334 lines
15 KiB
Plaintext
334 lines
15 KiB
Plaintext
|
|
|
|
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 "Extended 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 eithr poorly seated PCI cards, or,
|
|
unfortunately quite commonly, due 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 reseting 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. Followiing 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/ppc64/kernel/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 include/asm-ppc64/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/ppc64/kernel/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 ...
|
|
|
|
|