1PCI bus bridges have standardized Device Tree bindings:
2
3PCI Bus Binding to: IEEE Std 1275-1994
4http://www.devicetree.org/open-firmware/bindings/pci/pci2_1.pdf
5
6And for the interrupt mapping part:
7
8Open Firmware Recommended Practice: Interrupt Mapping
9http://www.devicetree.org/open-firmware/practice/imap/imap0_9d.pdf
10
11Additionally to the properties specified in the above standards a host bridge
12driver implementation may support the following properties:
13
14- linux,pci-domain:
15   If present this property assigns a fixed PCI domain number to a host bridge,
16   otherwise an unstable (across boots) unique number will be assigned.
17   It is required to either not set this property at all or set it for all
18   host bridges in the system, otherwise potentially conflicting domain numbers
19   may be assigned to root buses behind different host bridges.  The domain
20   number for each host bridge in the system must be unique.
21- max-link-speed:
22   If present this property specifies PCI gen for link capability.  Host
23   drivers could add this as a strategy to avoid unnecessary operation for
24   unsupported link speed, for instance, trying to do training for
25   unsupported link speed, etc.  Must be '4' for gen4, '3' for gen3, '2'
26   for gen2, and '1' for gen1. Any other values are invalid.
27- reset-gpios:
28   If present this property specifies PERST# GPIO. Host drivers can parse the
29   GPIO and apply fundamental reset to endpoints.
30- supports-clkreq:
31   If present this property specifies that CLKREQ signal routing exists from
32   root port to downstream device and host bridge drivers can do programming
33   which depends on CLKREQ signal existence. For example, programming root port
34   not to advertise ASPM L1 Sub-States support if there is no CLKREQ signal.
35
36PCI-PCI Bridge properties
37-------------------------
38
39PCIe root ports and switch ports may be described explicitly in the device
40tree, as children of the host bridge node. Even though those devices are
41discoverable by probing, it might be necessary to describe properties that
42aren't provided by standard PCIe capabilities.
43
44Required properties:
45
46- reg:
47   Identifies the PCI-PCI bridge. As defined in the IEEE Std 1275-1994
48   document, it is a five-cell address encoded as (phys.hi phys.mid
49   phys.lo size.hi size.lo). phys.hi should contain the device's BDF as
50   0b00000000 bbbbbbbb dddddfff 00000000. The other cells should be zero.
51
52   The bus number is defined by firmware, through the standard bridge
53   configuration mechanism. If this port is a switch port, then firmware
54   allocates the bus number and writes it into the Secondary Bus Number
55   register of the bridge directly above this port. Otherwise, the bus
56   number of a root port is the first number in the bus-range property,
57   defaulting to zero.
58
59   If firmware leaves the ARI Forwarding Enable bit set in the bridge
60   above this port, then phys.hi contains the 8-bit function number as
61   0b00000000 bbbbbbbb ffffffff 00000000. Note that the PCIe specification
62   recommends that firmware only leaves ARI enabled when it knows that the
63   OS is ARI-aware.
64
65Optional properties:
66
67- external-facing:
68   When present, the port is external-facing. All bridges and endpoints
69   downstream of this port are external to the machine. The OS can, for
70   example, use this information to identify devices that cannot be
71   trusted with relaxed DMA protection, as users could easily attach
72   malicious devices to this port.
73
74Example:
75
76pcie@10000000 {
77	compatible = "pci-host-ecam-generic";
78	...
79	pcie@0008 {
80		/* Root port 00:01.0 is external-facing */
81		reg = <0x00000800 0 0 0 0>;
82		external-facing;
83	};
84};
85