PCI-Express AER implemetation: aer howto document

   The PCI Express Advanced Error Reporting Driver Guide HOWTO

		T. Long Nguyen	<tom.l.nguyen@intel.com>

		Yanmin Zhang	<yanmin.zhang@intel.com>

				07/29/2006

1. Overview

1.1 About this guide

This guide describes the basics of the PCI Express Advanced Error

Reporting (AER) driver and provides information on how to use it, as

well as how to enable the drivers of endpoint devices to conform with

PCI Express AER driver.

1.2 Copyright © Intel Corporation 2006.

1.3 What is the PCI Express AER Driver?

PCI Express error signaling can occur on the PCI Express link itself

or on behalf of transactions initiated on the link. PCI Express

defines two error reporting paradigms: the baseline capability and

the Advanced Error Reporting capability. The baseline capability is

required of all PCI Express components providing a minimum defined

set of error reporting requirements. Advanced Error Reporting

capability is implemented with a PCI Express advanced error reporting

extended capability structure providing more robust error reporting.

The PCI Express AER driver provides the infrastructure to support PCI

Express Advanced Error Reporting capability. The PCI Express AER

driver provides three basic functions:

-	Gathers the comprehensive error information if errors occurred.

-	Reports error to the users.

-	Performs error recovery actions.

AER driver only attaches root ports which support PCI-Express AER

capability.

2. User Guide

2.1 Include the PCI Express AER Root Driver into the Linux Kernel

The PCI Express AER Root driver is a Root Port service driver attached

to the PCI Express Port Bus driver. If a user wants to use it, the driver

has to be compiled. Option CONFIG_PCIEAER supports this capability. It

depends on CONFIG_PCIEPORTBUS, so pls. set CONFIG_PCIEPORTBUS=y and

CONFIG_PCIEAER = y.

2.2 Load PCI Express AER Root Driver

There is a case where a system has AER support in BIOS. Enabling the AER

Root driver and having AER support in BIOS may result unpredictable

behavior. To avoid this conflict, a successful load of the AER Root driver

requires ACPI _OSC support in the BIOS to allow the AER Root driver to

request for native control of AER. See the PCI FW 3.0 Specification for

details regarding OSC usage. Currently, lots of firmwares don't provide

_OSC support while they use PCI Express. To support such firmwares,

forceload, a parameter of type bool, could enable AER to continue to

be initiated although firmwares have no _OSC support. To enable the

walkaround, pls. add aerdriver.forceload=y to kernel boot parameter line

when booting kernel. Note that forceload=n by default.

2.3 AER error output

When a PCI-E AER error is captured, an error message will be outputed to

console. If it's a correctable error, it is outputed as a warning.

Otherwise, it is printed as an error. So users could choose different

log level to filter out correctable error messages.

Below shows an example.

+------ PCI-Express Device Error -----+

Error Severity          : Uncorrected (Fatal)

PCIE Bus Error type     : Transaction Layer

Unsupported Request     : First

Requester ID            : 0500

VendorID=8086h, DeviceID=0329h, Bus=05h, Device=00h, Function=00h

TLB Header:

04000001 00200a03 05010000 00050100

In the example, 'Requester ID' means the ID of the device who sends

the error message to root port. Pls. refer to pci express specs for

other fields.

3. Developer Guide

To enable AER aware support requires a software driver to configure

the AER capability structure within its device and to provide callbacks.

To support AER better, developers need understand how AER does work

firstly.

PCI Express errors are classified into two types: correctable errors

and uncorrectable errors. This classification is based on the impacts

of those errors, which may result in degraded performance or function

failure.

Correctable errors pose no impacts on the functionality of the

interface. The PCI Express protocol can recover without any software

intervention or any loss of data. These errors are detected and

corrected by hardware. Unlike correctable errors, uncorrectable

errors impact functionality of the interface. Uncorrectable errors

can cause a particular transaction or a particular PCI Express link

to be unreliable. Depending on those error conditions, uncorrectable

errors are further classified into non-fatal errors and fatal errors.

Non-fatal errors cause the particular transaction to be unreliable,

but the PCI Express link itself is fully functional. Fatal errors, on

the other hand, cause the link to be unreliable.

When AER is enabled, a PCI Express device will automatically send an

error message to the PCIE root port above it when the device captures

an error. The Root Port, upon receiving an error reporting message,

internally processes and logs the error message in its PCI Express

capability structure. Error information being logged includes storing

the error reporting agent's requestor ID into the Error Source

Identification Registers and setting the error bits of the Root Error

Status Register accordingly. If AER error reporting is enabled in Root

Error Command Register, the Root Port generates an interrupt if an

error is detected.

Note that the errors as described above are related to the PCI Express

hierarchy and links. These errors do not include any device specific

errors because device specific errors will still get sent directly to

the device driver.

