.. SPDX-License-Identifier: GPL-2.0
.. include:: <isonum.txt>

==================================================
Reliability, Availability and Serviceability (RAS)
==================================================

This documents different aspects of the RAS functionality present in the
kernel.

RAS concepts
************

Reliability, Availability and Serviceability (RAS) is a concept used on
servers meant to measure their robustness.

Reliability
  is the probability that a system will produce correct outputs.

  * Generally measured as Mean Time Between Failures (MTBF)
  * Enhanced by features that help to avoid, detect and repair hardware faults

Availability
  is the probability that a system is operational at a given time

  * Generally measured as a percentage of downtime per a period of time
  * Often uses mechanisms to detect and correct hardware faults in
    runtime;

Serviceability (or maintainability)
  is the simplicity and speed with which a system can be repaired or
  maintained

  * Generally measured on Mean Time Between Repair (MTBR)

Improving RAS
-------------

In order to reduce systems downtime, a system should be capable of detecting
hardware errors, and, when possible correcting them in runtime. It should
also provide mechanisms to detect hardware degradation, in order to warn
the system administrator to take the action of replacing a component before
it causes data loss or system downtime.

Among the monitoring measures, the most usual ones include:

* CPU – detect errors at instruction execution and at L1/L2/L3 caches;
* Memory – add error correction logic (ECC) to detect and correct errors;
* I/O – add CRC checksums for transferred data;
* Storage – RAID, journal file systems, checksums,
  Self-Monitoring, Analysis and Reporting Technology (SMART).

By monitoring the number of occurrences of error detections, it is possible
to identify if the probability of hardware errors is increasing, and, on such
case, do a preventive maintenance to replace a degraded component while
those errors are correctable.

Types of errors
---------------

Most mechanisms used on modern systems use technologies like Hamming
Codes that allow error correction when the number of errors on a bit packet
is below a threshold. If the number of errors is above, those mechanisms
can indicate with a high degree of confidence that an error happened, but
they can't correct.

Also, sometimes an error occur on a component that it is not used. For
example, a part of the memory that it is not currently allocated.

That defines some categories of errors:

* **Correctable Error (CE)** - the error detection mechanism detected and
  corrected the error. Such errors are usually not fatal, although some
  Kernel mechanisms allow the system administrator to consider them as fatal.

* **Uncorrected Error (UE)** - the amount of errors happened above the error
  correction threshold, and the system was unable to auto-correct.

* **Fatal Error** - when an UE error happens on a critical component of the
  system (for example, a piece of the Kernel got corrupted by an UE), the
  only reliable way to avoid data corruption is to hang or reboot the machine.

* **Non-fatal Error** - when an UE error happens on an unused component,
  like a CPU in power down state or an unused memory bank, the system may
  still run, eventually replacing the affected hardware by a hot spare,
  if available.

  Also, when an error happens on a userspace process, it is also possible to
  kill such process and let userspace restart it.

The mechanism for handling non-fatal errors is usually complex and may
require the help of some userspace application, in order to apply the
policy desired by the system administrator.

Identifying a bad hardware component
------------------------------------

Just detecting a hardware flaw is usually not enough, as the system needs
to pinpoint to the minimal replaceable unit (MRU) that should be exchanged
to make the hardware reliable again.

So, it requires not only error logging facilities, but also mechanisms that
will translate the error message to the silkscreen or component label for
the MRU.

Typically, it is very complex for memory, as modern CPUs interlace memory
from different memory modules, in order to provide a better performance. The
DMI BIOS usually have a list of memory module labels, with can be obtained
using the ``dmidecode`` tool. For example, on a desktop machine, it shows::

	Memory Device
		Total Width: 64 bits
		Data Width: 64 bits
		Size: 16384 MB
		Form Factor: SODIMM
		Set: None
		Locator: ChannelA-DIMM0
		Bank Locator: BANK 0
		Type: DDR4
		Type Detail: Synchronous
		Speed: 2133 MHz
		Rank: 2
		Configured Clock Speed: 2133 MHz

On the above example, a DDR4 SO-DIMM memory module is located at the
system's memory labeled as "BANK 0", as given by the *bank locator* field.
Please notice that, on such system, the *total width* is equal to the
*data width*. It means that such memory module doesn't have error
detection/correction mechanisms.

Unfortunately, not all systems use the same field to specify the memory
bank. On this example, from an older server, ``dmidecode`` shows::

	Memory Device
		Array Handle: 0x1000
		Error Information Handle: Not Provided
		Total Width: 72 bits
		Data Width: 64 bits
		Size: 8192 MB
		Form Factor: DIMM
		Set: 1
		Locator: DIMM_A1
		Bank Locator: Not Specified
		Type: DDR3
		Type Detail: Synchronous Registered (Buffered)
		Speed: 1600 MHz
		Rank: 2
		Configured Clock Speed: 1600 MHz

There, the DDR3 RDIMM memory module is located at the system's memory labeled
as "DIMM_A1", as given by the *locator* field. Please notice that this
memory module has 64 bits of *data width* and 72 bits of *total width*. So,
it has 8 extra bits to be used by error detection and correction mechanisms.
Such kind of memory is called Error-correcting code memory (ECC memory).

