u-boot/doc/driver-model/pci-info.txt

PCI with Driver Model
=====================

How busses are scanned
----------------------

Any config read will end up at pci_read_config(). This uses
uclass_get_device_by_seq() to get the PCI bus for a particular bus number.
Bus number 0 will need to  be requested first, and the alias in the device
tree file will point to the correct device:


	aliases {
		pci0 = &pci;
	};

	pci: pci-controller {
		compatible = "sandbox,pci";
		...
	};


If there is no alias the devices will be numbered sequentially in the device
tree.

The call to uclass_get_device by seq() will cause the PCI bus to be probed.
This does a scan of the bus to locate available devices. These devices are
bound to their appropriate driver if available. If there is no driver, then
they are bound to a generic PCI driver which does nothing.

After probing a bus, the available devices will appear in the device tree
under that bus.

Note that this is all done on a lazy basis, as needed, so until something is
touched on PCI it will not be probed.

PCI devices can appear in the device tree. If they do this serves to specify
the driver to use for the device. In this case they will be bound at
start-up.


Sandbox
-------

With sandbox we need a device emulator for each device on the bus since there
is no real PCI bus. This works by looking in the device tree node for a
driver. For example:


	pci@1f,0 {
		compatible = "pci-generic";
		reg = <0xf800 0 0 0 0>;
		emul@1f,0 {
			compatible = "sandbox,swap-case";
		};
	};

This means that there is a 'sandbox,swap-case' driver at that bus position.
Note that the first cell in the 'reg' value is the bus/device/function. See
PCI_BDF() for the encoding (it is also specified in the IEEE Std 1275-1994
PCI bus binding document, v2.1)

When this bus is scanned we will end up with something like this:

`- * pci-controller @ 05c660c8, 0
 `-   pci@1f,0 @ 05c661c8, 63488
  `-   emul@1f,0 @ 05c662c8

When accesses go to the pci@1f,0 device they are forwarded to its child, the
emulator.