[PATCH 1/5] PCI-Express AER implemetation: aer howto document

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

Here are the updated patches. Thank Greg, Andi Kleen, Linas Vepstas and Arjan
for their comments.

The new howto document consists of overview, user guide and developer guide. I
also add more pointers to pci-error-recovery.txt.

The patches could be applied to 2.6.18-rc3 cleanly.

From: Zhang, Yanmin <yanmin.zhang@intel.com>

PCI-Express AER (Advanced Error Reporting) provides more robust error reporting.
The series of patches enable kernel support to AER.

The initial patches were written by Tom Long Nguyen. I ported them to the kernel
2.6.17. Many thanks to Rajesh Shah and Narayanan Chandramouli for their great
review comments and testing help.

Patch 1 consists of the pciaer-howto.txt document.

Signed-off-by: Zhang Yanmin <yanmin.zhang@intel.com> 

---

--- linux-2.6.17/Documentation/pcieaer-howto.txt	1970-01-01 08:00:00.000000000 +0800
+++ linux-2.6.17_aer/Documentation/pcieaer-howto.txt	2006-07-31 10:28:45.000000000 +0800
@@ -0,0 +1,253 @@
+   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.
+
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