Documentation/PCI/pci-error-recovery.rst - linux - Git at Google

 .. SPDX-License-Identifier: GPL-2.0

 ==================
 PCI Error Recovery
 ==================


 :Authors: - Linas Vepstas <linasvepstas@gmail.com>
           - Richard Lary <rlary@us.ibm.com>
           - Mike Mason <mmlnx@us.ibm.com>


 Many PCI bus controllers are able to detect a variety of hardware
 PCI errors on the bus, such as parity errors on the data and address
 buses, as well as SERR and PERR errors.  Some of the more advanced
 chipsets are able to deal with these errors; these include PCI-E chipsets,
 and the PCI-host bridges found on IBM Power4, Power5 and Power6-based
 pSeries boxes. A typical action taken is to disconnect the affected device,
 halting all I/O to it.  The goal of a disconnection is to avoid system
 corruption; for example, to halt system memory corruption due to DMAs
 to "wild" addresses. Typically, a reconnection mechanism is also
 offered, so that the affected PCI device(s) are reset and put back
 into working condition. The reset phase requires coordination
 between the affected device drivers and the PCI controller chip.
 This document describes a generic API for notifying device drivers
 of a bus disconnection, and then performing error recovery.
 This API is currently implemented in the 2.6.16 and later kernels.

 Reporting and recovery is performed in several steps. First, when
 a PCI hardware error has resulted in a bus disconnect, that event
 is reported as soon as possible to all affected device drivers,
 including multiple instances of a device driver on multi-function
 cards. This allows device drivers to avoid deadlocking in spinloops,
 waiting for some i/o-space register to change, when it never will.
 It also gives the drivers a chance to defer incoming I/O as
 needed.

 Next, recovery is performed in several stages. Most of the complexity
 is forced by the need to handle multi-function devices, that is,
 devices that have multiple device drivers associated with them.
 In the first stage, each driver is allowed to indicate what type
 of reset it desires, the choices being a simple re-enabling of I/O
 or requesting a slot reset.

 If any driver requests a slot reset, that is what will be done.

 After a reset and/or a re-enabling of I/O, all drivers are
 again notified, so that they may then perform any device setup/config
 that may be required.  After these have all completed, a final
 "resume normal operations" event is sent out.

 The biggest reason for choosing a kernel-based implementation rather
 than a user-space implementation was the need to deal with bus
 disconnects of PCI devices attached to storage media, and, in particular,
 disconnects from devices holding the root file system.  If the root
 file system is disconnected, a user-space mechanism would have to go
 through a large number of contortions to complete recovery. Almost all
 of the current Linux file systems are not tolerant of disconnection
 from/reconnection to their underlying block device. By contrast,
 bus errors are easy to manage in the device driver. Indeed, most
 device drivers already handle very similar recovery procedures;
 for example, the SCSI-generic layer already provides significant
 mechanisms for dealing with SCSI bus errors and SCSI bus resets.


 Detailed Design
 ===============

 Design and implementation details below, based on a chain of
 public email discussions with Ben Herrenschmidt, circa 5 April 2005.

 The error recovery API support is exposed to the driver in the form of
 a structure of function pointers pointed to by a new field in struct
 pci_driver. A driver that fails to provide the structure is "non-aware",
 and the actual recovery steps taken are platform dependent.  The
 arch/powerpc implementation will simulate a PCI hotplug remove/add.

 This structure has the form::

 	struct pci_error_handlers
 	{
 		int (*error_detected)(struct pci_dev *dev, pci_channel_state_t);
 		int (*mmio_enabled)(struct pci_dev *dev);
 		int (*slot_reset)(struct pci_dev *dev);
 		void (*resume)(struct pci_dev *dev);
 		void (*cor_error_detected)(struct pci_dev *dev);
 	};

 The possible channel states are::

 	typedef enum {
 		pci_channel_io_normal,  /* I/O channel is in normal state */
 		pci_channel_io_frozen,  /* I/O to channel is blocked */
 		pci_channel_io_perm_failure, /* PCI card is dead */
 	} pci_channel_state_t;

 Possible return values are::

 	enum pci_ers_result {
 		PCI_ERS_RESULT_NONE,        /* no result/none/not supported in device driver */
 		PCI_ERS_RESULT_CAN_RECOVER, /* Device driver can recover without slot reset */
 		PCI_ERS_RESULT_NEED_RESET,  /* Device driver wants slot to be reset. */
 		PCI_ERS_RESULT_DISCONNECT,  /* Device has completely failed, is unrecoverable */
 		PCI_ERS_RESULT_RECOVERED,   /* Device driver is fully recovered and operational */
 	};

 A driver does not have to implement all of these callbacks; however,
 if it implements any, it must implement error_detected(). If a callback
 is not implemented, the corresponding feature is considered unsupported.
 For example, if mmio_enabled() and resume() aren't there, then it
 is assumed that the driver is not doing any direct recovery and requires
 a slot reset.  Typically a driver will want to know about
 a slot_reset().

