8. The PCI Express Advanced Error Reporting Driver Guide HOWTO

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© 2006 Intel Corporation

8.1. Overview

8.1.1. About this guide

This guide describes the basics of the PCI Express (PCIe) 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 the PCIe AER driver.

8.1.2. What is the PCIe AER Driver?

PCIe error signaling can occur on the PCIe link itself or on behalf of transactions initiated on the link. PCIe defines two error reporting paradigms: the baseline capability and the Advanced Error Reporting capability. The baseline capability is required of all PCIe components providing a minimum defined set of error reporting requirements. Advanced Error Reporting capability is implemented with a PCIe Advanced Error Reporting extended capability structure providing more robust error reporting.

The PCIe AER driver provides the infrastructure to support PCIe Advanced Error Reporting capability. The PCIe AER driver provides three basic functions:

  • Gathers the comprehensive error information if errors occurred.

  • Reports error to the users.

  • Performs error recovery actions.

The AER driver only attaches to Root Ports and RCECs that support the PCIe AER capability.

8.2. User Guide

8.2.1. Include the PCIe AER Root Driver into the Linux Kernel

The PCIe AER driver is a Root Port service driver attached via the PCIe Port Bus driver. If a user wants to use it, the driver must be compiled. It is enabled with CONFIG_PCIEAER, which depends on CONFIG_PCIEPORTBUS.

8.2.2. Load PCIe AER Root Driver

Some systems have AER support in firmware. Enabling Linux AER support at the same time the firmware handles AER would result in unpredictable behavior. Therefore, Linux does not handle AER events unless the firmware grants AER control to the OS via the ACPI _OSC method. See the PCI Firmware Specification for details regarding _OSC usage.

8.2.3. AER error output

When a PCIe AER error is captured, an error message will be output to console. If it's a correctable error, it is output as an info message. Otherwise, it is printed as an error. So users could choose different log level to filter out correctable error messages.

Below shows an example:

0000:50:00.0: PCIe Bus Error: severity=Uncorrected (Fatal), type=Transaction Layer, id=0500(Requester ID)
0000:50:00.0:   device [8086:0329] error status/mask=00100000/00000000
0000:50:00.0:    [20] Unsupported Request    (First)
0000:50:00.0:   TLP Header: 04000001 00200a03 05010000 00050100

In the example, 'Requester ID' means the ID of the device that sent the error message to the Root Port. Please refer to PCIe specs for other fields.

8.2.4. AER Statistics / Counters

When PCIe AER errors are captured, the counters / statistics are also exposed in the form of sysfs attributes which are documented at Documentation/ABI/testing/sysfs-bus-pci-devices-aer_stats

8.3. Developer Guide

To enable error recovery, a software driver must provide callbacks.

To support AER better, developers need to understand how AER works.

PCIe errors are classified into two types: correctable errors and uncorrectable errors. This classification is based on the impact of those errors, which may result in degraded performance or function failure.

Correctable errors pose no impacts on the functionality of the interface. The PCIe 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 PCIe 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 PCIe link itself is fully functional. Fatal errors, on the other hand, cause the link to be unreliable.

When PCIe error reporting is enabled, a device will automatically send an error message to the Root Port above it when it captures an error. The Root Port, upon receiving an error reporting message, internally processes and logs the error message in its AER 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 the Root Error Command Register, the Root Port generates an interrupt when an error is detected.

Note that the errors as described above are related to the PCIe 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.

8.3.1. Provide callbacks

8.3.1.2. PCI error-recovery callbacks

The PCIe 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. The AER driver follows the rules defined in PCI Error Recovery except PCIe-specific parts (e.g. reset_link). Please refer to PCI Error Recovery for detailed definitions of the callbacks.

The sections below specify when to call the error callback functions.

8.3.1.3. Correctable errors

Correctable errors pose no impacts on the functionality of the interface. The PCIe 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.

8.3.1.4. 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 <==> Downstream Port B <==> Upstream Port A <==> Root Port

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 via callback parameter of pcie_do_recovery() function. If reset_link is not NULL, recovery function will use it to reset the link. If error_detected returns PCI_ERS_RESULT_CAN_RECOVER and reset_link returns PCI_ERS_RESULT_RECOVERED, the error handling goes to mmio_enabled.

8.3.2. Frequent Asked Questions

Q:

What happens if a PCIe 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.

8.4. Software error injection

Debugging PCIe AER error recovery code is quite difficult because it is hard to trigger real hardware errors. Software based error injection can be used to fake various kinds of PCIe errors.

First you should enable PCIe AER software error injection in kernel configuration, that is, following item should be in your .config.

CONFIG_PCIEAER_INJECT=y or CONFIG_PCIEAER_INJECT=m

After reboot with new kernel or insert the module, a device file named /dev/aer_inject should be created.

Then, you need a user space tool named aer-inject, which can be gotten from:

More information about aer-inject can be found in the document in its source code.