Root/Documentation/powerpc/eeh-pci-error-recovery.txt

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

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