

EDAC - Error Detection And Correction

Written by Doug Thompson <norsk5@xmission.com>
7 Dec 2005


EDAC was written by:
	Thayne Harbaugh,
	modified by Dave Peterson, Doug Thompson, et al,
	from the bluesmoke.sourceforge.net project.


============================================================================
EDAC PURPOSE

The 'edac' kernel module goal is to detect and report errors that occur
within the computer system. In the initial release, memory Correctable Errors
(CE) and Uncorrectable Errors (UE) are the primary errors being harvested.

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'.


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.

[There are patches in the kernel queue which will allow for storage of
quirks of PCI devices reporting false parity positives. The 2.6.18
kernel should have those patches included. When that becomes available,
then EDAC will be patched to utilize that information to "skip" such
devices.]

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_mc.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 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 'mc','mcX' and 'csrowX' directories are several
EDAC control and attribute files.


============================================================================
DIRECTORY 'mc'

In directory 'mc' are EDAC system overall control and attribute files:


Panic on UE control file:

	'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: panic_on_ue=[0|1]

	RUN TIME:  echo "1" >/sys/devices/system/edac/mc/panic_on_ue


Log UE control file:

	'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: log_ue=[0|1]

	RUN TIME: echo "1" >/sys/devices/system/edac/mc/log_ue


Log CE control file:

	'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: log_ce=[0|1]

	RUN TIME: echo "1" >/sys/devices/system/edac/mc/log_ce


Polling period control file:

	'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: poll_msec=[0|1]

	RUN TIME: echo "1000" >/sys/devices/system/edac/mc/poll_msec


============================================================================
'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


============================================================================
'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


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: panic_on_pci_parity=[0|1]

	Enable:
	echo "1" >/sys/devices/system/edac/pci/panic_on_pci_parity

	Disable:
	echo "0" >/sys/devices/system/edac/pci/panic_on_pci_parity


Parity Count:

	'pci_parity_count'

	This attribute file will display the number of parity errors that
	have been detected.


=======================================================================