About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / PCI / pci-error-recovery.txt




Custom Search

Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1	
2	                       PCI Error Recovery
3	                       ------------------
4	                        February 2, 2006
5	
6	                 Current document maintainer:
7	             Linas Vepstas <linasvepstas@gmail.com>
8	          updated by Richard Lary <rlary@us.ibm.com>
9	       and Mike Mason <mmlnx@us.ibm.com> on 27-Jul-2009
10	
11	
12	Many PCI bus controllers are able to detect a variety of hardware
13	PCI errors on the bus, such as parity errors on the data and address
14	busses, as well as SERR and PERR errors.  Some of the more advanced
15	chipsets are able to deal with these errors; these include PCI-E chipsets,
16	and the PCI-host bridges found on IBM Power4, Power5 and Power6-based
17	pSeries boxes. A typical action taken is to disconnect the affected device,
18	halting all I/O to it.  The goal of a disconnection is to avoid system
19	corruption; for example, to halt system memory corruption due to DMA's
20	to "wild" addresses. Typically, a reconnection mechanism is also
21	offered, so that the affected PCI device(s) are reset and put back
22	into working condition. The reset phase requires coordination
23	between the affected device drivers and the PCI controller chip.
24	This document describes a generic API for notifying device drivers
25	of a bus disconnection, and then performing error recovery.
26	This API is currently implemented in the 2.6.16 and later kernels.
27	
28	Reporting and recovery is performed in several steps. First, when
29	a PCI hardware error has resulted in a bus disconnect, that event
30	is reported as soon as possible to all affected device drivers,
31	including multiple instances of a device driver on multi-function
32	cards. This allows device drivers to avoid deadlocking in spinloops,
33	waiting for some i/o-space register to change, when it never will.
34	It also gives the drivers a chance to defer incoming I/O as
35	needed.
36	
37	Next, recovery is performed in several stages. Most of the complexity
38	is forced by the need to handle multi-function devices, that is,
39	devices that have multiple device drivers associated with them.
40	In the first stage, each driver is allowed to indicate what type
41	of reset it desires, the choices being a simple re-enabling of I/O
42	or requesting a slot reset.
43	
44	If any driver requests a slot reset, that is what will be done.
45	
46	After a reset and/or a re-enabling of I/O, all drivers are
47	again notified, so that they may then perform any device setup/config
48	that may be required.  After these have all completed, a final
49	"resume normal operations" event is sent out.
50	
51	The biggest reason for choosing a kernel-based implementation rather
52	than a user-space implementation was the need to deal with bus
53	disconnects of PCI devices attached to storage media, and, in particular,
54	disconnects from devices holding the root file system.  If the root
55	file system is disconnected, a user-space mechanism would have to go
56	through a large number of contortions to complete recovery. Almost all
57	of the current Linux file systems are not tolerant of disconnection
58	from/reconnection to their underlying block device. By contrast,
59	bus errors are easy to manage in the device driver. Indeed, most
60	device drivers already handle very similar recovery procedures;
61	for example, the SCSI-generic layer already provides significant
62	mechanisms for dealing with SCSI bus errors and SCSI bus resets.
63	
64	
65	Detailed Design
66	---------------
67	Design and implementation details below, based on a chain of
68	public email discussions with Ben Herrenschmidt, circa 5 April 2005.
69	
70	The error recovery API support is exposed to the driver in the form of
71	a structure of function pointers pointed to by a new field in struct
72	pci_driver. A driver that fails to provide the structure is "non-aware",
73	and the actual recovery steps taken are platform dependent.  The
74	arch/powerpc implementation will simulate a PCI hotplug remove/add.
