edac.txt

linux2.6.16版本

EDAC - Error Detection And Correction

Written by Doug Thompson <[e-mail]>
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 maintainence 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 addin 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.

The PCI Parity EDAC device has the ability to "skip" known flakey
cards during the parity scan. These are set by the parity "blacklist"
interface in the sysfs for PCI Parity. (See the PCI section in the sysfs
section below.) There is also a parity "whitelist" which is used as
an explicit list of devices to scan, while the blacklist is a list
of devices to skip.

EDAC will have future error detectors that will be added or integrated
into EDAC 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 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
layed out in a Chip-Select Row (csrowX) and Channel table (chX). There can
be multiple csrows and two 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.


		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 labled '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 similiaryly 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 about
	right for most uses.

	LOAD TIME: module/kernel parameter: poll_msec=[0|1]

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


Module Version read-only attribute file:

	'mc_version'

	The EDAC CORE modules's version and compile date are shown here to
	indicate what EDAC is running.



============================================================================
'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.



DIMM capability attribute file:

	'edac_capability'

	The EDAC (Error Detection and Correction) capabilities/modes of
	the memory controller hardware.


DIMM Current Capability attribute file:

	'edac_current_capability'

	The EDAC capabilities available with the hardware
	configuration.  This may not be the same as "EDAC capability"
	if the correct memory is not used.  If a memory controller is
	capable of EDAC, but DIMMs without check bits are in use, then
	Parity, SECDED, S4ECD4ED capabilities will not be available
	even though the memory controller might be capable of those
	modes with the proper memory loaded.


Memory Type supported on this controller attribute file:

	'supported_mem_type'

	This attribute file displays the memory type, usually
	buffered and unbuffered DIMMs.


Memory Controller name attribute file:

	'mc_name'

	This attribute file displays the type of memory controller
	that is being utilized.


Memory Controller Module name attribute file:

	'module_name'

	This attribute file displays the memory controller module name,
	version and date built.  The name of the memory controller
	hardware - some drivers work with multiple controllers and
	this field shows which hardware is present.


Total memory managed by this memory controller attribute file:

	'size_mb'