1.. SPDX-License-Identifier: GPL-2.0
2
3==================
4PCI Error Recovery
5==================
6
7
8:Authors: - Linas Vepstas <linasvepstas@gmail.com>
9          - Richard Lary <rlary@us.ibm.com>
10          - Mike Mason <mmlnx@us.ibm.com>
11
12
13Many PCI bus controllers are able to detect a variety of hardware
14PCI errors on the bus, such as parity errors on the data and address
15buses, as well as SERR and PERR errors.  Some of the more advanced
16chipsets are able to deal with these errors; these include PCI-E chipsets,
17and the PCI-host bridges found on IBM Power4, Power5 and Power6-based
18pSeries boxes. A typical action taken is to disconnect the affected device,
19halting all I/O to it.  The goal of a disconnection is to avoid system
20corruption; for example, to halt system memory corruption due to DMAs
21to "wild" addresses. Typically, a reconnection mechanism is also
22offered, so that the affected PCI device(s) are reset and put back
23into working condition. The reset phase requires coordination
24between the affected device drivers and the PCI controller chip.
25This document describes a generic API for notifying device drivers
26of a bus disconnection, and then performing error recovery.
27This API is currently implemented in the 2.6.16 and later kernels.
28
29Reporting and recovery is performed in several steps. First, when
30a PCI hardware error has resulted in a bus disconnect, that event
31is reported as soon as possible to all affected device drivers,
32including multiple instances of a device driver on multi-function
33cards. This allows device drivers to avoid deadlocking in spinloops,
34waiting for some i/o-space register to change, when it never will.
35It also gives the drivers a chance to defer incoming I/O as
36needed.
37
38Next, recovery is performed in several stages. Most of the complexity
39is forced by the need to handle multi-function devices, that is,
40devices that have multiple device drivers associated with them.
41In the first stage, each driver is allowed to indicate what type
42of reset it desires, the choices being a simple re-enabling of I/O
43or requesting a slot reset.
44
45If any driver requests a slot reset, that is what will be done.
46
47After a reset and/or a re-enabling of I/O, all drivers are
48again notified, so that they may then perform any device setup/config
49that may be required.  After these have all completed, a final
50"resume normal operations" event is sent out.
51
52The biggest reason for choosing a kernel-based implementation rather
53than a user-space implementation was the need to deal with bus
54disconnects of PCI devices attached to storage media, and, in particular,
55disconnects from devices holding the root file system.  If the root
56file system is disconnected, a user-space mechanism would have to go
57through a large number of contortions to complete recovery. Almost all
58of the current Linux file systems are not tolerant of disconnection
59from/reconnection to their underlying block device. By contrast,
60bus errors are easy to manage in the device driver. Indeed, most
61device drivers already handle very similar recovery procedures;
62for example, the SCSI-generic layer already provides significant
63mechanisms for dealing with SCSI bus errors and SCSI bus resets.
64
65
66Detailed Design
67===============
68
69Design and implementation details below, based on a chain of
70public email discussions with Ben Herrenschmidt, circa 5 April 2005.
71
72The error recovery API support is exposed to the driver in the form of
73a structure of function pointers pointed to by a new field in struct
74pci_driver. A driver that fails to provide the structure is "non-aware",
75and the actual recovery steps taken are platform dependent.  The
76arch/powerpc implementation will simulate a PCI hotplug remove/add.
77
78This structure has the form::
79
80	struct pci_error_handlers
81	{
82		int (*error_detected)(struct pci_dev *dev, pci_channel_state_t);
83		int (*mmio_enabled)(struct pci_dev *dev);
84		int (*slot_reset)(struct pci_dev *dev);
85		void (*resume)(struct pci_dev *dev);
86		void (*cor_error_detected)(struct pci_dev *dev);
87	};
88
89The possible channel states are::
90
91	typedef enum {
92		pci_channel_io_normal,  /* I/O channel is in normal state */
93		pci_channel_io_frozen,  /* I/O to channel is blocked */
94		pci_channel_io_perm_failure, /* PCI card is dead */
95	} pci_channel_state_t;
96
97Possible return values are::
98
99	enum pci_ers_result {
100		PCI_ERS_RESULT_NONE,        /* no result/none/not supported in device driver */
101		PCI_ERS_RESULT_CAN_RECOVER, /* Device driver can recover without slot reset */
102		PCI_ERS_RESULT_NEED_RESET,  /* Device driver wants slot to be reset. */
103		PCI_ERS_RESULT_DISCONNECT,  /* Device has completely failed, is unrecoverable */
104		PCI_ERS_RESULT_RECOVERED,   /* Device driver is fully recovered and operational */
105	};
106
107A driver does not have to implement all of these callbacks; however,
108if it implements any, it must implement error_detected(). If a callback
109is not implemented, the corresponding feature is considered unsupported.
