1 ==========================
2 PCI Bus EEH Error Recovery
3 ==========================
4
5 Linas Vepstas <linas@austin.ibm.com>
6
7 12 January 2005
8
9
10 Overview:
11 ---------
12 The IBM POWER-based pSeries and iSeries computers include PCI bus
13 controller chips that have extended capabilities for detecting and
14 reporting a large variety of PCI bus error conditions. These features
15 go under the name of "EEH", for "Enhanced Error Handling". The EEH
16 hardware features allow PCI bus errors to be cleared and a PCI
17 card to be "rebooted", without also having to reboot the operating
18 system.
19
20 This is in contrast to traditional PCI error handling, where the
21 PCI chip is wired directly to the CPU, and an error would cause
22 a CPU machine-check/check-stop condition, halting the CPU entirely.
23 Another "traditional" technique is to ignore such errors, which
24 can lead to data corruption, both of user data or of kernel data,
25 hung/unresponsive adapters, or system crashes/lockups. Thus,
26 the idea behind EEH is that the operating system can become more
27 reliable and robust by protecting it from PCI errors, and giving
28 the OS the ability to "reboot"/recover individual PCI devices.
29
30 Future systems from other vendors, based on the PCI-E specification,
31 may contain similar features.
32
33
34 Causes of EEH Errors
35 --------------------
36 EEH was originally designed to guard against hardware failure, such
37 as PCI cards dying from heat, humidity, dust, vibration and bad
38 electrical connections. The vast majority of EEH errors seen in
39 "real life" are due to either poorly seated PCI cards, or,
40 unfortunately quite commonly, due to device driver bugs, device firmware
41 bugs, and sometimes PCI card hardware bugs.
42
43 The most common software bug, is one that causes the device to
44 attempt to DMA to a location in system memory that has not been
45 reserved for DMA access for that card. This is a powerful feature,
46 as it prevents what; otherwise, would have been silent memory
47 corruption caused by the bad DMA. A number of device driver
48 bugs have been found and fixed in this way over the past few
49 years. Other possible causes of EEH errors include data or
50 address line parity errors (for example, due to poor electrical
51 connectivity due to a poorly seated card), and PCI-X split-completion
52 errors (due to software, device firmware, or device PCI hardware bugs).
53 The vast majority of "true hardware failures" can be cured by
54 physically removing and re-seating the PCI card.
55
56
57 Detection and Recovery
58 ----------------------
59 In the following discussion, a generic overview of how to detect
60 and recover from EEH errors will be presented. This is followed
61 by an overview of how the current implementation in the Linux
62 kernel does it. The actual implementation is subject to change,
63 and some of the finer points are still being debated. These
64 may in turn be swayed if or when other architectures implement
65 similar functionality.
66
67 When a PCI Host Bridge (PHB, the bus controller connecting the
68 PCI bus to the system CPU electronics complex) detects a PCI error
69 condition, it will "isolate" the affected PCI card. Isolation
70 will block all writes (either to the card from the system, or
71 from the card to the system), and it will cause all reads to
72 return all-ff's (0xff, 0xffff, 0xffffffff for 8/16/32-bit reads).
73 This value was chosen because it is the same value you would
74 get if the device was physically unplugged from the slot.
75 This includes access to PCI memory, I/O space, and PCI config
76 space. Interrupts; however, will continue to be delivered.
77
78 Detection and recovery are performed with the aid of ppc64
79 firmware. The programming interfaces in the Linux kernel
80 into the firmware are referred to as RTAS (Run-Time Abstraction
81 Services). The Linux kernel does not (should not) access
82 the EEH function in the PCI chipsets directly, primarily because
83 there are a number of different chipsets out there, each with
84 different interfaces and quirks. The firmware provides a
85 uniform abstraction layer that will work with all pSeries
86 and iSeries hardware (and be forwards-compatible).
87
88 If the OS or device driver suspects that a PCI slot has been
89 EEH-isolated, there is a firmware call it can make to determine if
90 this is the case. If so, then the device driver should put itself
91 into a consistent state (given that it won't be able to complete any
92 pending work) and start recovery of the card. Recovery normally
93 would consist of resetting the PCI device (holding the PCI #RST
94 line high for two seconds), followed by setting up the device
95 config space (the base address registers (BAR's), latency timer,
96 cache line size, interrupt line, and so on). This is followed by a
97 reinitialization of the device driver. In a worst-case scenario,
98 the power to the card can be toggled, at least on hot-plug-capable
99 slots. In principle, layers far above the device driver probably
100 do not need to know that the PCI card has been "rebooted" in this
101 way; ideally, there should be at most a pause in Ethernet/disk/USB
102 I/O while the card is being reset.
103
104 If the card cannot be recovered after three or four resets, the
105 kernel/device driver should assume the worst-case scenario, that the
106 card has died completely, and report this error to the sysadmin.
