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Author:	Tom L Nguyen tom.l.nguyen@intel.com 11/03/2004
Copyright:	© 2004 Intel Corporation

2.1. About this guide¶

This guide describes the basics of the PCI Express Port Bus driverand provides information on how to enable the service drivers toregister/unregister with the PCI Express Port Bus Driver.

2.2. What is the PCI Express Port Bus Driver¶

A PCI Express Port is a logical PCI-PCI Bridge structure. Thereare two types of PCI Express Port: the Root Port and the SwitchPort. The Root Port originates a PCI Express link from a PCI ExpressRoot Complex and the Switch Port connects PCI Express links tointernal logical PCI buses. The Switch Port, which has its secondarybus representing the switch’s internal routing logic, is called theswitch’s Upstream Port. The switch’s Downstream Port is bridging fromswitch’s internal routing bus to a bus representing the downstreamPCI Express link from the PCI Express Switch.

A PCI Express Port can provide up to four distinct functions,referred to in this document as services, depending on its port type.PCI Express Port’s services include native hotplug support (HP),power management event support (PME), advanced error reportingsupport (AER), and virtual channel support (VC). These services maybe handled by a single complex driver or be individually distributedand handled by corresponding service drivers.

2.3. Why use the PCI Express Port Bus Driver?¶

In existing Linux kernels, the Linux Device Driver Model allows aphysical device to be handled by only a single driver. The PCIExpress Port is a PCI-PCI Bridge device with multiple distinctservices. To maintain a clean and simple solution each servicemay have its own software service driver. In this case severalservice drivers will compete for a single PCI-PCI Bridge device.For example, if the PCI Express Root Port native hotplug servicedriver is loaded first, it claims a PCI-PCI Bridge Root Port. Thekernel therefore does not load other service drivers for that RootPort. In other words, it is impossible to have multiple servicedrivers load and run on a PCI-PCI Bridge device simultaneouslyusing the current driver model.

To enable multiple service drivers running simultaneously requireshaving a PCI Express Port Bus driver, which manages all populatedPCI Express Ports and distributes all provided service requeststo the corresponding service drivers as required. Some keyadvantages of using the PCI Express Port Bus driver are listed below:

Allow multiple service drivers to run simultaneously ona PCI-PCI Bridge Port device.
Allow service drivers implemented in an independentstaged approach.
Allow one service driver to run on multiple PCI-PCI BridgePort devices.
Manage and distribute resources of a PCI-PCI Bridge Portdevice to requested service drivers.

2.4. Configuring the PCI Express Port Bus Driver vs. Service Drivers¶

2.4.1. Including the PCI Express Port Bus Driver Support into the Kernel¶

Including the PCI Express Port Bus driver depends on whether the PCIExpress support is included in the kernel config. The kernel willautomatically include the PCI Express Port Bus driver as a kerneldriver when the PCI Express support is enabled in the kernel.

2.4.2. Enabling Service Driver Support¶

PCI device drivers are implemented based on Linux Device Driver Model.All service drivers are PCI device drivers. As discussed above, it isimpossible to load any service driver once the kernel has loaded thePCI Express Port Bus Driver. To meet the PCI Express Port Bus DriverModel requires some minimal changes on existing service drivers thatimposes no impact on the functionality of existing service drivers.

A service driver is required to use the two APIs shown below toregister its service with the PCI Express Port Bus driver (seesection 5.2.1 & 5.2.2). It is important that a service driverinitializes the pcie_port_service_driver data structure, included inheader file /include/linux/pcieport_if.h, before calling these APIs.Failure to do so will result an identity mismatch, which preventsthe PCI Express Port Bus driver from loading a service driver.

2.4.2.1. pcie_port_service_register¶

This API replaces the Linux Driver Model’s pci_register_driver API. Aservice driver should always calls pcie_port_service_register atmodule init. Note that after service driver being loaded, callssuch as pci_enable_device(dev) and pci_set_master(dev) are no longernecessary since these calls are executed by the PCI Port Bus driver.

2.4.2.2. pcie_port_service_unregister¶

pcie_port_service_unregister replaces the Linux Driver Model’spci_unregister_driver. It’s always called by service driver when amodule exits.

2.4.2.3. Sample Code¶

Below is sample service driver code to initialize the port servicedriver data structure.

Below is a sample code for registering/unregistering a servicedriver.

2.5. Possible Resource Conflicts¶

Since all service drivers of a PCI-PCI Bridge Port device areallowed to run simultaneously, below lists a few of possible resourceconflicts with proposed solutions.

2.5.1. MSI and MSI-X Vector Resource¶

Once MSI or MSI-X interrupts are enabled on a device, it stays in thismode until they are disabled again. Since service drivers of the samePCI-PCI Bridge port share the same physical device, if an individualservice driver enables or disables MSI/MSI-X mode it may resultunpredictable behavior.

To avoid this situation all service drivers are not permitted toswitch interrupt mode on its device. The PCI Express Port Bus driveris responsible for determining the interrupt mode and this should betransparent to service drivers. Service drivers need to know onlythe vector IRQ assigned to the field irq of struct pcie_device, whichis passed in when the PCI Express Port Bus driver probes each servicedriver. Service drivers should use (struct pcie_device*)dev->irq tocall request_irq/free_irq. In addition, the interrupt mode is storedin the field interrupt_mode of struct pcie_device.

2.5.2. PCI Memory/IO Mapped Regions¶

Service drivers for PCI Express Power Management (PME), AdvancedError Reporting (AER), Hot-Plug (HP) and Virtual Channel (VC) accessPCI configuration space on the PCI Express port. In all cases theregisters accessed are independent of each other. This patch assumesthat all service drivers will be well behaved and not overwriteother service driver’s configuration settings.

2.5.3. PCI Config Registers¶

Each service driver runs its PCI config operations on its owncapability structure except the PCI Express capability structure, inwhich Root Control register and Device Control register are sharedbetween PME and AER. This patch assumes that all service driverswill be well behaved and not overwrite other service driver’sconfiguration settings.

Authors:	Martin Mares <mj@ucw.cz> Grant Grundler <grundler@parisc-linux.org>

The world of PCI is vast and full of (mostly unpleasant) surprises.Since each CPU architecture implements different chip-sets and PCI deviceshave different requirements (erm, “features”), the result is the PCI supportin the Linux kernel is not as trivial as one would wish. This short papertries to introduce all potential driver authors to Linux APIs forPCI device drivers.

A more complete resource is the third edition of “Linux Device Drivers”by Jonathan Corbet, Alessandro Rubini, and Greg Kroah-Hartman.LDD3 is available for free (under Creative Commons License) from:https://lwn.net/Kernel/LDD3/.

However, keep in mind that all documents are subject to “bit rot”.Refer to the source code if things are not working as described here.

Please send questions/comments/patches about Linux PCI API to the“Linux PCI” <linux-pci@atrey.karlin.mff.cuni.cz> mailing list.

1.1. Structure of PCI drivers¶

PCI drivers “discover” PCI devices in a system via pci_register_driver().Actuall

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