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EDAC - Error Detection And Correction
Written by Doug Thompson <dougthompson@xmission.com>
7 Dec 2005
17 Jul 2007 Updated
EDAC is maintained and written by:
Doug Thompson, Dave Jiang, Dave Peterson et al,
original author: Thayne Harbaugh,
Contact:
website: bluesmoke.sourceforge.net
mailing list: bluesmoke-devel@lists.sourceforge.net
"bluesmoke" was the name for this device driver when it was "out-of-tree"
and maintained at sourceforge.net. When it was pushed into 2.6.16 for the
first time, it was renamed to 'EDAC'.
The bluesmoke project at sourceforge.net is now utilized as a 'staging area'
for EDAC development, before it is sent upstream to kernel.org
At the bluesmoke/EDAC project site is a series of quilt patches against
recent kernels, stored in a SVN repository. For easier downloading, there
is also a tarball snapshot available.
============================================================================
EDAC PURPOSE
The 'edac' kernel module goal is to detect and report errors that occur
within the computer system running under linux.
MEMORY
In the initial release, memory Correctable Errors (CE) and Uncorrectable
Errors (UE) are the primary errors being harvested. These types of errors
are harvested by the 'edac_mc' class of device.
Detecting CE events, then harvesting those events and reporting them,
CAN be a predictor of future UE events. With CE events, the system can
continue to operate, but with less safety. Preventive maintenance and
proactive part replacement of memory DIMMs exhibiting CEs can reduce
the likelihood of the dreaded UE events and system 'panics'.
NON-MEMORY
A new feature for EDAC, the edac_device class of device, was added in
the 2.6.23 version of the kernel.
This new device type allows for non-memory type of ECC hardware detectors
to have their states harvested and presented to userspace via the sysfs
interface.
Some architectures have ECC detectors for L1, L2 and L3 caches, along with DMA
engines, fabric switches, main data path switches, interconnections,
and various other hardware data paths. If the hardware reports it, then
a edac_device device probably can be constructed to harvest and present
that to userspace.
PCI BUS SCANNING
In addition, PCI Bus Parity and SERR Errors are scanned for on PCI devices
in order to determine if errors are occurring on data transfers.
The presence of PCI Parity errors must be examined with a grain of salt.
There are several add-in adapters that do NOT follow the PCI specification
with regards to Parity generation and reporting. The specification says
the vendor should tie the parity status bits to 0 if they do not intend
to generate parity. Some vendors do not do this, and thus the parity bit
can "float" giving false positives.
In the kernel there is a PCI device attribute located in sysfs that is
checked by the EDAC PCI scanning code. If that attribute is set,
PCI parity/error scanning is skipped for that device. The attribute
is:
broken_parity_status
as is located in /sys/devices/pci<XXX>/0000:XX:YY.Z directorys for
PCI devices.
FUTURE HARDWARE SCANNING
EDAC will have future error detectors that will be integrated with
EDAC or added to it, in the following list:
MCE Machine Check Exception
MCA Machine Check Architecture
NMI NMI notification of ECC errors
MSRs Machine Specific Register error cases
and other mechanisms.
These errors are usually bus errors, ECC errors, thermal throttling
and the like.
============================================================================
EDAC VERSIONING
EDAC is composed of a "core" module (edac_core.ko) and several Memory
Controller (MC) driver modules. On a given system, the CORE
is loaded and one MC driver will be loaded. Both the CORE and
the MC driver (or edac_device driver) have individual versions that reflect
current release level of their respective modules.
Thus, to "report" on what version a system is running, one must report both
the CORE's and the MC driver's versions.
LOADING
If 'edac' was statically linked with the kernel then no loading is
necessary. If 'edac' was built as modules then simply modprobe the
'edac' pieces that you need. You should be able to modprobe
hardware-specific modules and have the dependencies load the necessary core
modules.
Example:
$> modprobe amd76x_edac
loads both the amd76x_edac.ko memory controller module and the edac_mc.ko
core module.
============================================================================
EDAC sysfs INTERFACE
EDAC presents a 'sysfs' interface for control, reporting and attribute
reporting purposes.
EDAC lives in the /sys/devices/system/edac directory.
Within this directory there currently reside 2 'edac' components:
mc memory controller(s) system
pci PCI control and status system
============================================================================
Memory Controller (mc) Model
First a background on the memory controller's model abstracted in EDAC.
Each 'mc' device controls a set of DIMM memory modules. These modules are
laid out in a Chip-Select Row (csrowX) and Channel table (chX). There can
be multiple csrows and multiple channels.
Memory controllers allow for several csrows, with 8 csrows being a typical value.
Yet, the actual number of csrows depends on the electrical "loading"
of a given motherboard, memory controller and DIMM characteristics.