110For example, if mmio_enabled() and resume() aren't there, then it
111is assumed that the driver is not doing any direct recovery and requires
112a slot reset.  Typically a driver will want to know about
113a slot_reset().
114
115The actual steps taken by a platform to recover from a PCI error
116event will be platform-dependent, but will follow the general
117sequence described below.
118
119STEP 0: Error Event
120-------------------
121A PCI bus error is detected by the PCI hardware.  On powerpc, the slot
122is isolated, in that all I/O is blocked: all reads return 0xffffffff,
123all writes are ignored.
124
125
126STEP 1: Notification
127--------------------
128Platform calls the error_detected() callback on every instance of
129every driver affected by the error.
130
131At this point, the device might not be accessible anymore, depending on
132the platform (the slot will be isolated on powerpc). The driver may
133already have "noticed" the error because of a failing I/O, but this
134is the proper "synchronization point", that is, it gives the driver
135a chance to cleanup, waiting for pending stuff (timers, whatever, etc...)
136to complete; it can take semaphores, schedule, etc... everything but
137touch the device. Within this function and after it returns, the driver
138shouldn't do any new IOs. Called in task context. This is sort of a
139"quiesce" point. See note about interrupts at the end of this doc.
140
141All drivers participating in this system must implement this call.
142The driver must return one of the following result codes:
143
144  - PCI_ERS_RESULT_CAN_RECOVER
145      Driver returns this if it thinks it might be able to recover
146      the HW by just banging IOs or if it wants to be given
147      a chance to extract some diagnostic information (see
148      mmio_enable, below).
149  - PCI_ERS_RESULT_NEED_RESET
150      Driver returns this if it can't recover without a
151      slot reset.
152  - PCI_ERS_RESULT_DISCONNECT
153      Driver returns this if it doesn't want to recover at all.
154
155The next step taken will depend on the result codes returned by the
156drivers.
157
158If all drivers on the segment/slot return PCI_ERS_RESULT_CAN_RECOVER,
159then the platform should re-enable IOs on the slot (or do nothing in
160particular, if the platform doesn't isolate slots), and recovery
161proceeds to STEP 2 (MMIO Enable).
162
163If any driver requested a slot reset (by returning PCI_ERS_RESULT_NEED_RESET),
164then recovery proceeds to STEP 4 (Slot Reset).
165
166If the platform is unable to recover the slot, the next step
167is STEP 6 (Permanent Failure).
168
169.. note::
170
171   The current powerpc implementation assumes that a device driver will
172   *not* schedule or semaphore in this routine; the current powerpc
173   implementation uses one kernel thread to notify all devices;
174   thus, if one device sleeps/schedules, all devices are affected.
175   Doing better requires complex multi-threaded logic in the error
176   recovery implementation (e.g. waiting for all notification threads
177   to "join" before proceeding with recovery.)  This seems excessively
178   complex and not worth implementing.
179
180   The current powerpc implementation doesn't much care if the device
181   attempts I/O at this point, or not.  I/Os will fail, returning
182   a value of 0xff on read, and writes will be dropped. If more than
183   EEH_MAX_FAILS I/Os are attempted to a frozen adapter, EEH
184   assumes that the device driver has gone into an infinite loop
185   and prints an error to syslog.  A reboot is then required to
186   get the device working again.
187
188STEP 2: MMIO Enabled
189--------------------
190The platform re-enables MMIO to the device (but typically not the
191DMA), and then calls the mmio_enabled() callback on all affected
192device drivers.
193
194This is the "early recovery" call. IOs are allowed again, but DMA is
195not, with some restrictions. This is NOT a callback for the driver to
196start operations again, only to peek/poke at the device, extract diagnostic
197information, if any, and eventually do things like trigger a device local
198reset or some such, but not restart operations. This callback is made if
199all drivers on a segment agree that they can try to recover and if no automatic
200link reset was performed by the HW. If the platform can't just re-enable IOs
201without a slot reset or a link reset, it will not call this callback, and
202instead will have gone directly to STEP 3 (Link Reset) or STEP 4 (Slot Reset)
203
204.. note::
205
206   The following is proposed; no platform implements this yet:
207   Proposal: All I/Os should be done _synchronously_ from within
208   this callback, errors triggered by them will be returned via
209   the normal pci_check_whatever() API, no new error_detected()
210   callback will be issued due to an error happening here. However,
211   such an error might cause IOs to be re-blocked for the whole
212   segment, and thus invalidate the recovery that other devices
213   on the same segment might have done, forcing the whole segment
214   into one of the next states, that is, link reset or slot reset.
