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I got a question about the PCIe bifurcation setting when booting up in X86 system. Normally, we can set the PCIe controller to be X16, two X8, or four X4 in the UEFI shell menu. However, there is an auto detection way to do the same thing automatically. For example, if I put four X4 devices on the riser card, and put this riser card to the X16 PCIe slot, the system can automatically detect that the PCIe controller should be configured as a four X4 host controllers. Does anyone know the detail procedure to implement this function? I think it's not specified in the PCIe spec. Using PRSNT# pins hard wired to the PCH may be a way, but is there any other (or real case) way to achieve this goal? Thanks.

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  • \$\begingroup\$ Why not x64 system? \$\endgroup\$
    – Jason Han
    Aug 19, 2017 at 3:37
  • \$\begingroup\$ Easy way is to run the device with one lane or max lanes, test the device, then reset and redo initialization. \$\endgroup\$ Aug 19, 2017 at 5:48

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It depends on your processor family, but is auto configured by CFG 5/6 on i7.

This Intel video might help.

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  • \$\begingroup\$ Thanks for the video! So the bifurcation can only be set by the strap pin CFG5/6? Is there any other way to automatically set the configuration without manually set it? \$\endgroup\$
    – Nobody
    Aug 20, 2017 at 15:59
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    \$\begingroup\$ CFG5/6 is the automatic way. This allows YOU to configure a system using your jumpers and automatically use this instead of the BIOS/UEFI settings. \$\endgroup\$ Aug 20, 2017 at 16:32

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