75	
76	This structure has the form:
77	struct pci_error_handlers
78	{
79		int (*error_detected)(struct pci_dev *dev, enum pci_channel_state);
80		int (*mmio_enabled)(struct pci_dev *dev);
81		int (*link_reset)(struct pci_dev *dev);
82		int (*slot_reset)(struct pci_dev *dev);
83		void (*resume)(struct pci_dev *dev);
84	};
85	
86	The possible channel states are:
87	enum pci_channel_state {
88		pci_channel_io_normal,  /* I/O channel is in normal state */
89		pci_channel_io_frozen,  /* I/O to channel is blocked */
90		pci_channel_io_perm_failure, /* PCI card is dead */
91	};
92	
93	Possible return values are:
94	enum pci_ers_result {
95		PCI_ERS_RESULT_NONE,        /* no result/none/not supported in device driver */
96		PCI_ERS_RESULT_CAN_RECOVER, /* Device driver can recover without slot reset */
97		PCI_ERS_RESULT_NEED_RESET,  /* Device driver wants slot to be reset. */
98		PCI_ERS_RESULT_DISCONNECT,  /* Device has completely failed, is unrecoverable */
99		PCI_ERS_RESULT_RECOVERED,   /* Device driver is fully recovered and operational */
100	};
101	
102	A driver does not have to implement all of these callbacks; however,
103	if it implements any, it must implement error_detected(). If a callback
104	is not implemented, the corresponding feature is considered unsupported.
105	For example, if mmio_enabled() and resume() aren't there, then it
106	is assumed that the driver is not doing any direct recovery and requires
107	a slot reset. If link_reset() is not implemented, the card is assumed to
108	not care about link resets. Typically a driver will want to know about
109	a slot_reset().
110	
111	The actual steps taken by a platform to recover from a PCI error
112	event will be platform-dependent, but will follow the general
113	sequence described below.
114	
115	STEP 0: Error Event
116	-------------------
117	A PCI bus error is detected by the PCI hardware.  On powerpc, the slot
118	is isolated, in that all I/O is blocked: all reads return 0xffffffff,
119	all writes are ignored.
120	
121	
122	STEP 1: Notification
123	--------------------
124	Platform calls the error_detected() callback on every instance of
125	every driver affected by the error.
126	
127	At this point, the device might not be accessible anymore, depending on
128	the platform (the slot will be isolated on powerpc). The driver may
129	already have "noticed" the error because of a failing I/O, but this
130	is the proper "synchronization point", that is, it gives the driver
131	a chance to cleanup, waiting for pending stuff (timers, whatever, etc...)
132	to complete; it can take semaphores, schedule, etc... everything but
133	touch the device. Within this function and after it returns, the driver
134	shouldn't do any new IOs. Called in task context. This is sort of a
135	"quiesce" point. See note about interrupts at the end of this doc.
136	
137	All drivers participating in this system must implement this call.
138	The driver must return one of the following result codes:
139			- PCI_ERS_RESULT_CAN_RECOVER:
140			  Driver returns this if it thinks it might be able to recover
141			  the HW by just banging IOs or if it wants to be given
142			  a chance to extract some diagnostic information (see
143			  mmio_enable, below).
144			- PCI_ERS_RESULT_NEED_RESET:
145			  Driver returns this if it can't recover without a
146			  slot reset.
147			- PCI_ERS_RESULT_DISCONNECT:
148			  Driver returns this if it doesn't want to recover at all.
149	
150	The next step taken will depend on the result codes returned by the
151	drivers.
152	
153	If all drivers on the segment/slot return PCI_ERS_RESULT_CAN_RECOVER,
154	then the platform should re-enable IOs on the slot (or do nothing in
155	particular, if the platform doesn't isolate slots), and recovery
156	proceeds to STEP 2 (MMIO Enable).
157	
158	If any driver requested a slot reset (by returning PCI_ERS_RESULT_NEED_RESET),
159	then recovery proceeds to STEP 4 (Slot Reset).
160	
161	If the platform is unable to recover the slot, the next step
162	is STEP 6 (Permanent Failure).
163	
164	>>> The current powerpc implementation assumes that a device driver will
165	>>> *not* schedule or semaphore in this routine; the current powerpc
166	>>> implementation uses one kernel thread to notify all devices;
167	>>> thus, if one device sleeps/schedules, all devices are affected.
