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+
+                       PCI Error Recovery
+                       ------------------
+                        February 2, 2006
+
+                 Current document maintainer:
+             Linas Vepstas <linas@austin.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
+busses, 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 and Power5-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 DMA's
+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 hard reset (a full electrical #RST of the PCI card).
+If any driver requests a full reset, that is what will be done.
+
+After a full 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, enum pci_channel_state);
+        int (*mmio_enabled)(struct pci_dev *dev);
+        int (*link_reset)(struct pci_dev *dev);
+        int (*slot_reset)(struct pci_dev *dev);
+        void (*resume)(struct pci_dev *dev);
+};
+
+The possible channel states are:
+enum pci_channel_state {
+        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 */
+};
+
+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 reset. If link_reset() is not implemented, the card is assumed as
+not care about link resets. 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
+-------------------
+PCI bus error is detect 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 hard
+                  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).
+
+>>> 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/O's will fail, returning
+>>> a value of 0xff on read, and writes will be dropped. If the device
+>>> driver attempts more than 10K I/O's to a frozen adapter, it will
+>>> assume that the device driver has gone into an infinite loop, and
+>>> it will panic the kernel. There doesn't seem to be any other
+>>> way of stopping a device driver that insists on spinning on I/O.
+
+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 (hrm... to be discussed, I prefer 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 is 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 wont call this callback, and instead
+will have gone directly to STEP 3 (Link Reset) or STEP 4 (Slot Reset)
+
+>>> The following is proposed; no platform implements this yet:
+>>> Proposal: All I/O's 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 it's 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)
+
+>>> The current powerpc implementation does not implement this callback.
+
+
+STEP 3: Link Reset
+------------------
+The platform resets the link, and then calls the link_reset() callback
+on all affected device drivers.  This is a PCI-Express specific state
+and is done whenever a non-fatal error has been detected that can be
+"solved" by resetting the link. This call informs the driver of the
+reset and the driver should check to see if the device appears to be
+in working condition.
+
+The driver is not supposed to restart normal driver I/O operations
+at this point.  It should limit itself to "probing" the device to
+check it's recoverability status. If all is right, then the platform
+will call resume() once all drivers have ack'd link_reset().
+
+        Result codes:
+                (identical to STEP 3 (MMIO Enabled)
+
+The platform then proceeds to either STEP 4 (Slot Reset) or STEP 5
+(Resume Operations).
+
+>>> The current powerpc implementation does not implement this callback.
+
+
+STEP 4: Slot Reset
+------------------
+The platform performs a soft or hard reset of the device, and then
+calls the slot_reset() callback.
+
+A soft reset consists of asserting the adapter #RST line and then
+restoring the PCI BAR's 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.
+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 he card is in a fresh state and is fully functional. In
+particular, interrupt generation should work normally.
+
+Drivers should not yet 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.  It 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.
+
+Platform proceeds either to STEP 5 (Resume Operations) or STEP 6 (Permanent
+Failure).
+
+>>> The current powerpc implementation does not currently 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 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. DMA's to
+wild addresses or bogus split transactions due to programming
+errors. See the discussion in powerpc/eeh-pci-error-recovery.txt
+for additional detail on real-life experience of the causes of
+software errors.
+
+
+Conclusion; General Remarks
+---------------------------
+The way those 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
+resume callback is sent, 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 :)
+
+>>> Implementation details for the powerpc platform are discussed in
+>>> the file Documentation/powerpc/eeh-pci-error-recovery.txt
+
+>>> As of this writing, there are six 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.c
+>>> drivers/scsi/sym53cxx_2
+>>> drivers/next/e100.c
+>>> drivers/net/e1000
+>>> drivers/net/ixgb
+>>> drivers/net/s2io.c
+
+The End
+-------