 The actual steps taken by a platform to recover from a PCI error
 event will be platform-dependent, but will follow the general
 sequence described below.

 STEP 0: Error Event
 -------------------
 A PCI bus error is detected by the PCI hardware.  On powerpc, the slot
 is isolated, in that all I/O is blocked: all reads return 0xffffffff,
 all writes are ignored.


 STEP 1: Notification
 --------------------
 Platform calls the error_detected() callback on every instance of
 every driver affected by the error.

 At this point, the device might not be accessible anymore, depending on
 the platform (the slot will be isolated on powerpc). The driver may
 already have "noticed" the error because of a failing I/O, but this
 is the proper "synchronization point", that is, it gives the driver
 a chance to cleanup, waiting for pending stuff (timers, whatever, etc...)
 to complete; it can take semaphores, schedule, etc... everything but
 touch the device. Within this function and after it returns, the driver
 shouldn't do any new IOs. Called in task context. This is sort of a
 "quiesce" point. See note about interrupts at the end of this doc.

 All drivers participating in this system must implement this call.
 The driver must return one of the following result codes:

   - PCI_ERS_RESULT_CAN_RECOVER
       Driver returns this if it thinks it might be able to recover
       the HW by just banging IOs or if it wants to be given
       a chance to extract some diagnostic information (see
       mmio_enable, below).
   - PCI_ERS_RESULT_NEED_RESET
       Driver returns this if it can't recover without a
       slot reset.
   - PCI_ERS_RESULT_DISCONNECT
       Driver returns this if it doesn't want to recover at all.

 The next step taken will depend on the result codes returned by the
 drivers.

 If all drivers on the segment/slot return PCI_ERS_RESULT_CAN_RECOVER,
 then the platform should re-enable IOs on the slot (or do nothing in
 particular, if the platform doesn't isolate slots), and recovery
 proceeds to STEP 2 (MMIO Enable).

 If any driver requested a slot reset (by returning PCI_ERS_RESULT_NEED_RESET),
 then recovery proceeds to STEP 4 (Slot Reset).

 If the platform is unable to recover the slot, the next step
 is STEP 6 (Permanent Failure).

 .. note::

    The current powerpc implementation assumes that a device driver will
    *not* schedule or semaphore in this routine; the current powerpc
    implementation uses one kernel thread to notify all devices;
    thus, if one device sleeps/schedules, all devices are affected.
    Doing better requires complex multi-threaded logic in the error
    recovery implementation (e.g. waiting for all notification threads
    to "join" before proceeding with recovery.)  This seems excessively
    complex and not worth implementing.

    The current powerpc implementation doesn't much care if the device
    attempts I/O at this point, or not.  I/Os will fail, returning
    a value of 0xff on read, and writes will be dropped. If more than
    EEH_MAX_FAILS I/Os are attempted to a frozen adapter, EEH
    assumes that the device driver has gone into an infinite loop
    and prints an error to syslog.  A reboot is then required to
    get the device working again.

 STEP 2: MMIO Enabled
 --------------------
 The platform re-enables MMIO to the device (but typically not the
 DMA), and then calls the mmio_enabled() callback on all affected
 device drivers.

 This is the "early recovery" call. IOs are allowed again, but DMA is
 not, with some restrictions. This is NOT a callback for the driver to
 start operations again, only to peek/poke at the device, extract diagnostic
 information, if any, and eventually do things like trigger a device local
 reset or some such, but not restart operations. This callback is made if
 all drivers on a segment agree that they can try to recover and if no automatic
 link reset was performed by the HW. If the platform can't just re-enable IOs
 without a slot reset or a link reset, it will not call this callback, and
 instead will have gone directly to STEP 3 (Link Reset) or STEP 4 (Slot Reset)

 .. note::

    The following is proposed; no platform implements this yet:
    Proposal: All I/Os should be done _synchronously_ from within
    this callback, errors triggered by them will be returned via
    the normal pci_check_whatever() API, no new error_detected()
    callback will be issued due to an error happening here. However,
    such an error might cause IOs to be re-blocked for the whole
    segment, and thus invalidate the recovery that other devices
    on the same segment might have done, forcing the whole segment
    into one of the next states, that is, link reset or slot reset.

