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

===========================================================
The PCI Express Advanced Error Reporting Driver Guide HOWTO
===========================================================

:Authors: - T. Long Nguyen <tom.l.nguyen@intel.com>
          - Yanmin Zhang <yanmin.zhang@intel.com>

:Copyright: |copy| 2006 Intel Corporation

Overview
===========

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.


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.


User Guide
==========

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.

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.

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.

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

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.

Provide callbacks
-----------------

callback reset_link to reset PCIe link
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This callback is used to reset the PCIe 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 the PCIe link, so
Upstream Port drivers may provide their own reset_link functions.

Section 3.2.2.2 provides more detailed info on when to call
reset_link.

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.rst except PCIe-specific parts (e.g.
reset_link). Please refer to pci-error-recovery.rst for detailed
definitions of the callbacks.

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

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.

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.

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.


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:

    https://git.kernel.org/cgit/linux/kernel/git/gong.chen/aer-inject.git/

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