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+.. 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 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.
+
+
+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.
+
+
+User Guide
+==========
+
+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.
+
+Load PCI Express AER Root Driver
+--------------------------------
+
+Some systems have AER support in firmware. Enabling Linux AER support at
+the same time the firmware handles AER may 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 FW 3.0
+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 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::
+
+  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 who sends
+the error message to root port. Pls. refer to pci express 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 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.
+
+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.
+
+Provide callbacks
+-----------------
+
+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.
+
+Section 3.2.2.2 provides more detailed info on when to call
+reset_link.
+
+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.
+
+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.
+
+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 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.
+
+helper functions
+----------------
+::
+
+  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.
+
+::
+
+  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.
+
+::
+
+  int pci_aer_clear_nonfatal_status(struct pci_dev *dev);`
+
+pci_aer_clear_nonfatal_status clears non-fatal errors in the uncorrectable
+error status register.
+
+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.
+
+
+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 comes
+with its source code.