 The driver should return one of the following result codes:
   - PCI_ERS_RESULT_RECOVERED
       Driver returns this if it thinks the device is fully
       functional and thinks it is ready to start
       normal driver operations again. There is no
       guarantee that the driver will actually be
       allowed to proceed, as another driver on the
       same segment might have failed and thus triggered a
       slot reset on platforms that support it.

   - PCI_ERS_RESULT_NEED_RESET
       Driver returns this if it thinks the device is not
       recoverable in its current state and it needs a slot
       reset to proceed.

   - PCI_ERS_RESULT_DISCONNECT
       Same as above. Total failure, no recovery even after
       reset driver dead. (To be defined more precisely)

 The next step taken depends on the results returned by the drivers.
 If all drivers returned PCI_ERS_RESULT_RECOVERED, then the platform
 proceeds to either STEP3 (Link Reset) or to STEP 5 (Resume Operations).

 If any driver returned PCI_ERS_RESULT_NEED_RESET, then the platform
 proceeds to STEP 4 (Slot Reset)

 STEP 3: Link Reset
 ------------------
 The platform resets the link.  This is a PCI-Express specific step
 and is done whenever a fatal error has been detected that can be
 "solved" by resetting the link.

 STEP 4: Slot Reset
 ------------------

 In response to a return value of PCI_ERS_RESULT_NEED_RESET, the
 platform will perform a slot reset on the requesting PCI device(s).
 The actual steps taken by a platform to perform a slot reset
 will be platform-dependent. Upon completion of slot reset, the
 platform will call the device slot_reset() callback.

 Powerpc platforms implement two levels of slot reset:
 soft reset(default) and fundamental(optional) reset.

 Powerpc soft reset consists of asserting the adapter #RST line and then
 restoring the PCI BARs and PCI configuration header to a state
 that is equivalent to what it would be after a fresh system
 power-on followed by power-on BIOS/system firmware initialization.
 Soft reset is also known as hot-reset.

 Powerpc fundamental reset is supported by PCI Express cards only
 and results in device's state machines, hardware logic, port states and
 configuration registers to initialize to their default conditions.

 For most PCI devices, a soft reset will be sufficient for recovery.
 Optional fundamental reset is provided to support a limited number
 of PCI Express devices for which a soft reset is not sufficient
 for recovery.

 If the platform supports PCI hotplug, then the reset might be
 performed by toggling the slot electrical power off/on.

 It is important for the platform to restore the PCI config space
 to the "fresh poweron" state, rather than the "last state". After
 a slot reset, the device driver will almost always use its standard
 device initialization routines, and an unusual config space setup
 may result in hung devices, kernel panics, or silent data corruption.

 This call gives drivers the chance to re-initialize the hardware
 (re-download firmware, etc.).  At this point, the driver may assume
 that the card is in a fresh state and is fully functional. The slot
 is unfrozen and the driver has full access to PCI config space,
 memory mapped I/O space and DMA. Interrupts (Legacy, MSI, or MSI-X)
 will also be available.

 Drivers should not restart normal I/O processing operations
 at this point.  If all device drivers report success on this
 callback, the platform will call resume() to complete the sequence,
 and let the driver restart normal I/O processing.

 A driver can still return a critical failure for this function if
 it can't get the device operational after reset.  If the platform
 previously tried a soft reset, it might now try a hard reset (power
 cycle) and then call slot_reset() again.  If the device still can't
 be recovered, there is nothing more that can be done;  the platform
 will typically report a "permanent failure" in such a case.  The
 device will be considered "dead" in this case.

 Drivers for multi-function cards will need to coordinate among
 themselves as to which driver instance will perform any "one-shot"
 or global device initialization. For example, the Symbios sym53cxx2
 driver performs device init only from PCI function 0::

 	+       if (PCI_FUNC(pdev->devfn) == 0)
 	+               sym_reset_scsi_bus(np, 0);

 Result codes:
 	- PCI_ERS_RESULT_DISCONNECT
 	  Same as above.

