batches of 64, and also update for the last descriptor of the scatter gather list.
• C2H context:
  • bypass = 0 (Internal mode)
  • frcd_en = 1
  • qen = 1
  • wbk_en = 1
  • irq_en = irq_arm = int_aggr = 0
• C2H prefetch context:
  • pfch = 1
  • bypass = 0
  • valid = 1
• C2H CMPT context:
  • en_stat_desc = 1
  • en_int = 0 (Poll_mode)
  • int_aggr = 0 (Poll mode)
  • trig_mode = 4
  • counter_idx = corresponding to 64
  • timer_idx = corresponding to 3 μs
  • valid = 1
• H2C context:
  • bypass = 0 (Internal mode)
  • frcd_en = 0
  • fetch_max = 0
  • qen = 1
  • wbk_en = 1
  • wbi_chk = 1
  • wbi_intvl_en = 1
  • irq_en = 0 (Poll mode)
  • irq_arm = 0 (Poll mode)
  • int_aggr = 0 (Poll mode)

For optimal QDMA streaming performance, packet buffers of the descriptor ring should be aligned to at least 256 bytes.

Performance in Descriptor Bypass Mode

QDMA supports both internal mode and descriptor bypass mode. Depending on the number of active queues needed for the design, you need to select the Internal mode or Descriptor bypass mode. If the number of active queues are less than 64, then Internal mode works fine. If the number of queues are more than 64, it is better to use the descriptor bypass mode.

In the descriptor bypass mode, it is your responsibility to maintain descriptors for corresponding queues and need to control their priority in sending the descriptors back to the IP.

When the design is configured in descriptor bypass mode, all the above setting apply. The following information provides recommendations to improve performance in bypass mode.

• When bypass in dma<0/1>_h2c_byp_in_st_sdi ports is set, the QDMA IP generates the status write back for every packet. AMD recommends that this port be asserted once in 32 packets or 64 packets. And if there are no more descriptors left then assert dma<0/1>_h2c_byp_in_st_sdi at the last descriptor. This requirement is per queue basis, and applies to AXI4 (H2C and C2H) bypass transfers and AXI4-Stream H2C transfers.
• For AXI4-Stream C2H Simple bypass mode, the dma<0/1>_dsc_crdt_in_fence port should be set to 1 for performance reasons. This recommendation assumes the user design already coalesced credits for each queue and sent them to the IP. In internal mode, set the fence bit in the QDMA_C2H_PFCH_CFG_2 (0xA84) register.

Performance Optimization Based on Available Cache/Buffer Size