Dual channels allows for 128 bit data transfers to the CPU from memory.
Some newer chipsets allow for more than 2 channels, like Fully Buffered DIMMs
(FB-DIMMs). The following example will assume 2 channels:
Channel 0 Channel 1
===================================
csrow0 | DIMM_A0 | DIMM_B0 |
csrow1 | DIMM_A0 | DIMM_B0 |
===================================
===================================
csrow2 | DIMM_A1 | DIMM_B1 |
csrow3 | DIMM_A1 | DIMM_B1 |
===================================
In the above example table there are 4 physical slots on the motherboard
for memory DIMMs:
DIMM_A0
DIMM_B0
DIMM_A1
DIMM_B1
Labels for these slots are usually silk screened on the motherboard. Slots
labeled 'A' are channel 0 in this example. Slots labeled 'B'
are channel 1. Notice that there are two csrows possible on a
physical DIMM. These csrows are allocated their csrow assignment
based on the slot into which the memory DIMM is placed. Thus, when 1 DIMM
is placed in each Channel, the csrows cross both DIMMs.
Memory DIMMs come single or dual "ranked". A rank is a populated csrow.
Thus, 2 single ranked DIMMs, placed in slots DIMM_A0 and DIMM_B0 above
will have 1 csrow, csrow0. csrow1 will be empty. On the other hand,
when 2 dual ranked DIMMs are similarly placed, then both csrow0 and
csrow1 will be populated. The pattern repeats itself for csrow2 and
csrow3.
The representation of the above is reflected in the directory tree
in EDAC's sysfs interface. Starting in directory
/sys/devices/system/edac/mc each memory controller will be represented
by its own 'mcX' directory, where 'X" is the index of the MC.
..../edac/mc/
|
|->mc0
|->mc1
|->mc2
....
Under each 'mcX' directory each 'csrowX' is again represented by a
'csrowX', where 'X" is the csrow index:
.../mc/mc0/
|
|->csrow0
|->csrow2
|->csrow3
....
Notice that there is no csrow1, which indicates that csrow0 is
composed of a single ranked DIMMs. This should also apply in both
Channels, in order to have dual-channel mode be operational. Since
both csrow2 and csrow3 are populated, this indicates a dual ranked
set of DIMMs for channels 0 and 1.
Within each of the 'mcX' and 'csrowX' directories are several
EDAC control and attribute files.
============================================================================
'mcX' DIRECTORIES
In 'mcX' directories are EDAC control and attribute files for
this 'X" instance of the memory controllers:
Counter reset control file:
'reset_counters'
This write-only control file will zero all the statistical counters
for UE and CE errors. Zeroing the counters will also reset the timer
indicating how long since the last counter zero. This is useful
for computing errors/time. Since the counters are always reset at
driver initialization time, no module/kernel parameter is available.
RUN TIME: echo "anything" >/sys/devices/system/edac/mc/mc0/counter_reset
This resets the counters on memory controller 0
Seconds since last counter reset control file:
'seconds_since_reset'
This attribute file displays how many seconds have elapsed since the
last counter reset. This can be used with the error counters to
measure error rates.
Memory Controller name attribute file:
'mc_name'
This attribute file displays the type of memory controller
that is being utilized.
Total memory managed by this memory controller attribute file:
'size_mb'
This attribute file displays, in count of megabytes, of memory
that this instance of memory controller manages.
Total Uncorrectable Errors count attribute file:
'ue_count'
This attribute file displays the total count of uncorrectable
errors that have occurred on this memory controller. If panic_on_ue
is set this counter will not have a chance to increment,
since EDAC will panic the system.
Total UE count that had no information attribute fileY:
'ue_noinfo_count'
This attribute file displays the number of UEs that
have occurred have occurred with no informations as to which DIMM
slot is having errors.
Total Correctable Errors count attribute file:
'ce_count'
This attribute file displays the total count of correctable
errors that have occurred on this memory controller. This
count is very important to examine. CEs provide early
indications that a DIMM is beginning to fail. This count
field should be monitored for non-zero values and report
such information to the system administrator.
Total Correctable Errors count attribute file:
'ce_noinfo_count'
This attribute file displays the number of CEs that
have occurred wherewith no informations as to which DIMM slot
is having errors. Memory is handicapped, but operational,
yet no information is available to indicate which slot
the failing memory is in. This count field should be also
be monitored for non-zero values.
Device Symlink:
'device'
Symlink to the memory controller device.
Sdram memory scrubbing rate:
'sdram_scrub_rate'
Read/Write attribute file that controls memory scrubbing. The scrubbing
rate is set by writing a minimum bandwidth in bytes/sec to the attribute
file. The rate will be translated to an internal value that gives at
least the specified rate.