3.1 Configure the AER capability structure

AER aware drivers of PCI Express component need change the device

control registers to enable AER. They also could change AER registers,

including mask and severity registers. Helper function

pci_enable_pcie_error_reporting could be used to enable AER. See

section 3.3.

3.2. Provide callbacks

3.2.1 callback reset_link to reset pci express link

This callback is used to reset the pci express physical link when a

fatal error happens. The root port aer service driver provides a

default reset_link function, but different upstream ports might

have different specifications to reset pci express link, so all

upstream ports should provide their own reset_link functions.

In struct pcie_port_service_driver, a new pointer, reset_link, is

added.

pci_ers_result_t (*reset_link) (struct pci_dev *dev);

Section 3.2.2.2 provides more detailed info on when to call

reset_link.

3.2.2 PCI error-recovery callbacks

The PCI Express AER Root driver uses error callbacks to coordinate

with downstream device drivers associated with a hierarchy in question

when performing error recovery actions.

Data struct pci_driver has a pointer, err_handler, to point to

pci_error_handlers who consists of a couple of callback function

pointers. AER driver follows the rules defined in

pci-error-recovery.txt except pci express specific parts (e.g.

reset_link). Pls. refer to pci-error-recovery.txt for detailed

definitions of the callbacks.

Below sections specify when to call the error callback functions.

3.2.2.1 Correctable errors

Correctable errors pose no impacts on the functionality of

the interface. The PCI Express protocol can recover without any

software intervention or any loss of data. These errors do not

require any recovery actions. The AER driver clears the device's

correctable error status register accordingly and logs these errors.

3.2.2.2 Non-correctable (non-fatal and fatal) errors

If an error message indicates a non-fatal error, performing link reset

at upstream is not required. The AER driver calls error_detected(dev,

pci_channel_io_normal) to all drivers associated within a hierarchy in

question. for example,

EndPoint<==>DownstreamPort B<==>UpstreamPort A<==>RootPort.

If Upstream port A captures an AER error, the hierarchy consists of

Downstream port B and EndPoint.

A driver may return PCI_ERS_RESULT_CAN_RECOVER,

PCI_ERS_RESULT_DISCONNECT, or PCI_ERS_RESULT_NEED_RESET, depending on

whether it can recover or the AER driver calls mmio_enabled as next.

If an error message indicates a fatal error, kernel will broadcast

error_detected(dev, pci_channel_io_frozen) to all drivers within

a hierarchy in question. Then, performing link reset at upstream is

necessary. As different kinds of devices might use different approaches

to reset link, AER port service driver is required to provide the

function to reset link. Firstly, kernel looks for if the upstream

component has an aer driver. If it has, kernel uses the reset_link

callback of the aer driver. If the upstream component has no aer driver

and the port is downstream port, we will use the aer driver of the

root port who reports the AER error. As for upstream ports,

they should provide their own aer service drivers with reset_link

function. If error_detected returns PCI_ERS_RESULT_CAN_RECOVER and

reset_link returns PCI_ERS_RESULT_RECOVERED, the error handling goes

to mmio_enabled.

3.3 helper functions

3.3.1 int pci_find_aer_capability(struct pci_dev *dev);

pci_find_aer_capability locates the PCI Express AER capability

in the device configuration space. If the device doesn't support

PCI-Express AER, the function returns 0.

3.3.2 int pci_enable_pcie_error_reporting(struct pci_dev *dev);

pci_enable_pcie_error_reporting enables the device to send error

messages to root port when an error is detected. Note that devices

don't enable the error reporting by default, so device drivers need

call this function to enable it.

3.3.3 int pci_disable_pcie_error_reporting(struct pci_dev *dev);

pci_disable_pcie_error_reporting disables the device to send error

messages to root port when an error is detected.

3.3.4 int pci_cleanup_aer_uncorrect_error_status(struct pci_dev *dev);

pci_cleanup_aer_uncorrect_error_status cleanups the uncorrectable

error status register.

3.4 Frequent Asked Questions

Q: What happens if a PCI Express device driver does not provide an

error recovery handler (pci_driver->err_handler is equal to NULL)?

A: The devices attached with the driver won't be recovered. If the

error is fatal, kernel will print out warning messages. Please refer

to section 3 for more information.

Q: What happens if an upstream port service driver does not provide

callback reset_link?

A: Fatal error recovery will fail if the errors are reported by the

upstream ports who are attached by the service driver.

Q: How does this infrastructure deal with driver that is not PCI

Express aware?

A: This infrastructure calls the error callback functions of the

driver when an error happens. But if the driver is not aware of

PCI Express, the device might not report its own errors to root

port.

Q: What modifications will that driver need to make it compatible

with the PCI Express AER Root driver?

A: It could call the helper functions to enable AER in devices and

cleanup uncorrectable status register. Pls. refer to section 3.3.