337 lines
15 KiB
ReStructuredText
337 lines
15 KiB
ReStructuredText
==========================
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PCI Bus EEH Error Recovery
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==========================
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Linas Vepstas <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 continue 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()
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assert the PCI #RST line for 1/8th of a second
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rtas_configure_bridge()
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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():
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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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