The fu740 PCIe core does not probe any devices on the SiFive Unmatched
board without this fix (or having U-Boot explicitly start the PCIe via
either boot-script or user command). The fix is to start the link at
2.5GT/s speeds and once the link is up then change the maximum speed back
to the default.
The U-Boot driver claims to set the link-speed to 2.5GT/s to get the probe
to work (and U-Boot does print link up at 2.5GT/s) in the following code:
https://source.denx.de/u-boot/u-boot/-/blob/master/drivers/pci/pcie_dw_sifive.c?id=v2022.01#L271
Signed-off-by: Ben Dooks <ben.dooks@xxxxxxxxxxxxxxx>
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Note, this patch has had significant re-work since the previous 4
sets, including trying to fix style, message, reliance on the U-Boot
fix and the comments about usage of LINK_CAP and reserved fields.