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diff --git a/Documentation/PCI/pci-error-recovery.txt b/Documentation/PCI/pci-error-recovery.txt new file mode 100644 index 0000000..6650af4 --- /dev/null +++ b/Documentation/PCI/pci-error-recovery.txt @@ -0,0 +1,396 @@ + + 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 +------- |