168	>>> Doing better requires complex multi-threaded logic in the error
169	>>> recovery implementation (e.g. waiting for all notification threads
170	>>> to "join" before proceeding with recovery.)  This seems excessively
171	>>> complex and not worth implementing.
172	
173	>>> The current powerpc implementation doesn't much care if the device
174	>>> attempts I/O at this point, or not.  I/O's will fail, returning
175	>>> a value of 0xff on read, and writes will be dropped. If more than
176	>>> EEH_MAX_FAILS I/O's are attempted to a frozen adapter, EEH
177	>>> assumes that the device driver has gone into an infinite loop
178	>>> and prints an error to syslog.  A reboot is then required to 
179	>>> get the device working again.
180	
181	STEP 2: MMIO Enabled
182	-------------------
183	The platform re-enables MMIO to the device (but typically not the
184	DMA), and then calls the mmio_enabled() callback on all affected
185	device drivers.
186	
187	This is the "early recovery" call. IOs are allowed again, but DMA is
188	not, with some restrictions. This is NOT a callback for the driver to
189	start operations again, only to peek/poke at the device, extract diagnostic
190	information, if any, and eventually do things like trigger a device local
191	reset or some such, but not restart operations. This callback is made if
192	all drivers on a segment agree that they can try to recover and if no automatic
193	link reset was performed by the HW. If the platform can't just re-enable IOs
194	without a slot reset or a link reset, it will not call this callback, and
195	instead will have gone directly to STEP 3 (Link Reset) or STEP 4 (Slot Reset)
196	
197	>>> The following is proposed; no platform implements this yet:
198	>>> Proposal: All I/O's should be done _synchronously_ from within
199	>>> this callback, errors triggered by them will be returned via
200	>>> the normal pci_check_whatever() API, no new error_detected()
201	>>> callback will be issued due to an error happening here. However,
202	>>> such an error might cause IOs to be re-blocked for the whole
203	>>> segment, and thus invalidate the recovery that other devices
204	>>> on the same segment might have done, forcing the whole segment
205	>>> into one of the next states, that is, link reset or slot reset.
206	
207	The driver should return one of the following result codes:
208			- PCI_ERS_RESULT_RECOVERED
209			  Driver returns this if it thinks the device is fully
210			  functional and thinks it is ready to start
211			  normal driver operations again. There is no
212			  guarantee that the driver will actually be
213			  allowed to proceed, as another driver on the
214			  same segment might have failed and thus triggered a
215			  slot reset on platforms that support it.
216	
217			- PCI_ERS_RESULT_NEED_RESET
218			  Driver returns this if it thinks the device is not
219			  recoverable in its current state and it needs a slot
220			  reset to proceed.
221	
222			- PCI_ERS_RESULT_DISCONNECT
223			  Same as above. Total failure, no recovery even after
224			  reset driver dead. (To be defined more precisely)
225	
226	The next step taken depends on the results returned by the drivers.
227	If all drivers returned PCI_ERS_RESULT_RECOVERED, then the platform
228	proceeds to either STEP3 (Link Reset) or to STEP 5 (Resume Operations).
229	
230	If any driver returned PCI_ERS_RESULT_NEED_RESET, then the platform
231	proceeds to STEP 4 (Slot Reset)
232	
233	STEP 3: Link Reset
234	------------------
235	The platform resets the link, and then calls the link_reset() callback
236	on all affected device drivers.  This is a PCI-Express specific state
237	and is done whenever a non-fatal error has been detected that can be
238	"solved" by resetting the link. This call informs the driver of the
239	reset and the driver should check to see if the device appears to be
240	in working condition.
241	
242	The driver is not supposed to restart normal driver I/O operations
243	at this point.  It should limit itself to "probing" the device to
244	check its recoverability status. If all is right, then the platform
245	will call resume() once all drivers have ack'd link_reset().
246	
247		Result codes:
248			(identical to STEP 3 (MMIO Enabled)
249	
250	The platform then proceeds to either STEP 4 (Slot Reset) or STEP 5
251	(Resume Operations).
