This driver supports NTB of cappable IDT PCIe-switches.

Some of the pre-initializations must be made before IDT PCIe-switch
exposes it NT-functions correctly. It should be done by either proper
initialisation of EEPROM connected to master smbus of the switch or
by BIOS using slave-SMBus interface changing corresponding registers
value. Evidently it must be done before PCI bus enumeration is
finished in Linux kernel.

First of all partitions must be activated and properly assigned to all
the ports with NT-functions intended to be activated (see SWPARTxCTL
and SWPORTxCTL registers). Then all NT-function BARs must be enabled
with chosen valid aperture. For memory windows related BARs the
aperture settings shall determine the maximum size of memory windows
accepted by a BAR. Note that BAR0 must map PCI configuration space
registers.

It's worth to note, that since a part of this driver relies on the
BAR settings of peer NT-functions, the BAR setups can't be done over
kernel PCI fixups. That's why the alternative pre-initialization
techniques like BIOS using SMBus interface or EEPROM should be
utilized.

If unsure, say N.

source code:

drivers/ntb/hw/idt/ntb_hw_idt.c