Reading the file will return the actual scrubbing rate employed.
If configuration fails or memory scrubbing is not implemented, the value
of the attribute file will be -1.
============================================================================
'csrowX' DIRECTORIES
In the 'csrowX' directories are EDAC control and attribute files for
this 'X" instance of csrow:
Total Uncorrectable Errors count attribute file:
'ue_count'
This attribute file displays the total count of uncorrectable
errors that have occurred on this csrow. If panic_on_ue is set
this counter will not have a chance to increment, since EDAC
will panic the system.
Total Correctable Errors count attribute file:
'ce_count'
This attribute file displays the total count of correctable
errors that have occurred on this csrow. This
count is very important to examine. CEs provide early
indications that a DIMM is beginning to fail. This count
field should be monitored for non-zero values and report
such information to the system administrator.
Total memory managed by this csrow attribute file:
'size_mb'
This attribute file displays, in count of megabytes, of memory
that this csrow contains.
Memory Type attribute file:
'mem_type'
This attribute file will display what type of memory is currently
on this csrow. Normally, either buffered or unbuffered memory.
Examples:
Registered-DDR
Unbuffered-DDR
EDAC Mode of operation attribute file:
'edac_mode'
This attribute file will display what type of Error detection
and correction is being utilized.
Device type attribute file:
'dev_type'
This attribute file will display what type of DRAM device is
being utilized on this DIMM.
Examples:
x1
x2
x4
x8
Channel 0 CE Count attribute file:
'ch0_ce_count'
This attribute file will display the count of CEs on this
DIMM located in channel 0.
Channel 0 UE Count attribute file:
'ch0_ue_count'
This attribute file will display the count of UEs on this
DIMM located in channel 0.
Channel 0 DIMM Label control file:
'ch0_dimm_label'
This control file allows this DIMM to have a label assigned
to it. With this label in the module, when errors occur
the output can provide the DIMM label in the system log.
This becomes vital for panic events to isolate the
cause of the UE event.
DIMM Labels must be assigned after booting, with information
that correctly identifies the physical slot with its
silk screen label. This information is currently very
motherboard specific and determination of this information
must occur in userland at this time.
Channel 1 CE Count attribute file:
'ch1_ce_count'
This attribute file will display the count of CEs on this
DIMM located in channel 1.
Channel 1 UE Count attribute file:
'ch1_ue_count'
This attribute file will display the count of UEs on this
DIMM located in channel 0.
Channel 1 DIMM Label control file:
'ch1_dimm_label'
This control file allows this DIMM to have a label assigned
to it. With this label in the module, when errors occur
the output can provide the DIMM label in the system log.
This becomes vital for panic events to isolate the
cause of the UE event.
DIMM Labels must be assigned after booting, with information
that correctly identifies the physical slot with its
silk screen label. This information is currently very
motherboard specific and determination of this information
must occur in userland at this time.
============================================================================
SYSTEM LOGGING
If logging for UEs and CEs are enabled then system logs will have
error notices indicating errors that have been detected:
EDAC MC0: CE page 0x283, offset 0xce0, grain 8, syndrome 0x6ec3, row 0,
channel 1 "DIMM_B1": amd76x_edac
EDAC MC0: CE page 0x1e5, offset 0xfb0, grain 8, syndrome 0xb741, row 0,
channel 1 "DIMM_B1": amd76x_edac
The structure of the message is:
the memory controller (MC0)
Error type (CE)
memory page (0x283)
offset in the page (0xce0)
the byte granularity (grain 8)
or resolution of the error
the error syndrome (0xb741)
memory row (row 0)
memory channel (channel 1)
DIMM label, if set prior (DIMM B1
and then an optional, driver-specific message that may
have additional information.
Both UEs and CEs with no info will lack all but memory controller,
error type, a notice of "no info" and then an optional,
driver-specific error message.
============================================================================
PCI Bus Parity Detection
On Header Type 00 devices the primary status is looked at
for any parity error regardless of whether Parity is enabled on the
device. (The spec indicates parity is generated in some cases).
On Header Type 01 bridges, the secondary status register is also
looked at to see if parity occurred on the bus on the other side of
the bridge.
SYSFS CONFIGURATION
Under /sys/devices/system/edac/pci are control and attribute files as follows:
Enable/Disable PCI Parity checking control file:
'check_pci_parity'
This control file enables or disables the PCI Bus Parity scanning
operation. Writing a 1 to this file enables the scanning. Writing
a 0 to this file disables the scanning.
Enable:
echo "1" >/sys/devices/system/edac/pci/check_pci_parity
Disable:
echo "0" >/sys/devices/system/edac/pci/check_pci_parity
Parity Count:
'pci_parity_count'
This attribute file will display the number of parity errors that
have been detected.