 Drivers for PCI Express cards that require a fundamental reset must
 set the needs_freset bit in the pci_dev structure in their probe function.
 For example, the QLogic qla2xxx driver sets the needs_freset bit for certain
 PCI card types::

 	+	/* Set EEH reset type to fundamental if required by hba  */
 	+	if (IS_QLA24XX(ha) || IS_QLA25XX(ha) || IS_QLA81XX(ha))
 	+		pdev->needs_freset = 1;
 	+

 Platform proceeds either to STEP 5 (Resume Operations) or STEP 6 (Permanent
 Failure).

 .. note::

    The current powerpc implementation does not try a power-cycle
    reset if the driver returned PCI_ERS_RESULT_DISCONNECT.
    However, it probably should.


 STEP 5: Resume Operations
 -------------------------
 The platform will call the resume() callback on all affected device
 drivers if all drivers on the segment have returned
 PCI_ERS_RESULT_RECOVERED from one of the 3 previous callbacks.
 The goal of this callback is to tell the driver to restart activity,
 that everything is back and running. This callback does not return
 a result code.

 At this point, if a new error happens, the platform will restart
 a new error recovery sequence.

 STEP 6: Permanent Failure
 -------------------------
 A "permanent failure" has occurred, and the platform cannot recover
 the device.  The platform will call error_detected() with a
 pci_channel_state_t value of pci_channel_io_perm_failure.

 The device driver should, at this point, assume the worst. It should
 cancel all pending I/O, refuse all new I/O, returning -EIO to
 higher layers. The device driver should then clean up all of its
 memory and remove itself from kernel operations, much as it would
 during system shutdown.

 The platform will typically notify the system operator of the
 permanent failure in some way.  If the device is hotplug-capable,
 the operator will probably want to remove and replace the device.
 Note, however, not all failures are truly "permanent". Some are
 caused by over-heating, some by a poorly seated card. Many
 PCI error events are caused by software bugs, e.g. DMAs to
 wild addresses or bogus split transactions due to programming
 errors. See the discussion in Documentation/powerpc/eeh-pci-error-recovery.rst
 for additional detail on real-life experience of the causes of
 software errors.


 Conclusion; General Remarks
 ---------------------------
 The way the callbacks are called is platform policy. A platform with
 no slot reset capability may want to just "ignore" drivers that can't
 recover (disconnect them) and try to let other cards on the same segment
 recover. Keep in mind that in most real life cases, though, there will
 be only one driver per segment.

 Now, a note about interrupts. If you get an interrupt and your
 device is dead or has been isolated, there is a problem :)
 The current policy is to turn this into a platform policy.
 That is, the recovery API only requires that:

  - There is no guarantee that interrupt delivery can proceed from any
    device on the segment starting from the error detection and until the
    slot_reset callback is called, at which point interrupts are expected
    to be fully operational.

  - There is no guarantee that interrupt delivery is stopped, that is,
    a driver that gets an interrupt after detecting an error, or that detects
    an error within the interrupt handler such that it prevents proper
    ack'ing of the interrupt (and thus removal of the source) should just
    return IRQ_NOTHANDLED. It's up to the platform to deal with that
    condition, typically by masking the IRQ source during the duration of
    the error handling. It is expected that the platform "knows" which
    interrupts are routed to error-management capable slots and can deal
    with temporarily disabling that IRQ number during error processing (this
    isn't terribly complex). That means some IRQ latency for other devices
    sharing the interrupt, but there is simply no other way. High end
    platforms aren't supposed to share interrupts between many devices
    anyway :)

 .. note::

    Implementation details for the powerpc platform are discussed in
    the file Documentation/powerpc/eeh-pci-error-recovery.rst

    As of this writing, there is a growing list of device drivers with
    patches implementing error recovery. Not all of these patches are in
    mainline yet. These may be used as "examples":

    - drivers/scsi/ipr
    - drivers/scsi/sym53c8xx_2
    - drivers/scsi/qla2xxx
    - drivers/scsi/lpfc
    - drivers/next/bnx2.c
    - drivers/next/e100.c
    - drivers/net/e1000
    - drivers/net/e1000e
    - drivers/net/ixgbe
    - drivers/net/cxgb3
    - drivers/net/s2io.c

    The cor_error_detected() callback is invoked in handle_error_source() when
    the error severity is "correctable". The callback is optional and allows
    additional logging to be done if desired. See example:

    - drivers/cxl/pci.c

 The End
 -------
	.. SPDX-License-Identifier: GPL-2.0

	==================
	PCI Error Recovery
	==================


	:Authors: - Linas Vepstas <linasvepstas@gmail.com>
	- Richard Lary <rlary@us.ibm.com>
	- Mike Mason <mmlnx@us.ibm.com>


	Many PCI bus controllers are able to detect a variety of hardware
	PCI errors on the bus, such as parity errors on the data and address
	buses, as well as SERR and PERR errors. Some of the more advanced
	chipsets are able to deal with these errors; these include PCI-E chipsets,
	and the PCI-host bridges found on IBM Power4, Power5 and Power6-based
	pSeries boxes. A typical action taken is to disconnect the affected device,
	halting all I/O to it. The goal of a disconnection is to avoid system
	corruption; for example, to halt system memory corruption due to DMAs
	to "wild" addresses. Typically, a reconnection mechanism is also
	offered, so that the affected PCI device(s) are reset and put back
	into working condition. The reset phase requires coordination
	between the affected device drivers and the PCI controller chip.
	This document describes a generic API for notifying device drivers
	of a bus disconnection, and then performing error recovery.
	This API is currently implemented in the 2.6.16 and later kernels.

	Reporting and recovery is performed in several steps. First, when
	a PCI hardware error has resulted in a bus disconnect, that event
	is reported as soon as possible to all affected device drivers,
	including multiple instances of a device driver on multi-function
	cards. This allows device drivers to avoid deadlocking in spinloops,
	waiting for some i/o-space register to change, when it never will.
	It also gives the drivers a chance to defer incoming I/O as
	needed.

	Next, recovery is performed in several stages. Most of the complexity
	is forced by the need to handle multi-function devices, that is,
	devices that have multiple device drivers associated with them.
	In the first stage, each driver is allowed to indicate what type
	of reset it desires, the choices being a simple re-enabling of I/O
	or requesting a slot reset.

	If any driver requests a slot reset, that is what will be done.

	After a reset and/or a re-enabling of I/O, all drivers are
	again notified, so that they may then perform any device setup/config
	that may be required. After these have all completed, a final
	"resume normal operations" event is sent out.

	The biggest reason for choosing a kernel-based implementation rather
	than a user-space implementation was the need to deal with bus
	disconnects of PCI devices attached to storage media, and, in particular,
	disconnects from devices holding the root file system. If the root
	file system is disconnected, a user-space mechanism would have to go
	through a large number of contortions to complete recovery. Almost all
	of the current Linux file systems are not tolerant of disconnection
	from/reconnection to their underlying block device. By contrast,
	bus errors are easy to manage in the device driver. Indeed, most
	device drivers already handle very similar recovery procedures;
	for example, the SCSI-generic layer already provides significant
	mechanisms for dealing with SCSI bus errors and SCSI bus resets.


	Detailed Design
	===============

	Design and implementation details below, based on a chain of
	public email discussions with Ben Herrenschmidt, circa 5 April 2005.

	The error recovery API support is exposed to the driver in the form of
	a structure of function pointers pointed to by a new field in struct
	pci_driver. A driver that fails to provide the structure is "non-aware",
	and the actual recovery steps taken are platform dependent. The
	arch/powerpc implementation will simulate a PCI hotplug remove/add.

	This structure has the form::

	struct pci_error_handlers
	{
	int (error_detected)(struct pci_dev dev, pci_channel_state_t);
	int (mmio_enabled)(struct pci_dev dev);
	int (slot_reset)(struct pci_dev dev);
	void (resume)(struct pci_dev dev);
	void (cor_error_detected)(struct pci_dev dev);
	};

	The possible channel states are::

	typedef enum {
	pci_channel_io_normal, /* I/O channel is in normal state */
	pci_channel_io_frozen, /* I/O to channel is blocked */
	pci_channel_io_perm_failure, /* PCI card is dead */
	} pci_channel_state_t;

	Possible return values are::

	enum pci_ers_result {
	PCI_ERS_RESULT_NONE, /* no result/none/not supported in device driver */
	PCI_ERS_RESULT_CAN_RECOVER, /* Device driver can recover without slot reset */
	PCI_ERS_RESULT_NEED_RESET, /* Device driver wants slot to be reset. */
	PCI_ERS_RESULT_DISCONNECT, /* Device has completely failed, is unrecoverable */
	PCI_ERS_RESULT_RECOVERED, /* Device driver is fully recovered and operational */
	};

	A driver does not have to implement all of these callbacks; however,
	if it implements any, it must implement error_detected(). If a callback
	is not implemented, the corresponding feature is considered unsupported.
	For example, if mmio_enabled() and resume() aren't there, then it
	is assumed that the driver is not doing any direct recovery and requires
	a slot reset. Typically a driver will want to know about
	a slot_reset().