107 In addition, error messages are reported through RTAS and also through
108 syslogd (/var/log/messages) to alert the sysadmin of PCI resets.
109 The correct way to deal with failed adapters is to use the standard
110 PCI hotplug tools to remove and replace the dead card.
111
112
113 Current PPC64 Linux EEH Implementation
114 --------------------------------------
115 At this time, a generic EEH recovery mechanism has been implemented,
116 so that individual device drivers do not need to be modified to support
117 EEH recovery. This generic mechanism piggy-backs on the PCI hotplug
118 infrastructure, and percolates events up through the userspace/udev
119 infrastructure. Following is a detailed description of how this is
120 accomplished.
121
122 EEH must be enabled in the PHB's very early during the boot process,
123 and if a PCI slot is hot-plugged. The former is performed by
124 eeh_init() in arch/powerpc/platforms/pseries/eeh.c, and the later by
125 drivers/pci/hotplug/pSeries_pci.c calling in to the eeh.c code.
126 EEH must be enabled before a PCI scan of the device can proceed.
127 Current Power5 hardware will not work unless EEH is enabled;
128 although older Power4 can run with it disabled. Effectively,
129 EEH can no longer be turned off. PCI devices *must* be
130 registered with the EEH code; the EEH code needs to know about
131 the I/O address ranges of the PCI device in order to detect an
132 error. Given an arbitrary address, the routine
133 pci_get_device_by_addr() will find the pci device associated
134 with that address (if any).
135
136 The default arch/powerpc/include/asm/io.h macros readb(), inb(), insb(),
137 etc. include a check to see if the i/o read returned all-0xff's.
138 If so, these make a call to eeh_dn_check_failure(), which in turn
139 asks the firmware if the all-ff's value is the sign of a true EEH
140 error. If it is not, processing continues as normal. The grand
141 total number of these false alarms or "false positives" can be
142 seen in /proc/ppc64/eeh (subject to change). Normally, almost
143 all of these occur during boot, when the PCI bus is scanned, where
144 a large number of 0xff reads are part of the bus scan procedure.
145
146 If a frozen slot is detected, code in
147 arch/powerpc/platforms/pseries/eeh.c will print a stack trace to
148 syslog (/var/log/messages). This stack trace has proven to be very
149 useful to device-driver authors for finding out at what point the EEH
150 error was detected, as the error itself usually occurs slightly
151 beforehand.
152
153 Next, it uses the Linux kernel notifier chain/work queue mechanism to
154 allow any interested parties to find out about the failure. Device
155 drivers, or other parts of the kernel, can use
156 `eeh_register_notifier(struct notifier_block *)` to find out about EEH
157 events. The event will include a pointer to the pci device, the
158 device node and some state info. Receivers of the event can "do as
159 they wish"; the default handler will be described further in this
160 section.
161
162 To assist in the recovery of the device, eeh.c exports the
163 following functions:
164
165 rtas_set_slot_reset()
166 assert the PCI #RST line for 1/8th of a second
167 rtas_configure_bridge()
168 ask firmware to configure any PCI bridges
169 located topologically under the pci slot.
170 eeh_save_bars() and eeh_restore_bars():
171 save and restore the PCI
172 config-space info for a device and any devices under it.
173
174
175 A handler for the EEH notifier_block events is implemented in
176 drivers/pci/hotplug/pSeries_pci.c, called handle_eeh_events().
177 It saves the device BAR's and then calls rpaphp_unconfig_pci_adapter().
178 This last call causes the device driver for the card to be stopped,
179 which causes uevents to go out to user space. This triggers
180 user-space scripts that might issue commands such as "ifdown eth0"
181 for ethernet cards, and so on. This handler then sleeps for 5 seconds,
182 hoping to give the user-space scripts enough time to complete.
183 It then resets the PCI card, reconfigures the device BAR's, and
184 any bridges underneath. It then calls rpaphp_enable_pci_slot(),
185 which restarts the device driver and triggers more user-space
186 events (for example, calling "ifup eth0" for ethernet cards).
187
188
189 Device Shutdown and User-Space Events
190 -------------------------------------
191 This section documents what happens when a pci slot is unconfigured,
192 focusing on how the device driver gets shut down, and on how the
193 events get delivered to user-space scripts.
194
195 Following is an example sequence of events that cause a device driver
196 close function to be called during the first phase of an EEH reset.
