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Appendix E
Limitations for CPM4 and CPM5
Speed Change Related Issue #1
• Description: Repeated speed changes can result in the link not coming up to the intended
targeted speed.
• Workaround: A follow-on attempt should bring the link back. In extremely rare scenarios, a
full reboot might be required.
Speed Change Related Issue #2
• Description: In extremely rare cases repeated link Rate changes might also result in the
following:
1. PCIe access becoming unresponsive.
2. While traffic is going on in system and PM D3 is also enabled with rate changes, the host
might receive completion timeout for the read when the pre-read is done before the PM
D3 sequence is targeted to the EP ECAM space.
• Workaround: In the case of PM D3, Xilinx recommends that any valid EP address be used
except ECAM space in the pre-read before initiating PM D3 sequence.
In all other cases, waiting approximately 20 msec after the link rate and before attempting any
PCIe access may help.
However, in scenarios where the transaction still does not complete, a full reboot (power cycle
and re-programming image) would be required.
Speed Change Related Issue #3
• Description: In RP configuration with core clock of 1GHz, PCIe link rate changes from Gen1/
Gen2 to Gen3/Gen4/Gen5, it can fail to reach the intended speed or link can go down in rare
cases.
• Workaround: An additional write with value 1 to the Perform Equalization bit in Link Control
3 register on the Root complex PCIe configuration space is required when the rate change is
performed to Gen3, Gen4, or Gen5 speeds from Gen1/Gen2.
Appendix E: Limitations for CPM4 and CPM5
PG346 (v3.3) November 16, 2022
CPM Mode for PCI Express
273