[PATCH] PCI Error Recovery: documentation

Various PCI bus errors can be signaled by newer PCI controllers.
Recovering from those errors requires an infrastructure to notify
affected device drivers of the error, and a way of walking through
a reset sequence.  This patch adds documentation describing the
current error recovery proposal.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 246 insertions, 0 deletions

diff --git a/Documentation/pci-error-recovery.txt b/Documentation/pci-error-recovery.txt
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+
+                       PCI Error Recovery
+                       ------------------
+                         May 31, 2005
+
+               Current document maintainer:
+           Linas Vepstas <linas@austin.ibm.com>
+
+
+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 <linas@austin.ibm.com>