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IBM online documentation for EEH uses "extended error handling" and "enhanced error handling" to refer to the same thing, in different places. The only place mentioning it as "enhanced error handling" in the kernel is the MAINTAINERS file, and it's "extended" in some documentation. IBM originally defined EEH as "enhanced error handling", so standardise all mentions of EEH to use that term. Signed-off-by: Russell Currey <ruscur@russell.cc> Acked-by: Gavin Shan <gwshan@linux.vnet.ibm.com> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
335 lines
15 KiB
Plaintext
335 lines
15 KiB
Plaintext
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PCI Bus EEH Error Recovery
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--------------------------
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Linas Vepstas
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<linas@austin.ibm.com>
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12 January 2005
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Overview:
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---------
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The IBM POWER-based pSeries and iSeries computers include PCI bus
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controller chips that have extended capabilities for detecting and
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reporting a large variety of PCI bus error conditions. These features
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go under the name of "EEH", for "Enhanced Error Handling". The EEH
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hardware features allow PCI bus errors to be cleared and a PCI
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card to be "rebooted", without also having to reboot the operating
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system.
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This is in contrast to traditional PCI error handling, where the
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PCI chip is wired directly to the CPU, and an error would cause
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a CPU machine-check/check-stop condition, halting the CPU entirely.
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Another "traditional" technique is to ignore such errors, which
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can lead to data corruption, both of user data or of kernel data,
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hung/unresponsive adapters, or system crashes/lockups. Thus,
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the idea behind EEH is that the operating system can become more
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reliable and robust by protecting it from PCI errors, and giving
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the OS the ability to "reboot"/recover individual PCI devices.
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Future systems from other vendors, based on the PCI-E specification,
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may contain similar features.
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Causes of EEH Errors
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--------------------
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EEH was originally designed to guard against hardware failure, such
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as PCI cards dying from heat, humidity, dust, vibration and bad
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electrical connections. The vast majority of EEH errors seen in
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"real life" are due to either poorly seated PCI cards, or,
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unfortunately quite commonly, due to device driver bugs, device firmware
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bugs, and sometimes PCI card hardware bugs.
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The most common software bug, is one that causes the device to
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attempt to DMA to a location in system memory that has not been
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reserved for DMA access for that card. This is a powerful feature,
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as it prevents what; otherwise, would have been silent memory
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corruption caused by the bad DMA. A number of device driver
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bugs have been found and fixed in this way over the past few
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years. Other possible causes of EEH errors include data or
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address line parity errors (for example, due to poor electrical
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connectivity due to a poorly seated card), and PCI-X split-completion
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errors (due to software, device firmware, or device PCI hardware bugs).
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The vast majority of "true hardware failures" can be cured by
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physically removing and re-seating the PCI card.
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Detection and Recovery
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----------------------
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In the following discussion, a generic overview of how to detect
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and recover from EEH errors will be presented. This is followed
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by an overview of how the current implementation in the Linux
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kernel does it. The actual implementation is subject to change,
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and some of the finer points are still being debated. These
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may in turn be swayed if or when other architectures implement
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similar functionality.
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When a PCI Host Bridge (PHB, the bus controller connecting the
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PCI bus to the system CPU electronics complex) detects a PCI error
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condition, it will "isolate" the affected PCI card. Isolation
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will block all writes (either to the card from the system, or
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from the card to the system), and it will cause all reads to
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return all-ff's (0xff, 0xffff, 0xffffffff for 8/16/32-bit reads).
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This value was chosen because it is the same value you would
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get if the device was physically unplugged from the slot.
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This includes access to PCI memory, I/O space, and PCI config
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space. Interrupts; however, will continued to be delivered.
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Detection and recovery are performed with the aid of ppc64
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firmware. The programming interfaces in the Linux kernel
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into the firmware are referred to as RTAS (Run-Time Abstraction
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Services). The Linux kernel does not (should not) access
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the EEH function in the PCI chipsets directly, primarily because
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there are a number of different chipsets out there, each with
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different interfaces and quirks. The firmware provides a
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uniform abstraction layer that will work with all pSeries
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and iSeries hardware (and be forwards-compatible).
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If the OS or device driver suspects that a PCI slot has been
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EEH-isolated, there is a firmware call it can make to determine if
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this is the case. If so, then the device driver should put itself
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into a consistent state (given that it won't be able to complete any
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pending work) and start recovery of the card. Recovery normally
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would consist of resetting the PCI device (holding the PCI #RST
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line high for two seconds), followed by setting up the device
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config space (the base address registers (BAR's), latency timer,
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cache line size, interrupt line, and so on). This is followed by a
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reinitialization of the device driver. In a worst-case scenario,
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the power to the card can be toggled, at least on hot-plug-capable
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slots. In principle, layers far above the device driver probably
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do not need to know that the PCI card has been "rebooted" in this
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way; ideally, there should be at most a pause in Ethernet/disk/USB
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I/O while the card is being reset.