	The actual steps taken by a platform to recover from a PCI error
	event will be platform-dependent, but will follow the general
	sequence described below.

	STEP 0: Error Event
	-------------------
	A PCI bus error is detected by the PCI hardware. On powerpc, the slot
	is isolated, in that all I/O is blocked: all reads return 0xffffffff,
	all writes are ignored.


	STEP 1: Notification
	--------------------
	Platform calls the error_detected() callback on every instance of
	every driver affected by the error.

	At this point, the device might not be accessible anymore, depending on
	the platform (the slot will be isolated on powerpc). The driver may
	already have "noticed" the error because of a failing I/O, but this
	is the proper "synchronization point", that is, it gives the driver
	a chance to cleanup, waiting for pending stuff (timers, whatever, etc...)
	to complete; it can take semaphores, schedule, etc... everything but
	touch the device. Within this function and after it returns, the driver
	shouldn't do any new IOs. Called in task context. This is sort of a
	"quiesce" point. See note about interrupts at the end of this doc.

	All drivers participating in this system must implement this call.
	The driver must return one of the following result codes:

	- PCI_ERS_RESULT_CAN_RECOVER
	Driver returns this if it thinks it might be able to recover
	the HW by just banging IOs or if it wants to be given
	a chance to extract some diagnostic information (see
	mmio_enable, below).
	- PCI_ERS_RESULT_NEED_RESET
	Driver returns this if it can't recover without a
	slot reset.
	- PCI_ERS_RESULT_DISCONNECT
	Driver returns this if it doesn't want to recover at all.

	The next step taken will depend on the result codes returned by the
	drivers.

	If all drivers on the segment/slot return PCI_ERS_RESULT_CAN_RECOVER,
	then the platform should re-enable IOs on the slot (or do nothing in
	particular, if the platform doesn't isolate slots), and recovery
	proceeds to STEP 2 (MMIO Enable).

	If any driver requested a slot reset (by returning PCI_ERS_RESULT_NEED_RESET),
	then recovery proceeds to STEP 4 (Slot Reset).

	If the platform is unable to recover the slot, the next step
	is STEP 6 (Permanent Failure).

	.. note::

	The current powerpc implementation assumes that a device driver will
	not schedule or semaphore in this routine; the current powerpc
	implementation uses one kernel thread to notify all devices;
	thus, if one device sleeps/schedules, all devices are affected.
	Doing better requires complex multi-threaded logic in the error
	recovery implementation (e.g. waiting for all notification threads
	to "join" before proceeding with recovery.) This seems excessively
	complex and not worth implementing.

	The current powerpc implementation doesn't much care if the device
	attempts I/O at this point, or not. I/Os will fail, returning
	a value of 0xff on read, and writes will be dropped. If more than
	EEH_MAX_FAILS I/Os are attempted to a frozen adapter, EEH
	assumes that the device driver has gone into an infinite loop
	and prints an error to syslog. A reboot is then required to
	get the device working again.

	STEP 2: MMIO Enabled
	--------------------
	The platform re-enables MMIO to the device (but typically not the
	DMA), and then calls the mmio_enabled() callback on all affected
	device drivers.

	This is the "early recovery" call. IOs are allowed again, but DMA is
	not, with some restrictions. This is NOT a callback for the driver to
	start operations again, only to peek/poke at the device, extract diagnostic
	information, if any, and eventually do things like trigger a device local
	reset or some such, but not restart operations. This callback is made if
	all drivers on a segment agree that they can try to recover and if no automatic
	link reset was performed by the HW. If the platform can't just re-enable IOs
	without a slot reset or a link reset, it will not call this callback, and
	instead will have gone directly to STEP 3 (Link Reset) or STEP 4 (Slot Reset)

	.. note::

	The following is proposed; no platform implements this yet:
	Proposal: All I/Os should be done _synchronously_ from within
	this callback, errors triggered by them will be returned via
	the normal pci_check_whatever() API, no new error_detected()
	callback will be issued due to an error happening here. However,
	such an error might cause IOs to be re-blocked for the whole
	segment, and thus invalidate the recovery that other devices
	on the same segment might have done, forcing the whole segment
	into one of the next states, that is, link reset or slot reset.

