sb/intel/bd82x6x: Disable unused PCIe root ports

Follow the PCH BIOS spec more closely by porting the broadwell
and braswell PCIe downstream device detection. To safe power
disable PCIe root ports that have no downstream device connected.

By setting the FLAGS_SLOT bit in register PCI_EXP_FLAGS the
PCI_EXP_SLTSTA_PDS bit will be updated with in band device
detection from the PCIe PHY. While this is primarly used for PCIe
hot-plug detection, it is more reliable than probing for downstream
devices by reading DID/VID PCI registers.

The FLAGS_SLOT bit should stay cleared for integrated devices,
as those are known to be present, but to simplify the code all
PCIe ports will have the FLAGS_SLOT bit set. There currently
used devicetrees might also be lacking integrated devices on
the PCH root ports...

The SLOTCAP field must be updated by BIOS when the FLAGS_SLOT
is set, but it shouldn't be filled for integrated devices. Until
now the SLOTCAP field has always been populated and it never
was a problem.

- Set FLAGS_SLOT "Slot Implemented" bit early.
- Read bit PCI_EXP_SLTSTA_PDS to detect connected downstream
  devices as done on braswell.
- Disable unused PCIe slots that are not hotplugable.
- Set BIT26 in register 0x338 and wait for bits in register 0x328
  to clear as done on broadwell.

Test: Tested on Lenovo X220. Unused root ports are disabled and port
      that are in used or marked hot-plug are kept enabled.

Change-Id: I8ccfcab2e0e4faba8322755a4f8c2108d9b007ac
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78226
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

0 files changed, 0 insertions, 0 deletions