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 ===========================================================
 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.

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