============================================================================
MODULE PARAMETERS
Panic on UE control file:
'edac_mc_panic_on_ue'
An uncorrectable error will cause a machine panic. This is usually
desirable. It is a bad idea to continue when an uncorrectable error
occurs - it is indeterminate what was uncorrected and the operating
system context might be so mangled that continuing will lead to further
corruption. If the kernel has MCE configured, then EDAC will never
notice the UE.
LOAD TIME: module/kernel parameter: edac_mc_panic_on_ue=[0|1]
RUN TIME: echo "1" > /sys/module/edac_core/parameters/edac_mc_panic_on_ue
Log UE control file:
'edac_mc_log_ue'
Generate kernel messages describing uncorrectable errors. These errors
are reported through the system message log system. UE statistics
will be accumulated even when UE logging is disabled.
LOAD TIME: module/kernel parameter: edac_mc_log_ue=[0|1]
RUN TIME: echo "1" > /sys/module/edac_core/parameters/edac_mc_log_ue
Log CE control file:
'edac_mc_log_ce'
Generate kernel messages describing correctable errors. These
errors are reported through the system message log system.
CE statistics will be accumulated even when CE logging is disabled.
LOAD TIME: module/kernel parameter: edac_mc_log_ce=[0|1]
RUN TIME: echo "1" > /sys/module/edac_core/parameters/edac_mc_log_ce
Polling period control file:
'edac_mc_poll_msec'
The time period, in milliseconds, for polling for error information.
Too small a value wastes resources. Too large a value might delay
necessary handling of errors and might loose valuable information for
locating the error. 1000 milliseconds (once each second) is the current
default. Systems which require all the bandwidth they can get, may
increase this.
LOAD TIME: module/kernel parameter: edac_mc_poll_msec=[0|1]
RUN TIME: echo "1000" > /sys/module/edac_core/parameters/edac_mc_poll_msec
Panic on PCI PARITY Error:
'panic_on_pci_parity'
This control files enables or disables panicking when a parity
error has been detected.
module/kernel parameter: edac_panic_on_pci_pe=[0|1]
Enable:
echo "1" > /sys/module/edac_core/parameters/edac_panic_on_pci_pe
Disable:
echo "0" > /sys/module/edac_core/parameters/edac_panic_on_pci_pe
=======================================================================
EDAC_DEVICE type of device
In the header file, edac_core.h, there is a series of edac_device structures
and APIs for the EDAC_DEVICE.
User space access to an edac_device is through the sysfs interface.
At the location /sys/devices/system/edac (sysfs) new edac_device devices will
appear.
There is a three level tree beneath the above 'edac' directory. For example,
the 'test_device_edac' device (found at the bluesmoke.sourceforget.net website)
installs itself as:
/sys/devices/systm/edac/test-instance
in this directory are various controls, a symlink and one or more 'instance'
directorys.
The standard default controls are:
log_ce boolean to log CE events
log_ue boolean to log UE events
panic_on_ue boolean to 'panic' the system if an UE is encountered
(default off, can be set true via startup script)
poll_msec time period between POLL cycles for events
The test_device_edac device adds at least one of its own custom control:
test_bits which in the current test driver does nothing but
show how it is installed. A ported driver can
add one or more such controls and/or attributes
for specific uses.
One out-of-tree driver uses controls here to allow
for ERROR INJECTION operations to hardware
injection registers
The symlink points to the 'struct dev' that is registered for this edac_device.
INSTANCES
One or more instance directories are present. For the 'test_device_edac' case:
test-instance0
In this directory there are two default counter attributes, which are totals of
counter in deeper subdirectories.
ce_count total of CE events of subdirectories
ue_count total of UE events of subdirectories
BLOCKS
At the lowest directory level is the 'block' directory. There can be 0, 1
or more blocks specified in each instance.
test-block0
In this directory the default attributes are:
ce_count which is counter of CE events for this 'block'
of hardware being monitored
ue_count which is counter of UE events for this 'block'
of hardware being monitored
The 'test_device_edac' device adds 4 attributes and 1 control:
test-block-bits-0 for every POLL cycle this counter
is incremented
test-block-bits-1 every 10 cycles, this counter is bumped once,
and test-block-bits-0 is set to 0
test-block-bits-2 every 100 cycles, this counter is bumped once,
and test-block-bits-1 is set to 0
test-block-bits-3 every 1000 cycles, this counter is bumped once,
and test-block-bits-2 is set to 0
reset-counters writing ANY thing to this control will
reset all the above counters.
Use of the 'test_device_edac' driver should any others to create their own
unique drivers for their hardware systems.
The 'test_device_edac' sample driver is located at the
bluesmoke.sourceforge.net project site for EDAC.
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