	The driver should return one of the following result codes:
	- PCI_ERS_RESULT_RECOVERED
	Driver returns this if it thinks the device is fully
	functional and thinks it is ready to start
	normal driver operations again. There is no
	guarantee that the driver will actually be
	allowed to proceed, as another driver on the
	same segment might have failed and thus triggered a
	slot reset on platforms that support it.

	- PCI_ERS_RESULT_NEED_RESET
	Driver returns this if it thinks the device is not
	recoverable in its current state and it needs a slot
	reset to proceed.

	- PCI_ERS_RESULT_DISCONNECT
	Same as above. Total failure, no recovery even after
	reset driver dead. (To be defined more precisely)

	The next step taken depends on the results returned by the drivers.
	If all drivers returned PCI_ERS_RESULT_RECOVERED, then the platform
	proceeds to either STEP3 (Link Reset) or to STEP 5 (Resume Operations).

	If any driver returned PCI_ERS_RESULT_NEED_RESET, then the platform
	proceeds to STEP 4 (Slot Reset)

	STEP 3: Link Reset
	------------------
	The platform resets the link. This is a PCI-Express specific step
	and is done whenever a fatal error has been detected that can be
	"solved" by resetting the link.

	STEP 4: Slot Reset
	------------------

	In response to a return value of PCI_ERS_RESULT_NEED_RESET, the
	platform will perform a slot reset on the requesting PCI device(s).
	The actual steps taken by a platform to perform a slot reset
	will be platform-dependent. Upon completion of slot reset, the
	platform will call the device slot_reset() callback.

	Powerpc platforms implement two levels of slot reset:
	soft reset(default) and fundamental(optional) reset.

	Powerpc soft reset consists of asserting the adapter #RST line and then
	restoring the PCI BARs and PCI configuration header to a state
	that is equivalent to what it would be after a fresh system
	power-on followed by power-on BIOS/system firmware initialization.
	Soft reset is also known as hot-reset.

	Powerpc fundamental reset is supported by PCI Express cards only
	and results in device's state machines, hardware logic, port states and
	configuration registers to initialize to their default conditions.

	For most PCI devices, a soft reset will be sufficient for recovery.
	Optional fundamental reset is provided to support a limited number
	of PCI Express devices for which a soft reset is not sufficient
	for recovery.

	If the platform supports PCI hotplug, then the reset might be
	performed by toggling the slot electrical power off/on.

	It is important for the platform to restore the PCI config space
	to the "fresh poweron" state, rather than the "last state". After
	a slot reset, the device driver will almost always use its standard
	device initialization routines, and an unusual config space setup
	may result in hung devices, kernel panics, or silent data corruption.

	This call gives drivers the chance to re-initialize the hardware
	(re-download firmware, etc.). At this point, the driver may assume
	that the card is in a fresh state and is fully functional. The slot
	is unfrozen and the driver has full access to PCI config space,
	memory mapped I/O space and DMA. Interrupts (Legacy, MSI, or MSI-X)
	will also be available.

	Drivers should not restart normal I/O processing operations
	at this point. If all device drivers report success on this
	callback, the platform will call resume() to complete the sequence,
	and let the driver restart normal I/O processing.

	A driver can still return a critical failure for this function if
	it can't get the device operational after reset. If the platform
	previously tried a soft reset, it might now try a hard reset (power
	cycle) and then call slot_reset() again. If the device still can't
	be recovered, there is nothing more that can be done; the platform
	will typically report a "permanent failure" in such a case. The
	device will be considered "dead" in this case.

	Drivers for multi-function cards will need to coordinate among
	themselves as to which driver instance will perform any "one-shot"
	or global device initialization. For example, the Symbios sym53cxx2
	driver performs device init only from PCI function 0::

	+ if (PCI_FUNC(pdev->devfn) == 0)
	+ sym_reset_scsi_bus(np, 0);

	Result codes:
	- PCI_ERS_RESULT_DISCONNECT
	Same as above.

