linux-next/Documentation/PCI/pciebus-howto.rst

.. SPDX-License-Identifier: GPL-2.0
.. include:: <isonum.txt>

===========================================
The PCI Express Port Bus Driver Guide HOWTO
===========================================

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

About this guide
================

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


What is the PCI Express Port Bus Driver
=======================================

A PCI Express Port is a logical PCI-PCI Bridge structure. There
are two types of PCI Express Port: the Root Port and the Switch
Port. The Root Port originates a PCI Express link from a PCI Express
Root Complex and the Switch Port connects PCI Express links to
internal logical PCI buses. The Switch Port, which has its secondary
bus representing the switch's internal routing logic, is called the
switch's Upstream Port. The switch's Downstream Port is bridging from
switch's internal routing bus to a bus representing the downstream
PCI 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 reporting
support (AER), and virtual channel support (VC). These services may
be handled by a single complex driver or be individually distributed
and handled by corresponding service drivers.

Why use the PCI Express Port Bus Driver?
========================================

In existing Linux kernels, the Linux Device Driver Model allows a
physical device to be handled by only a single driver. The PCI
Express Port is a PCI-PCI Bridge device with multiple distinct
services. To maintain a clean and simple solution each service
may have its own software service driver. In this case several
service drivers will compete for a single PCI-PCI Bridge device.
For example, if the PCI Express Root Port native hotplug service
driver is loaded first, it claims a PCI-PCI Bridge Root Port. The
kernel therefore does not load other service drivers for that Root
Port. In other words, it is impossible to have multiple service
drivers load and run on a PCI-PCI Bridge device simultaneously
using the current driver model.

To enable multiple service drivers running simultaneously requires
having a PCI Express Port Bus driver, which manages all populated
PCI Express Ports and distributes all provided service requests
to the corresponding service drivers as required. Some key
advantages of using the PCI Express Port Bus driver are listed below:

  - Allow multiple service drivers to run simultaneously on
    a PCI-PCI Bridge Port device.

  - Allow service drivers implemented in an independent
    staged approach.

  - Allow one service driver to run on multiple PCI-PCI Bridge
    Port devices.

  - Manage and distribute resources of a PCI-PCI Bridge Port
    device to requested service drivers.

Configuring the PCI Express Port Bus Driver vs. Service Drivers
===============================================================

Including the PCI Express Port Bus Driver Support into the Kernel
-----------------------------------------------------------------

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

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 is
impossible to load any service driver once the kernel has loaded the
PCI Express Port Bus Driver. To meet the PCI Express Port Bus Driver
Model requires some minimal changes on existing service drivers that
imposes no impact on the functionality of existing service drivers.

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

pcie_port_service_register
~~~~~~~~~~~~~~~~~~~~~~~~~~
::

  int pcie_port_service_register(struct pcie_port_service_driver *new)

This API replaces the Linux Driver Model's pci_register_driver API. A
service driver should always calls pcie_port_service_register at
module init. Note that after service driver being loaded, calls
such as pci_enable_device(dev) and pci_set_master(dev) are no longer
necessary since these calls are executed by the PCI Port Bus driver.

pcie_port_service_unregister
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
::

  void pcie_port_service_unregister(struct pcie_port_service_driver *new)

pcie_port_service_unregister replaces the Linux Driver Model's
pci_unregister_driver. It's always called by service driver when a
module exits.

Sample Code
~~~~~~~~~~~

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

  static struct pcie_port_service_id service_id[] = { {
    .vendor = PCI_ANY_ID,
    .device = PCI_ANY_ID,
    .port_type = PCIE_RC_PORT,
    .service_type = PCIE_PORT_SERVICE_AER,
    }, { /* end: all zeroes */ }
  };

  static struct pcie_port_service_driver root_aerdrv = {
    .name		= (char *)device_name,
    .id_table	= &service_id[0],

    .probe		= aerdrv_load,
    .remove		= aerdrv_unload,

    .suspend	= aerdrv_suspend,
    .resume		= aerdrv_resume,
  };

Below is a sample code for registering/unregistering a service
driver.
::

  static int __init aerdrv_service_init(void)
  {
    int retval = 0;

    retval = pcie_port_service_register(&root_aerdrv);
    if (!retval) {
      /*
      * FIX ME
      */
    }
    return retval;
  }

  static void __exit aerdrv_service_exit(void)
  {
    pcie_port_service_unregister(&root_aerdrv);
  }

  module_init(aerdrv_service_init);
  module_exit(aerdrv_service_exit);

Possible Resource Conflicts
===========================

Since all service drivers of a PCI-PCI Bridge Port device are
allowed to run simultaneously, below lists a few of possible resource
conflicts with proposed solutions.

MSI and MSI-X Vector Resource
-----------------------------

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

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

PCI Memory/IO Mapped Regions
----------------------------

Service drivers for PCI Express Power Management (PME), Advanced
Error Reporting (AER), Hot-Plug (HP) and Virtual Channel (VC) access
PCI configuration space on the PCI Express port. In all cases the
registers accessed are independent of each other. This patch assumes
that all service drivers will be well behaved and not overwrite
other service driver's configuration settings.

PCI Config Registers
--------------------

Each service driver runs its PCI config operations on its own
capability structure except the PCI Express capability structure,
that is shared between many drivers including the service drivers.
RMW Capability accessors (pcie_capability_clear_and_set_word(),
pcie_capability_set_word(), and pcie_capability_clear_word()) protect
a selected set of PCI Express Capability Registers (Link Control
Register and Root Control Register). Any change to those registers
should be performed using RMW accessors to avoid problems due to
concurrent updates. For the up-to-date list of protected registers,
see pcie_capability_clear_and_set_word().