PCI Error Recovery ------------------ May 31, 2005 Current document maintainer: Linas Vepstas Some PCI bus controllers are able to detect certain "hard" PCI errors on the bus, such as parity errors on the data and address busses, as well as SERR and PERR errors. These chipsets are then able to disable I/O to/from the affected device, so that, for example, a bad DMA address doesn't end up corrupting system memory. These same chipsets are also able to reset the affected PCI device, and return it to working condition. This document describes a generic API form performing error recovery. The core idea is that after a PCI error has been detected, there must be a way for the kernel to coordinate with all affected device drivers so that the pci card can be made operational again, possibly after performing a full electrical #RST of the PCI card. The API below provides a generic API for device drivers to be notified of PCI errors, and to be notified of, and respond to, a reset sequence. Preliminary sketch of API, cut-n-pasted-n-modified email from 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. The absence of this pointer in pci_driver denotes an "non-aware" driver, behaviour on these is platform dependant. Platforms like ppc64 can try to simulate pci hotplug remove/add. The definition of "pci_error_token" is not covered here. It is based on Seto's work on the synchronous error detection. We still need to define functions for extracting infos out of an opaque error token. This is separate from this API. This structure has the form: struct pci_error_handlers { int (*error_detected)(struct pci_dev *dev, pci_error_token error); int (*mmio_enabled)(struct pci_dev *dev); int (*resume)(struct pci_dev *dev); int (*link_reset)(struct pci_dev *dev); int (*slot_reset)(struct pci_dev *dev); }; A driver doesn't have to implement all of these callbacks. The only mandatory one is 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 the driver is assumed as not doing any direct recovery and requires a reset. If link_reset() is not implemented, the card is assumed as not caring about link resets, in which case, if recover is supported, the core can try recover (but not slot_reset() unless it really did reset the slot). If slot_reset() is not supported, link_reset() can be called instead on a slot reset. At first, the call will always be : 1) error_detected() Error detected. This is sent once after an error has been detected. At this point, the device might not be accessible anymore depending on the platform (the slot will be isolated on ppc64). The driver may already have "noticed" the error because of a failing IO, but this is the proper "synchronisation point", that is, it gives a chance to the driver 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. Result codes: - PCIERR_RESULT_CAN_RECOVER: Driever 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 informations (see below). - PCIERR_RESULT_NEED_RESET: Driver returns this if it thinks it can't recover unless the slot is reset. - PCIERR_RESULT_DISCONNECT: Return this if driver thinks it won't recover at all, (this will detach the driver ? or just leave it dangling ? to be decided) So at this point, we have called error_detected() for all drivers on the segment that had the error. On ppc64, the slot is isolated. What happens now typically depends on the result from the drivers. If all drivers on the segment/slot return PCIERR_RESULT_CAN_RECOVER, we would re-enable IOs on the slot (or do nothing special if the platform doesn't isolate slots) and call 2). If not and we can reset slots, we go to 4), if neither, we have a dead slot. If it's an hotplug slot, we might "simulate" reset by triggering HW unplug/replug though. >>> Current ppc64 implementation assumes that a device driver will >>> *not* schedule or semaphore in this routine; the current ppc64 >>> implementation uses one kernel thread to notify all devices; >>> thus, of 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 ppc64 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 the kernel. 2) mmio_enabled() 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 sent if all drivers on a segment agree that they can try to recover and 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 doesn't call this callback and goes directly to 3) or 4). All IOs 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. Result codes: - PCIERR_RESULT_RECOVERED Driver returns this if it thinks the device is fully functionnal 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. - PCIERR_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. - PCIERR_RESULT_DISCONNECT Same as above. Total failure, no recovery even after reset driver dead. (To be defined more precisely) >>> The current ppc64 implementation does not implement this callback. 3) link_reset() This is called after the link has been reset. This is typically a PCI Express specific state at this point 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 if the device appears to be in working condition. This function acts a bit like 2) mmio_enabled(), in that the driver is not supposed to restart normal driver I/O operations right away. Instead, it should just "probe" the device to check it's recoverability status. If all is right, then the core will call resume() once all drivers have ack'd link_reset(). Result codes: (identical to mmio_enabled) >>> The current ppc64 implementation does not implement this callback. 4) slot_reset() This is called after the slot has been soft or hard reset by the platform. A soft reset consists of asserting the adapter #RST line and then restoring the PCI BARs and PCI configuration header. If the platform supports PCI hotplug, then it might instead perform a hard reset by toggling power on the slot off/on. This call gives drivers the chance to re-initialize the hardware (re-download firmware, etc.), but drivers shouldn't restart normal I/O processing operations at this point. (See note about interrupts; interrupts aren't guaranteed to be delivered until the resume() callback has been called). If all device drivers report success on this callback, the patform will call resume() to complete the error handling 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 migh 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. Result codes: - PCIERR_RESULT_DISCONNECT Same as above. >>> The current ppc64 implementation does not try a power-cycle reset >>> if the driver returned PCIERR_RESULT_DISCONNECT. However, it should. 5) resume() This is called if all drivers on the segment have returned PCIERR_RESULT_RECOVERED from one of the 3 prevous callbacks. That basically tells the driver to restart activity, tht everything is back and running. No result code is taken into account here. If a new error happens, it will restart a new error handling process. That's it. I think this covers all the possibilities. The way those callbacks are called is platform policy. A platform with no slot reset capability for example 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, there is a note about interrupts. If you get an interrupt and your device is dead or has been isolated, there is a problem :) After much thinking, I decided to leave that to the platform. That is, the recovery API only precies that: - There is no guarantee that interrupt delivery can proceed from any device on the segment starting from the error detection and until the restart callback is sent, at which point interrupts are expected to be fully operational. - There is no guarantee that interrupt delivery is stopped, that is, ad river that gets an interrupts after detecting an error, or that detects and 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 taht 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 :) Revised: 31 May 2005 Linas Vepstas