215
216The driver should return one of the following result codes:
217  - PCI_ERS_RESULT_RECOVERED
218      Driver returns this if it thinks the device is fully
219      functional and thinks it is ready to start
220      normal driver operations again. There is no
221      guarantee that the driver will actually be
222      allowed to proceed, as another driver on the
223      same segment might have failed and thus triggered a
224      slot reset on platforms that support it.
225
226  - PCI_ERS_RESULT_NEED_RESET
227      Driver returns this if it thinks the device is not
228      recoverable in its current state and it needs a slot
229      reset to proceed.
230
231  - PCI_ERS_RESULT_DISCONNECT
232      Same as above. Total failure, no recovery even after
233      reset driver dead. (To be defined more precisely)
234
235The next step taken depends on the results returned by the drivers.
236If all drivers returned PCI_ERS_RESULT_RECOVERED, then the platform
237proceeds to either STEP3 (Link Reset) or to STEP 5 (Resume Operations).
238
239If any driver returned PCI_ERS_RESULT_NEED_RESET, then the platform
240proceeds to STEP 4 (Slot Reset)
241
242STEP 3: Link Reset
243------------------
244The platform resets the link.  This is a PCI-Express specific step
245and is done whenever a fatal error has been detected that can be
246"solved" by resetting the link.
247
248STEP 4: Slot Reset
249------------------
250
251In response to a return value of PCI_ERS_RESULT_NEED_RESET, the
252platform will perform a slot reset on the requesting PCI device(s).
253The actual steps taken by a platform to perform a slot reset
254will be platform-dependent. Upon completion of slot reset, the
255platform will call the device slot_reset() callback.
256
257Powerpc platforms implement two levels of slot reset:
258soft reset(default) and fundamental(optional) reset.
259
260Powerpc soft reset consists of asserting the adapter #RST line and then
261restoring the PCI BARs and PCI configuration header to a state
262that is equivalent to what it would be after a fresh system
263power-on followed by power-on BIOS/system firmware initialization.
264Soft reset is also known as hot-reset.
265
266Powerpc fundamental reset is supported by PCI Express cards only
267and results in device's state machines, hardware logic, port states and
268configuration registers to initialize to their default conditions.
269
270For most PCI devices, a soft reset will be sufficient for recovery.
271Optional fundamental reset is provided to support a limited number
272of PCI Express devices for which a soft reset is not sufficient
273for recovery.
274
275If the platform supports PCI hotplug, then the reset might be
276performed by toggling the slot electrical power off/on.
277
278It is important for the platform to restore the PCI config space
279to the "fresh poweron" state, rather than the "last state". After
280a slot reset, the device driver will almost always use its standard
281device initialization routines, and an unusual config space setup
282may result in hung devices, kernel panics, or silent data corruption.
283
284This call gives drivers the chance to re-initialize the hardware
285(re-download firmware, etc.).  At this point, the driver may assume
286that the card is in a fresh state and is fully functional. The slot
287is unfrozen and the driver has full access to PCI config space,
288memory mapped I/O space and DMA. Interrupts (Legacy, MSI, or MSI-X)
289will also be available.
290
291Drivers should not restart normal I/O processing operations
292at this point.  If all device drivers report success on this
293callback, the platform will call resume() to complete the sequence,
294and let the driver restart normal I/O processing.
295
296A driver can still return a critical failure for this function if
297it can't get the device operational after reset.  If the platform
298previously tried a soft reset, it might now try a hard reset (power
299cycle) and then call slot_reset() again.  If the device still can't
300be recovered, there is nothing more that can be done;  the platform
301will typically report a "permanent failure" in such a case.  The
302device will be considered "dead" in this case.
303
304Drivers for multi-function cards will need to coordinate among
305themselves as to which driver instance will perform any "one-shot"
306or global device initialization. For example, the Symbios sym53cxx2
307driver performs device init only from PCI function 0::
308
309	+       if (PCI_FUNC(pdev->devfn) == 0)
310	+               sym_reset_scsi_bus(np, 0);
311
312Result codes:
313	- PCI_ERS_RESULT_DISCONNECT
314	  Same as above.
315
316Drivers for PCI Express cards that require a fundamental reset must
317set the needs_freset bit in the pci_dev structure in their probe function.
