PCI bus bridges have standardized Device Tree bindings:
PCI Bus Binding to: IEEE Std 1275-1994
And for the interrupt mapping part:
Open Firmware Recommended Practice: Interrupt Mapping
Additionally to the properties specified in the above standards a host bridge
driver implementation may support the following properties:
If present this property assigns a fixed PCI domain number to a host bridge,
otherwise an unstable (across boots) unique number will be assigned.
It is required to either not set this property at all or set it for all
host bridges in the system, otherwise potentially conflicting domain numbers
may be assigned to root buses behind different host bridges. The domain
number for each host bridge in the system must be unique.
If present this property specifies PCI gen for link capability. Host
drivers could add this as a strategy to avoid unnecessary operation for
unsupported link speed, for instance, trying to do training for
unsupported link speed, etc. Must be '4' for gen4, '3' for gen3, '2'
for gen2, and '1' for gen1. Any other values are invalid.
PCI-PCI Bridge properties
PCIe root ports and switch ports may be described explicitly in the device
tree, as children of the host bridge node. Even though those devices are
discoverable by probing, it might be necessary to describe properties that
aren't provided by standard PCIe capabilities.
Identifies the PCI-PCI bridge. As defined in the IEEE Std 1275-1994
document, it is a five-cell address encoded as (phys.hi phys.mid
phys.lo size.hi size.lo). phys.hi should contain the device's BDF as
0b00000000 bbbbbbbb dddddfff 00000000. The other cells should be zero.
The bus number is defined by firmware, through the standard bridge
configuration mechanism. If this port is a switch port, then firmware
allocates the bus number and writes it into the Secondary Bus Number
register of the bridge directly above this port. Otherwise, the bus
number of a root port is the first number in the bus-range property,
defaulting to zero.
If firmware leaves the ARI Forwarding Enable bit set in the bridge
above this port, then phys.hi contains the 8-bit function number as
0b00000000 bbbbbbbb ffffffff 00000000. Note that the PCIe specification
recommends that firmware only leaves ARI enabled when it knows that the
OS is ARI-aware.
When present, the port is external-facing. All bridges and endpoints
downstream of this port are external to the machine. The OS can, for
example, use this information to identify devices that cannot be
trusted with relaxed DMA protection, as users could easily attach
malicious devices to this port.
compatible = "pci-host-ecam-generic";
/* Root port 00:01.0 is external-facing */
reg = <0x00000800 0 0 0 0>;