[U-Boot] BayTrail PCIe x4 slot (Soft-Strap?)

[Stefan Roese <sr@denx.de>]

Hi!

I do have BayTrail / FSP related question. I'm currently trying
to use a DFI QSeven SoM which has one x4 PCIe slot instead
of the usual 4 x1 slots. So all 4 PCIe lanes are used by the
first PCIe controller. With the current U-Boot, all 4 PCIe
controllers are enabled by the FSP :

00:1c.0 PCI bridge: Intel Corporation Atom Processor E3800 Series PCI Express Root Port 1 (rev 11)
00:1c.1 PCI bridge: Intel Corporation Atom Processor E3800 Series PCI Express Root Port 2 (rev 11)
00:1c.2 PCI bridge: Intel Corporation Atom Processor E3800 Series PCI Express Root Port 3 (rev 11)
00:1c.3 PCI bridge: Intel Corporation Atom Processor E3800 Series PCI Express Root Port 4 (rev 11)

In this configuration, the x4 PCIe card that is installed in the
PCIe slot is not detected. The system always generated hotplug
events for all for ports, but the link is not established.

The original DFI BIOS only enables the first PCIe controller. The
controllers 1...3 are not visible via lspci. Here the 4x link
is established and the 4x PCIe card is detected correctly.

My question now is, how can I enable this 4x link on the first
PCIe controller via U-Boot / FSP? I have found no option on how
to configure the PCIe controllers in the FSP dts properties.
So that only the first controller is enabled and visible via
lspci etc. The BayTrail datasheet mentions this to configure the
PCIe setup in chapter "23.2.1 Root Port Configurations":

"
Root port configurations are set by SoftStraps stored in SPI flash,
and the default option is ?(4) x1?. Links for each root port will
train automatically to the maximum possible for each port.
"

Where is this SoftStraps in the SPI flash located? I've found this
page mentioning that its a offset 0x100:

https://embedded.communities.intel.com/thread/8539

But I fail to find any documentation for all those Soft-Strap
Registers / Values in the SPI flash. Does anyone have some further
infos / documentation on this? How to enable 4x PCIe lanes
for one PCIe controller on BayTrail / Atom?

Thanks,
Stefan