252	
253	>>> The current powerpc implementation does not implement this callback.
254	
255	STEP 4: Slot Reset
256	------------------
257	
258	In response to a return value of PCI_ERS_RESULT_NEED_RESET, the
259	the platform will peform a slot reset on the requesting PCI device(s). 
260	The actual steps taken by a platform to perform a slot reset
261	will be platform-dependent. Upon completion of slot reset, the
262	platform will call the device slot_reset() callback.
263	
264	Powerpc platforms implement two levels of slot reset:
265	soft reset(default) and fundamental(optional) reset.
266	
267	Powerpc soft reset consists of asserting the adapter #RST line and then
268	restoring the PCI BAR's and PCI configuration header to a state
269	that is equivalent to what it would be after a fresh system
270	power-on followed by power-on BIOS/system firmware initialization.
271	Soft reset is also known as hot-reset.
272	
273	Powerpc fundamental reset is supported by PCI Express cards only
274	and results in device's state machines, hardware logic, port states and
275	configuration registers to initialize to their default conditions.
276	
277	For most PCI devices, a soft reset will be sufficient for recovery.
278	Optional fundamental reset is provided to support a limited number
279	of PCI Express PCI devices  for which a soft reset is not sufficient
280	for recovery.
281	
282	If the platform supports PCI hotplug, then the reset might be
283	performed by toggling the slot electrical power off/on.
284	
285	It is important for the platform to restore the PCI config space
286	to the "fresh poweron" state, rather than the "last state". After
287	a slot reset, the device driver will almost always use its standard
288	device initialization routines, and an unusual config space setup
289	may result in hung devices, kernel panics, or silent data corruption.
290	
291	This call gives drivers the chance to re-initialize the hardware
292	(re-download firmware, etc.).  At this point, the driver may assume
293	that the card is in a fresh state and is fully functional. The slot
294	is unfrozen and the driver has full access to PCI config space,
295	memory mapped I/O space and DMA. Interrupts (Legacy, MSI, or MSI-X)
296	will also be available.
297	
298	Drivers should not restart normal I/O processing operations
299	at this point.  If all device drivers report success on this
300	callback, the platform will call resume() to complete the sequence,
301	and let the driver restart normal I/O processing.
302	
303	A driver can still return a critical failure for this function if
304	it can't get the device operational after reset.  If the platform
305	previously tried a soft reset, it might now try a hard reset (power
306	cycle) and then call slot_reset() again.  It the device still can't
307	be recovered, there is nothing more that can be done;  the platform
308	will typically report a "permanent failure" in such a case.  The
309	device will be considered "dead" in this case.
310	
311	Drivers for multi-function cards will need to coordinate among
312	themselves as to which driver instance will perform any "one-shot"
313	or global device initialization. For example, the Symbios sym53cxx2
314	driver performs device init only from PCI function 0:
315	
316	+       if (PCI_FUNC(pdev->devfn) == 0)
317	+               sym_reset_scsi_bus(np, 0);
318	
319		Result codes:
320			- PCI_ERS_RESULT_DISCONNECT
321			Same as above.
322	
323	Drivers for PCI Express cards that require a fundamental reset must
324	set the needs_freset bit in the pci_dev structure in their probe function.  
325	For example, the QLogic qla2xxx driver sets the needs_freset bit for certain
326	PCI card types:
327	
328	+	/* Set EEH reset type to fundamental if required by hba  */
329	+	if (IS_QLA24XX(ha) || IS_QLA25XX(ha) || IS_QLA81XX(ha))
330	+		pdev->needs_freset = 1;
331	+
332	
333	Platform proceeds either to STEP 5 (Resume Operations) or STEP 6 (Permanent
334	Failure).
335	
336	>>> The current powerpc implementation does not try a power-cycle
337	>>> reset if the driver returned PCI_ERS_RESULT_DISCONNECT.
338	>>> However, it probably should.
