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1 The PCI Express Advanced Error Reporting Driver Guide HOWTO 2 T. Long Nguyen <email@example.com> 3 Yanmin Zhang <firstname.lastname@example.org> 4 07/29/2006 5 6 7 1. Overview 8 9 1.1 About this guide 10 11 This guide describes the basics of the PCI Express Advanced Error 12 Reporting (AER) driver and provides information on how to use it, as 13 well as how to enable the drivers of endpoint devices to conform with 14 PCI Express AER driver. 15 16 1.2 Copyright (C) Intel Corporation 2006. 17 18 1.3 What is the PCI Express AER Driver? 19 20 PCI Express error signaling can occur on the PCI Express link itself 21 or on behalf of transactions initiated on the link. PCI Express 22 defines two error reporting paradigms: the baseline capability and 23 the Advanced Error Reporting capability. The baseline capability is 24 required of all PCI Express components providing a minimum defined 25 set of error reporting requirements. Advanced Error Reporting 26 capability is implemented with a PCI Express advanced error reporting 27 extended capability structure providing more robust error reporting. 28 29 The PCI Express AER driver provides the infrastructure to support PCI 30 Express Advanced Error Reporting capability. The PCI Express AER 31 driver provides three basic functions: 32 33 - Gathers the comprehensive error information if errors occurred. 34 - Reports error to the users. 35 - Performs error recovery actions. 36 37 AER driver only attaches root ports which support PCI-Express AER 38 capability. 39 40 41 2. User Guide 42 43 2.1 Include the PCI Express AER Root Driver into the Linux Kernel 44 45 The PCI Express AER Root driver is a Root Port service driver attached 46 to the PCI Express Port Bus driver. If a user wants to use it, the driver 47 has to be compiled. Option CONFIG_PCIEAER supports this capability. It 48 depends on CONFIG_PCIEPORTBUS, so pls. set CONFIG_PCIEPORTBUS=y and 49 CONFIG_PCIEAER = y. 50 51 2.2 Load PCI Express AER Root Driver 52 53 Some systems have AER support in firmware. Enabling Linux AER support at 54 the same time the firmware handles AER may result in unpredictable 55 behavior. Therefore, Linux does not handle AER events unless the firmware 56 grants AER control to the OS via the ACPI _OSC method. See the PCI FW 3.0 57 Specification for details regarding _OSC usage. 58 59 2.3 AER error output 60 61 When a PCIe AER error is captured, an error message will be output to 62 console. If it's a correctable error, it is output as a warning. 63 Otherwise, it is printed as an error. So users could choose different 64 log level to filter out correctable error messages. 65 66 Below shows an example: 67 0000:50:00.0: PCIe Bus Error: severity=Uncorrected (Fatal), type=Transaction Layer, id=0500(Requester ID) 68 0000:50:00.0: device [8086:0329] error status/mask=00100000/00000000 69 0000:50:00.0:  Unsupported Request (First) 70 0000:50:00.0: TLP Header: 04000001 00200a03 05010000 00050100 71 72 In the example, 'Requester ID' means the ID of the device who sends 73 the error message to root port. Pls. refer to pci express specs for 74 other fields. 75 76 77 3. Developer Guide 78 79 To enable AER aware support requires a software driver to configure 80 the AER capability structure within its device and to provide callbacks. 81 82 To support AER better, developers need understand how AER does work 83 firstly. 84 85 PCI Express errors are classified into two types: correctable errors 86 and uncorrectable errors. This classification is based on the impacts 87 of those errors, which may result in degraded performance or function 88 failure. 89 90 Correctable errors pose no impacts on the functionality of the 91 interface. The PCI Express protocol can recover without any software 92 intervention or any loss of data. These errors are detected and 93 corrected by hardware. Unlike correctable errors, uncorrectable 94 errors impact functionality of the interface. Uncorrectable errors 95 can cause a particular transaction or a particular PCI Express link 96 to be unreliable. Depending on those error conditions, uncorrectable 97 errors are further classified into non-fatal errors and fatal errors. 98 Non-fatal errors cause the particular transaction to be unreliable, 99 but the PCI Express link itself is fully functional. Fatal errors, on 100 the other hand, cause the link to be unreliable. 101 102 When AER is enabled, a PCI Express device will automatically send an 103 error message to the PCIe root port above it when the device captures 104 an error. The Root Port, upon receiving an error reporting message, 105 internally processes and logs the error message in its PCI Express 106 capability structure. Error information being logged includes storing 107 the error reporting agent's requestor ID into the Error Source 108 Identification Registers and setting the error bits of the Root Error 109 Status Register accordingly. If AER error reporting is enabled in Root 110 Error Command Register, the Root Port generates an interrupt if an 111 error is detected. 112 113 Note that the errors as described above are related to the PCI Express 114 hierarchy and links. These errors do not include any device specific 115 errors because device specific errors will still get sent directly to 116 the device driver. 117 118 3.1 Configure the AER capability structure 119 120 AER aware drivers of PCI Express component need change the device 121 control registers to enable AER. They also could change AER registers, 122 including mask and severity registers. Helper function 123 pci_enable_pcie_error_reporting could be used to enable AER. See 124 section 3.3. 125 126 3.2. Provide callbacks 127 128 3.2.1 callback reset_link to reset pci express link 129 130 This callback is used to reset the pci express physical link when a 131 fatal error happens. The root port aer service driver provides a 132 default reset_link function, but different upstream ports might 133 have different specifications to reset pci express link, so all 134 upstream ports should provide their own reset_link functions. 