197 The following sequence is an example of the pcnet32 device driver::
198
199 rpa_php_unconfig_pci_adapter (struct slot *) // in rpaphp_pci.c
200 {
201 calls
202 pci_remove_bus_device (struct pci_dev *) // in /drivers/pci/remove.c
203 {
204 calls
205 pci_destroy_dev (struct pci_dev *)
206 {
207 calls
208 device_unregister (&dev->dev) // in /drivers/base/core.c
209 {
210 calls
211 device_del (struct device *)
212 {
213 calls
214 bus_remove_device() // in /drivers/base/bus.c
215 {
216 calls
217 device_release_driver()
218 {
219 calls
220 struct device_driver->remove() which is just
221 pci_device_remove() // in /drivers/pci/pci_driver.c
222 {
223 calls
224 struct pci_driver->remove() which is just
225 pcnet32_remove_one() // in /drivers/net/pcnet32.c
226 {
227 calls
228 unregister_netdev() // in /net/core/dev.c
229 {
230 calls
231 dev_close() // in /net/core/dev.c
232 {
233 calls dev->stop();
234 which is just pcnet32_close() // in pcnet32.c
235 {
236 which does what you wanted
237 to stop the device
238 }
239 }
240 }
241 which
242 frees pcnet32 device driver memory
243 }
244 }}}}}}
245
246
247 in drivers/pci/pci_driver.c,
248 struct device_driver->remove() is just pci_device_remove()
249 which calls struct pci_driver->remove() which is pcnet32_remove_one()
250 which calls unregister_netdev() (in net/core/dev.c)
251 which calls dev_close() (in net/core/dev.c)
252 which calls dev->stop() which is pcnet32_close()
253 which then does the appropriate shutdown.
254
255 ---
256
257 Following is the analogous stack trace for events sent to user-space
258 when the pci device is unconfigured::
259
260 rpa_php_unconfig_pci_adapter() { // in rpaphp_pci.c
261 calls
262 pci_remove_bus_device (struct pci_dev *) { // in /drivers/pci/remove.c
263 calls
264 pci_destroy_dev (struct pci_dev *) {
265 calls
266 device_unregister (&dev->dev) { // in /drivers/base/core.c
267 calls
268 device_del(struct device * dev) { // in /drivers/base/core.c
269 calls
270 kobject_del() { //in /libs/kobject.c
271 calls
272 kobject_uevent() { // in /libs/kobject.c
273 calls
274 kset_uevent() { // in /lib/kobject.c
275 calls
276 kset->uevent_ops->uevent() // which is really just
277 a call to
278 dev_uevent() { // in /drivers/base/core.c
279 calls
280 dev->bus->uevent() which is really just a call to
281 pci_uevent () { // in drivers/pci/hotplug.c
282 which prints device name, etc....
283 }
284 }
285 then kobject_uevent() sends a netlink uevent to userspace
286 --> userspace uevent
287 (during early boot, nobody listens to netlink events and
288 kobject_uevent() executes uevent_helper[], which runs the
289 event process /sbin/hotplug)
290 }
291 }
292 kobject_del() then calls sysfs_remove_dir(), which would
293 trigger any user-space daemon that was watching /sysfs,
294 and notice the delete event.
295
296
297 Pro's and Con's of the Current Design
298 -------------------------------------
299 There are several issues with the current EEH software recovery design,
300 which may be addressed in future revisions. But first, note that the
301 big plus of the current design is that no changes need to be made to
302 individual device drivers, so that the current design throws a wide net.
303 The biggest negative of the design is that it potentially disturbs
304 network daemons and file systems that didn't need to be disturbed.
305
306 - A minor complaint is that resetting the network card causes
307 user-space back-to-back ifdown/ifup burps that potentially disturb
308 network daemons, that didn't need to even know that the pci
309 card was being rebooted.
310
311 - A more serious concern is that the same reset, for SCSI devices,
312 causes havoc to mounted file systems. Scripts cannot post-facto
313 unmount a file system without flushing pending buffers, but this
314 is impossible, because I/O has already been stopped. Thus,
315 ideally, the reset should happen at or below the block layer,
316 so that the file systems are not disturbed.
317
318 Reiserfs does not tolerate errors returned from the block device.
319 Ext3fs seems to be tolerant, retrying reads/writes until it does
320 succeed. Both have been only lightly tested in this scenario.
321
322 The SCSI-generic subsystem already has built-in code for performing
323 SCSI device resets, SCSI bus resets, and SCSI host-bus-adapter
324 (HBA) resets. These are cascaded into a chain of attempted
325 resets if a SCSI command fails. These are completely hidden
326 from the block layer. It would be very natural to add an EEH
327 reset into this chain of events.
328
329 - If a SCSI error occurs for the root device, all is lost unless
330 the sysadmin had the foresight to run /bin, /sbin, /etc, /var
331 and so on, out of ramdisk/tmpfs.
332
333
334 Conclusions
335 -----------
336 There's forward progress ...
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