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