[coreboot] Question about PCIe separate reference clock

[Zheng Bao]

Our VPX design uses separate reference clock source, which is Si52111-B5 (No spread), instead of common ref clock from CPU.
Now The system is unstable. Reading PCIE configuration space is unstable too. (If we add some fly wire to make it work with common ref clock, the system becomes stable.)

(abstracted from PCIe spec: 12 Slot Clock Configuration - This bit indicates that the
component uses the same physical reference clock that the
platform provides on the connector. If the device uses an
independent clock irrespective of the presence of a reference
clock on the connector, this bit must be clear.
For a multi-Function device, each Function must report the
same value for this bit.)

Based on my understanding, the BIOS need to read bit "Slot Clock Configurationclear" to see if
separate ref clock is used.  BIOS then write bit "Common Clock Configuration".

On our board, the bit "Slot Clock Configuration" is always 1, which I assume should be 0.

My question is, how the hardware affect the bit "Slot Clock Configuration"? How do we need to design our board to make the bit "Slot Clock Configuration" be 0?

Thanks.

Zheng

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