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If the card cannot be recovered after three or four resets, the
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kernel/device driver should assume the worst-case scenario, that the
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card has died completely, and report this error to the sysadmin.
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In addition, error messages are reported through RTAS and also through
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syslogd (/var/log/messages) to alert the sysadmin of PCI resets.
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The correct way to deal with failed adapters is to use the standard
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PCI hotplug tools to remove and replace the dead card.
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Current PPC64 Linux EEH Implementation
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--------------------------------------
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At this time, a generic EEH recovery mechanism has been implemented,
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so that individual device drivers do not need to be modified to support
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EEH recovery. This generic mechanism piggy-backs on the PCI hotplug
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infrastructure, and percolates events up through the userspace/udev
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infrastructure. Following is a detailed description of how this is
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accomplished.
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EEH must be enabled in the PHB's very early during the boot process,
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and if a PCI slot is hot-plugged. The former is performed by
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eeh_init() in arch/powerpc/platforms/pseries/eeh.c, and the later by
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drivers/pci/hotplug/pSeries_pci.c calling in to the eeh.c code.
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EEH must be enabled before a PCI scan of the device can proceed.
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Current Power5 hardware will not work unless EEH is enabled;
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although older Power4 can run with it disabled. Effectively,
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EEH can no longer be turned off. PCI devices *must* be
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registered with the EEH code; the EEH code needs to know about
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the I/O address ranges of the PCI device in order to detect an
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error. Given an arbitrary address, the routine
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pci_get_device_by_addr() will find the pci device associated
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with that address (if any).
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The default arch/powerpc/include/asm/io.h macros readb(), inb(), insb(),
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etc. include a check to see if the i/o read returned all-0xff's.
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If so, these make a call to eeh_dn_check_failure(), which in turn
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asks the firmware if the all-ff's value is the sign of a true EEH
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error. If it is not, processing continues as normal. The grand
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total number of these false alarms or "false positives" can be
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seen in /proc/ppc64/eeh (subject to change). Normally, almost
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all of these occur during boot, when the PCI bus is scanned, where
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a large number of 0xff reads are part of the bus scan procedure.
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If a frozen slot is detected, code in
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arch/powerpc/platforms/pseries/eeh.c will print a stack trace to
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syslog (/var/log/messages). This stack trace has proven to be very
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useful to device-driver authors for finding out at what point the EEH
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error was detected, as the error itself usually occurs slightly
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beforehand.
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Next, it uses the Linux kernel notifier chain/work queue mechanism to
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allow any interested parties to find out about the failure. Device
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drivers, or other parts of the kernel, can use
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eeh_register_notifier(struct notifier_block *) to find out about EEH
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events. The event will include a pointer to the pci device, the
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device node and some state info. Receivers of the event can "do as
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they wish"; the default handler will be described further in this
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section.
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To assist in the recovery of the device, eeh.c exports the
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following functions:
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rtas_set_slot_reset() -- assert the PCI #RST line for 1/8th of a second
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rtas_configure_bridge() -- ask firmware to configure any PCI bridges
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located topologically under the pci slot.
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eeh_save_bars() and eeh_restore_bars(): save and restore the PCI
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config-space info for a device and any devices under it.
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A handler for the EEH notifier_block events is implemented in
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drivers/pci/hotplug/pSeries_pci.c, called handle_eeh_events().
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It saves the device BAR's and then calls rpaphp_unconfig_pci_adapter().
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This last call causes the device driver for the card to be stopped,
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which causes uevents to go out to user space. This triggers
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user-space scripts that might issue commands such as "ifdown eth0"
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for ethernet cards, and so on. This handler then sleeps for 5 seconds,
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hoping to give the user-space scripts enough time to complete.
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It then resets the PCI card, reconfigures the device BAR's, and
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any bridges underneath. It then calls rpaphp_enable_pci_slot(),
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which restarts the device driver and triggers more user-space
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events (for example, calling "ifup eth0" for ethernet cards).
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Device Shutdown and User-Space Events
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-------------------------------------
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This section documents what happens when a pci slot is unconfigured,
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focusing on how the device driver gets shut down, and on how the
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events get delivered to user-space scripts.
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Following is an example sequence of events that cause a device driver
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close function to be called during the first phase of an EEH reset.