	Drivers for PCI Express cards that require a fundamental reset must
	set the needs_freset bit in the pci_dev structure in their probe function.
	For example, the QLogic qla2xxx driver sets the needs_freset bit for certain
	PCI card types::

	+ /* Set EEH reset type to fundamental if required by hba */
	+ if (IS_QLA24XX(ha) \|\| IS_QLA25XX(ha) \|\| IS_QLA81XX(ha))
	+ pdev->needs_freset = 1;
	+

	Platform proceeds either to STEP 5 (Resume Operations) or STEP 6 (Permanent
	Failure).

	.. note::

	The current powerpc implementation does not try a power-cycle
	reset if the driver returned PCI_ERS_RESULT_DISCONNECT.
	However, it probably should.


	STEP 5: Resume Operations
	-------------------------
	The platform will call the resume() callback on all affected device
	drivers if all drivers on the segment have returned
	PCI_ERS_RESULT_RECOVERED from one of the 3 previous callbacks.
	The goal of this callback is to tell the driver to restart activity,
	that everything is back and running. This callback does not return
	a result code.

	At this point, if a new error happens, the platform will restart
	a new error recovery sequence.

	STEP 6: Permanent Failure
	-------------------------
	A "permanent failure" has occurred, and the platform cannot recover
	the device. The platform will call error_detected() with a
	pci_channel_state_t value of pci_channel_io_perm_failure.

	The device driver should, at this point, assume the worst. It should
	cancel all pending I/O, refuse all new I/O, returning -EIO to
	higher layers. The device driver should then clean up all of its
	memory and remove itself from kernel operations, much as it would
	during system shutdown.

	The platform will typically notify the system operator of the
	permanent failure in some way. If the device is hotplug-capable,
	the operator will probably want to remove and replace the device.
	Note, however, not all failures are truly "permanent". Some are
	caused by over-heating, some by a poorly seated card. Many
	PCI error events are caused by software bugs, e.g. DMAs to
	wild addresses or bogus split transactions due to programming
	errors. See the discussion in Documentation/powerpc/eeh-pci-error-recovery.rst
	for additional detail on real-life experience of the causes of
	software errors.


	Conclusion; General Remarks
	---------------------------
	The way the callbacks are called is platform policy. A platform with
	no slot reset capability may want to just "ignore" drivers that can't
	recover (disconnect them) and try to let other cards on the same segment
	recover. Keep in mind that in most real life cases, though, there will
	be only one driver per segment.

	Now, a note about interrupts. If you get an interrupt and your
	device is dead or has been isolated, there is a problem :)
	The current policy is to turn this into a platform policy.
	That is, the recovery API only requires that:

	- There is no guarantee that interrupt delivery can proceed from any
	device on the segment starting from the error detection and until the
	slot_reset callback is called, at which point interrupts are expected
	to be fully operational.

	- There is no guarantee that interrupt delivery is stopped, that is,
	a driver that gets an interrupt after detecting an error, or that detects
	an error within the interrupt handler such that it prevents proper
	ack'ing of the interrupt (and thus removal of the source) should just
	return IRQ_NOTHANDLED. It's up to the platform to deal with that
	condition, typically by masking the IRQ source during the duration of
	the error handling. It is expected that the platform "knows" which
	interrupts are routed to error-management capable slots and can deal
	with temporarily disabling that IRQ number during error processing (this
	isn't terribly complex). That means some IRQ latency for other devices
	sharing the interrupt, but there is simply no other way. High end
	platforms aren't supposed to share interrupts between many devices
	anyway :)

	.. note::

	Implementation details for the powerpc platform are discussed in
	the file Documentation/powerpc/eeh-pci-error-recovery.rst

	As of this writing, there is a growing list of device drivers with
	patches implementing error recovery. Not all of these patches are in
	mainline yet. These may be used as "examples":

	- drivers/scsi/ipr
	- drivers/scsi/sym53c8xx_2
	- drivers/scsi/qla2xxx
	- drivers/scsi/lpfc
	- drivers/next/bnx2.c
	- drivers/next/e100.c
	- drivers/net/e1000
	- drivers/net/e1000e
	- drivers/net/ixgbe
	- drivers/net/cxgb3
	- drivers/net/s2io.c

	The cor_error_detected() callback is invoked in handle_error_source() when
	the error severity is "correctable". The callback is optional and allows
	additional logging to be done if desired. See example:

	- drivers/cxl/pci.c

	The End
	-------