318For example, the QLogic qla2xxx driver sets the needs_freset bit for certain
319PCI card types::
320
321	+	/* Set EEH reset type to fundamental if required by hba  */
322	+	if (IS_QLA24XX(ha) || IS_QLA25XX(ha) || IS_QLA81XX(ha))
323	+		pdev->needs_freset = 1;
324	+
325
326Platform proceeds either to STEP 5 (Resume Operations) or STEP 6 (Permanent
327Failure).
328
329.. note::
330
331   The current powerpc implementation does not try a power-cycle
332   reset if the driver returned PCI_ERS_RESULT_DISCONNECT.
333   However, it probably should.
334
335
336STEP 5: Resume Operations
337-------------------------
338The platform will call the resume() callback on all affected device
339drivers if all drivers on the segment have returned
340PCI_ERS_RESULT_RECOVERED from one of the 3 previous callbacks.
341The goal of this callback is to tell the driver to restart activity,
342that everything is back and running. This callback does not return
343a result code.
344
345At this point, if a new error happens, the platform will restart
346a new error recovery sequence.
347
348STEP 6: Permanent Failure
349-------------------------
350A "permanent failure" has occurred, and the platform cannot recover
351the device.  The platform will call error_detected() with a
352pci_channel_state_t value of pci_channel_io_perm_failure.
353
354The device driver should, at this point, assume the worst. It should
355cancel all pending I/O, refuse all new I/O, returning -EIO to
356higher layers. The device driver should then clean up all of its
357memory and remove itself from kernel operations, much as it would
358during system shutdown.
359
360The platform will typically notify the system operator of the
361permanent failure in some way.  If the device is hotplug-capable,
362the operator will probably want to remove and replace the device.
363Note, however, not all failures are truly "permanent". Some are
364caused by over-heating, some by a poorly seated card. Many
365PCI error events are caused by software bugs, e.g. DMAs to
366wild addresses or bogus split transactions due to programming
367errors. See the discussion in Documentation/powerpc/eeh-pci-error-recovery.rst
368for additional detail on real-life experience of the causes of
369software errors.
370
371
372Conclusion; General Remarks
373---------------------------
374The way the callbacks are called is platform policy. A platform with
375no slot reset capability may want to just "ignore" drivers that can't
376recover (disconnect them) and try to let other cards on the same segment
377recover. Keep in mind that in most real life cases, though, there will
378be only one driver per segment.
379
380Now, a note about interrupts. If you get an interrupt and your
381device is dead or has been isolated, there is a problem :)
382The current policy is to turn this into a platform policy.
383That is, the recovery API only requires that:
384
385 - There is no guarantee that interrupt delivery can proceed from any
386   device on the segment starting from the error detection and until the
387   slot_reset callback is called, at which point interrupts are expected
388   to be fully operational.
389
390 - There is no guarantee that interrupt delivery is stopped, that is,
391   a driver that gets an interrupt after detecting an error, or that detects
392   an error within the interrupt handler such that it prevents proper
393   ack'ing of the interrupt (and thus removal of the source) should just
394   return IRQ_NOTHANDLED. It's up to the platform to deal with that
395   condition, typically by masking the IRQ source during the duration of
396   the error handling. It is expected that the platform "knows" which
397   interrupts are routed to error-management capable slots and can deal
398   with temporarily disabling that IRQ number during error processing (this
399   isn't terribly complex). That means some IRQ latency for other devices
400   sharing the interrupt, but there is simply no other way. High end
401   platforms aren't supposed to share interrupts between many devices
402   anyway :)
403
404.. note::
405
406   Implementation details for the powerpc platform are discussed in
407   the file Documentation/powerpc/eeh-pci-error-recovery.rst
408
409   As of this writing, there is a growing list of device drivers with
410   patches implementing error recovery. Not all of these patches are in
411   mainline yet. These may be used as "examples":
412
413   - drivers/scsi/ipr
414   - drivers/scsi/sym53c8xx_2
415   - drivers/scsi/qla2xxx
416   - drivers/scsi/lpfc
417   - drivers/next/bnx2.c
418   - drivers/next/e100.c
419   - drivers/net/e1000
420   - drivers/net/e1000e
421   - drivers/net/ixgbe
422   - drivers/net/cxgb3
423   - drivers/net/s2io.c
424
425   The cor_error_detected() callback is invoked in handle_error_source() when
426   the error severity is "correctable". The callback is optional and allows
427   additional logging to be done if desired. See example:
428
429   - drivers/cxl/pci.c
430
431The End
432-------
433