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The following sequence is an example of the pcnet32 device driver.
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rpa_php_unconfig_pci_adapter (struct slot *) // in rpaphp_pci.c
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{
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calls
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pci_remove_bus_device (struct pci_dev *) // in /drivers/pci/remove.c
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{
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calls
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pci_destroy_dev (struct pci_dev *)
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{
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calls
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device_unregister (&dev->dev) // in /drivers/base/core.c
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{
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calls
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device_del (struct device *)
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{
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calls
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bus_remove_device() // in /drivers/base/bus.c
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{
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calls
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device_release_driver()
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{
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calls
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struct device_driver->remove() which is just
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pci_device_remove() // in /drivers/pci/pci_driver.c
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{
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calls
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struct pci_driver->remove() which is just
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pcnet32_remove_one() // in /drivers/net/pcnet32.c
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{
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calls
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unregister_netdev() // in /net/core/dev.c
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{
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calls
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dev_close() // in /net/core/dev.c
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{
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calls dev->stop();
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which is just pcnet32_close() // in pcnet32.c
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{
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which does what you wanted
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to stop the device
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}
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}
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}
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which
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frees pcnet32 device driver memory
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}
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}}}}}}
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in drivers/pci/pci_driver.c,
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struct device_driver->remove() is just pci_device_remove()
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which calls struct pci_driver->remove() which is pcnet32_remove_one()
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which calls unregister_netdev() (in net/core/dev.c)
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which calls dev_close() (in net/core/dev.c)
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which calls dev->stop() which is pcnet32_close()
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which then does the appropriate shutdown.
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---
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Following is the analogous stack trace for events sent to user-space
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when the pci device is unconfigured.
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rpa_php_unconfig_pci_adapter() { // in rpaphp_pci.c
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calls
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pci_remove_bus_device (struct pci_dev *) { // in /drivers/pci/remove.c
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calls
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pci_destroy_dev (struct pci_dev *) {
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calls
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device_unregister (&dev->dev) { // in /drivers/base/core.c
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calls
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device_del(struct device * dev) { // in /drivers/base/core.c
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calls
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kobject_del() { //in /libs/kobject.c
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calls
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kobject_uevent() { // in /libs/kobject.c
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calls
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kset_uevent() { // in /lib/kobject.c
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calls
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kset->uevent_ops->uevent() // which is really just
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a call to
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dev_uevent() { // in /drivers/base/core.c
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calls
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dev->bus->uevent() which is really just a call to
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pci_uevent () { // in drivers/pci/hotplug.c
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which prints device name, etc....
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}
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}
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then kobject_uevent() sends a netlink uevent to userspace
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--> userspace uevent
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(during early boot, nobody listens to netlink events and
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kobject_uevent() executes uevent_helper[], which runs the
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event process /sbin/hotplug)
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}
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}
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kobject_del() then calls sysfs_remove_dir(), which would
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trigger any user-space daemon that was watching /sysfs,
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and notice the delete event.
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Pro's and Con's of the Current Design
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-------------------------------------
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There are several issues with the current EEH software recovery design,
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which may be addressed in future revisions. But first, note that the
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big plus of the current design is that no changes need to be made to
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individual device drivers, so that the current design throws a wide net.
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The biggest negative of the design is that it potentially disturbs
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network daemons and file systems that didn't need to be disturbed.
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-- A minor complaint is that resetting the network card causes
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user-space back-to-back ifdown/ifup burps that potentially disturb
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network daemons, that didn't need to even know that the pci
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card was being rebooted.
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-- A more serious concern is that the same reset, for SCSI devices,
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causes havoc to mounted file systems. Scripts cannot post-facto
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unmount a file system without flushing pending buffers, but this
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is impossible, because I/O has already been stopped. Thus,
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ideally, the reset should happen at or below the block layer,
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so that the file systems are not disturbed.
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Reiserfs does not tolerate errors returned from the block device.
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Ext3fs seems to be tolerant, retrying reads/writes until it does
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succeed. Both have been only lightly tested in this scenario.
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The SCSI-generic subsystem already has built-in code for performing
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SCSI device resets, SCSI bus resets, and SCSI host-bus-adapter
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(HBA) resets. These are cascaded into a chain of attempted
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resets if a SCSI command fails. These are completely hidden
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from the block layer. It would be very natural to add an EEH
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reset into this chain of events.
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-- If a SCSI error occurs for the root device, all is lost unless
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the sysadmin had the foresight to run /bin, /sbin, /etc, /var
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and so on, out of ramdisk/tmpfs.
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Conclusions
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-----------
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There's forward progress ...
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