339	
340	
341	STEP 5: Resume Operations
342	-------------------------
343	The platform will call the resume() callback on all affected device
344	drivers if all drivers on the segment have returned
345	PCI_ERS_RESULT_RECOVERED from one of the 3 previous callbacks.
346	The goal of this callback is to tell the driver to restart activity,
347	that everything is back and running. This callback does not return
348	a result code.
349	
350	At this point, if a new error happens, the platform will restart
351	a new error recovery sequence.
352	
353	STEP 6: Permanent Failure
354	-------------------------
355	A "permanent failure" has occurred, and the platform cannot recover
356	the device.  The platform will call error_detected() with a
357	pci_channel_state value of pci_channel_io_perm_failure.
358	
359	The device driver should, at this point, assume the worst. It should
360	cancel all pending I/O, refuse all new I/O, returning -EIO to
361	higher layers. The device driver should then clean up all of its
362	memory and remove itself from kernel operations, much as it would
363	during system shutdown.
364	
365	The platform will typically notify the system operator of the
366	permanent failure in some way.  If the device is hotplug-capable,
367	the operator will probably want to remove and replace the device.
368	Note, however, not all failures are truly "permanent". Some are
369	caused by over-heating, some by a poorly seated card. Many
370	PCI error events are caused by software bugs, e.g. DMA's to
371	wild addresses or bogus split transactions due to programming
372	errors. See the discussion in powerpc/eeh-pci-error-recovery.txt
373	for additional detail on real-life experience of the causes of
374	software errors.
375	
376	
377	Conclusion; General Remarks
378	---------------------------
379	The way the callbacks are called is platform policy. A platform with
380	no slot reset capability may want to just "ignore" drivers that can't
381	recover (disconnect them) and try to let other cards on the same segment
382	recover. Keep in mind that in most real life cases, though, there will
383	be only one driver per segment.
384	
385	Now, a note about interrupts. If you get an interrupt and your
386	device is dead or has been isolated, there is a problem :)
387	The current policy is to turn this into a platform policy.
388	That is, the recovery API only requires that:
389	
390	 - There is no guarantee that interrupt delivery can proceed from any
391	device on the segment starting from the error detection and until the
392	slot_reset callback is called, at which point interrupts are expected
393	to be fully operational.
394	
395	 - There is no guarantee that interrupt delivery is stopped, that is,
396	a driver that gets an interrupt after detecting an error, or that detects
397	an error within the interrupt handler such that it prevents proper
398	ack'ing of the interrupt (and thus removal of the source) should just
399	return IRQ_NOTHANDLED. It's up to the platform to deal with that
400	condition, typically by masking the IRQ source during the duration of
401	the error handling. It is expected that the platform "knows" which
402	interrupts are routed to error-management capable slots and can deal
403	with temporarily disabling that IRQ number during error processing (this
404	isn't terribly complex). That means some IRQ latency for other devices
405	sharing the interrupt, but there is simply no other way. High end
406	platforms aren't supposed to share interrupts between many devices
407	anyway :)
408	
409	>>> Implementation details for the powerpc platform are discussed in
410	>>> the file Documentation/powerpc/eeh-pci-error-recovery.txt
411	
412	>>> As of this writing, there is a growing list of device drivers with
413	>>> patches implementing error recovery. Not all of these patches are in
414	>>> mainline yet. These may be used as "examples":
415	>>>
416	>>> drivers/scsi/ipr
417	>>> drivers/scsi/sym53c8xx_2
418	>>> drivers/scsi/qla2xxx
419	>>> drivers/scsi/lpfc
420	>>> drivers/next/bnx2.c
421	>>> drivers/next/e100.c
422	>>> drivers/net/e1000
423	>>> drivers/net/e1000e
424	>>> drivers/net/ixgb
425	>>> drivers/net/ixgbe
426	>>> drivers/net/cxgb3
427	>>> drivers/net/s2io.c
428	>>> drivers/net/qlge
429	
430	The End
431	-------
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.