135 136 In struct pcie_port_service_driver, a new pointer, reset_link, is 137 added. 138 139 pci_ers_result_t (*reset_link) (struct pci_dev *dev); 140 141 Section 220.127.116.11 provides more detailed info on when to call 142 reset_link. 143 144 3.2.2 PCI error-recovery callbacks 145 146 The PCI Express AER Root driver uses error callbacks to coordinate 147 with downstream device drivers associated with a hierarchy in question 148 when performing error recovery actions. 149 150 Data struct pci_driver has a pointer, err_handler, to point to 151 pci_error_handlers who consists of a couple of callback function 152 pointers. AER driver follows the rules defined in 153 pci-error-recovery.txt except pci express specific parts (e.g. 154 reset_link). Pls. refer to pci-error-recovery.txt for detailed 155 definitions of the callbacks. 156 157 Below sections specify when to call the error callback functions. 158 159 18.104.22.168 Correctable errors 160 161 Correctable errors pose no impacts on the functionality of 162 the interface. The PCI Express protocol can recover without any 163 software intervention or any loss of data. These errors do not 164 require any recovery actions. The AER driver clears the device's 165 correctable error status register accordingly and logs these errors. 166 167 22.214.171.124 Non-correctable (non-fatal and fatal) errors 168 169 If an error message indicates a non-fatal error, performing link reset 170 at upstream is not required. The AER driver calls error_detected(dev, 171 pci_channel_io_normal) to all drivers associated within a hierarchy in 172 question. for example, 173 EndPoint<==>DownstreamPort B<==>UpstreamPort A<==>RootPort. 174 If Upstream port A captures an AER error, the hierarchy consists of 175 Downstream port B and EndPoint. 176 177 A driver may return PCI_ERS_RESULT_CAN_RECOVER, 178 PCI_ERS_RESULT_DISCONNECT, or PCI_ERS_RESULT_NEED_RESET, depending on 179 whether it can recover or the AER driver calls mmio_enabled as next. 180 181 If an error message indicates a fatal error, kernel will broadcast 182 error_detected(dev, pci_channel_io_frozen) to all drivers within 183 a hierarchy in question. Then, performing link reset at upstream is 184 necessary. As different kinds of devices might use different approaches 185 to reset link, AER port service driver is required to provide the 186 function to reset link. Firstly, kernel looks for if the upstream 187 component has an aer driver. If it has, kernel uses the reset_link 188 callback of the aer driver. If the upstream component has no aer driver 189 and the port is downstream port, we will perform a hot reset as the 190 default by setting the Secondary Bus Reset bit of the Bridge Control 191 register associated with the downstream port. As for upstream ports, 192 they should provide their own aer service drivers with reset_link 193 function. If error_detected returns PCI_ERS_RESULT_CAN_RECOVER and 194 reset_link returns PCI_ERS_RESULT_RECOVERED, the error handling goes 195 to mmio_enabled. 196 197 3.3 helper functions 198 199 3.3.1 int pci_enable_pcie_error_reporting(struct pci_dev *dev); 200 pci_enable_pcie_error_reporting enables the device to send error 201 messages to root port when an error is detected. Note that devices 202 don't enable the error reporting by default, so device drivers need 203 call this function to enable it. 204 205 3.3.2 int pci_disable_pcie_error_reporting(struct pci_dev *dev); 206 pci_disable_pcie_error_reporting disables the device to send error 207 messages to root port when an error is detected. 208 209 3.3.3 int pci_cleanup_aer_uncorrect_error_status(struct pci_dev *dev); 210 pci_cleanup_aer_uncorrect_error_status cleanups the uncorrectable 211 error status register. 212 213 3.4 Frequent Asked Questions 214 215 Q: What happens if a PCI Express device driver does not provide an 216 error recovery handler (pci_driver->err_handler is equal to NULL)? 217 218 A: The devices attached with the driver won't be recovered. If the 219 error is fatal, kernel will print out warning messages. Please refer 220 to section 3 for more information. 221 222 Q: What happens if an upstream port service driver does not provide 223 callback reset_link? 224 225 A: Fatal error recovery will fail if the errors are reported by the 226 upstream ports who are attached by the service driver. 227 228 Q: How does this infrastructure deal with driver that is not PCI 229 Express aware? 230 231 A: This infrastructure calls the error callback functions of the 232 driver when an error happens. But if the driver is not aware of 233 PCI Express, the device might not report its own errors to root 234 port. 235 236 Q: What modifications will that driver need to make it compatible 237 with the PCI Express AER Root driver? 238 239 A: It could call the helper functions to enable AER in devices and 240 cleanup uncorrectable status register. Pls. refer to section 3.3. 241 242 243 4. Software error injection 244 245 Debugging PCIe AER error recovery code is quite difficult because it 246 is hard to trigger real hardware errors. Software based error 247 injection can be used to fake various kinds of PCIe errors. 248 249 First you should enable PCIe AER software error injection in kernel 250 configuration, that is, following item should be in your .config. 251 252 CONFIG_PCIEAER_INJECT=y or CONFIG_PCIEAER_INJECT=m 253 254 After reboot with new kernel or insert the module, a device file named 255 /dev/aer_inject should be created. 256 257 Then, you need a user space tool named aer-inject, which can be gotten 258 from: 259 http://www.kernel.org/pub/linux/utils/pci/aer-inject/ 260 261 More information about aer-inject can be found in the document comes 262 with its source code.