To make things even worse, it is not uncommon that systems with different
labels on their system's board to use exactly the same BIOS, meaning that
the labels provided by the BIOS won't match the real ones.

ECC memory
----------

As mentioned in the previous section, ECC memory has extra bits to be
used for error correction. In the above example, a memory module has
64 bits of *data width*, and 72 bits of *total width*.  The extra 8
bits which are used for the error detection and correction mechanisms
are referred to as the *syndrome*\ [#f1]_\ [#f2]_.

So, when the cpu requests the memory controller to write a word with
*data width*, the memory controller calculates the *syndrome* in real time,
using Hamming code, or some other error correction code, like SECDED+,
producing a code with *total width* size. Such code is then written
on the memory modules.

At read, the *total width* bits code is converted back, using the same
ECC code used on write, producing a word with *data width* and a *syndrome*.
The word with *data width* is sent to the CPU, even when errors happen.

The memory controller also looks at the *syndrome* in order to check if
there was an error, and if the ECC code was able to fix such error.
If the error was corrected, a Corrected Error (CE) happened. If not, an
Uncorrected Error (UE) happened.

The information about the CE/UE errors is stored on some special registers
at the memory controller and can be accessed by reading such registers,
either by BIOS, by some special CPUs or by Linux EDAC driver. On x86 64
bit CPUs, such errors can also be retrieved via the Machine Check
Architecture (MCA)\ [#f3]_.

.. [#f1] Please notice that several memory controllers allow operation on a
  mode called "Lock-Step", where it groups two memory modules together,
  doing 128-bit reads/writes. That gives 16 bits for error correction, with
  significantly improves the error correction mechanism, at the expense
  that, when an error happens, there's no way to know what memory module is
  to blame. So, it has to blame both memory modules.

.. [#f2] Some memory controllers also allow using memory in mirror mode.
  On such mode, the same data is written to two memory modules. At read,
  the system checks both memory modules, in order to check if both provide
  identical data. On such configuration, when an error happens, there's no
  way to know what memory module is to blame. So, it has to blame both
  memory modules (or 4 memory modules, if the system is also on Lock-step
  mode).

.. [#f3] For more details about the Machine Check Architecture (MCA),
  please read Documentation/arch/x86/x86_64/machinecheck.rst at the Kernel tree.

EDAC - Error Detection And Correction
*************************************

.. note::

   "bluesmoke" was the name for this device driver subsystem when it
   was "out-of-tree" and maintained at http://bluesmoke.sourceforge.net.
   That site is mostly archaic now and can be used only for historical
   purposes.

   When the subsystem was pushed upstream for the first time, on
   Kernel 2.6.16, it was renamed to ``EDAC``.

Purpose
-------

The ``edac`` kernel module's goal is to detect and report hardware errors
that occur within the computer system running under linux.

Memory
------

Memory Correctable Errors (CE) and Uncorrectable Errors (UE) are the
primary errors being harvested. These types of errors are harvested by
the ``edac_mc`` device.

Detecting CE events, then harvesting those events and reporting them,
**can** but must not necessarily be a predictor of future UE events. With
CE events only, the system can and will continue to operate as no data
has been damaged yet.

However, preventive maintenance and proactive part replacement of memory
modules exhibiting CEs can reduce the likelihood of the dreaded UE events
and system panics.

Other hardware elements
-----------------------

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 an edac_device device probably can be constructed to
harvest and present that to userspace.


PCI bus scanning
----------------

In addition, PCI devices are scanned for PCI Bus Parity and SERR Errors
in order to determine if errors are occurring during 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 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

and is located in ``/sys/devices/pci<XXX>/0000:XX:YY.Z`` directories for
PCI devices.


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.


Sysfs interface
---------------

EDAC presents a ``sysfs`` interface for control and reporting purposes. It
lives in the /sys/devices/system/edac directory.

Within this directory there currently reside 2 components:

	======= ==============================
	mc	memory controller(s) system
	pci	PCI control and status system
	======= ==============================


Memory Controller (mc) Model
----------------------------

Each ``mc`` device controls a set of memory modules [#f4]_. These modules
are laid out in a Chip-Select Row (``csrowX``) and Channel table (``chX``).
There can be multiple csrows and multiple channels.

.. [#f4] Nowadays, the term DIMM (Dual In-line Memory Module) is widely
  used to refer to a memory module, although there are other memory
  packaging alternatives, like SO-DIMM, SIMM, etc. The UEFI
  specification (Version 2.7) defines a memory module in the Common
  Platform Error Record (CPER) section to be an SMBIOS Memory Device
  (Type 17). Along this document, and inside the EDAC subsystem, the term
  "dimm" is used for all memory modules, even when they use a
  different kind of packaging.

Memory controllers allow for several csrows, with 8 csrows being a
typical value. Yet, the actual number of csrows depends on the layout of
a given motherboard, memory controller and memory module characteristics.

Dual channels allow for dual data length (e. g. 128 bits, on 64 bit systems)
data transfers to/from the CPU from/to memory. Some newer chipsets allow
for more than 2 channels, like Fully Buffered DIMMs (FB-DIMMs) memory
controllers. The following example will assume 2 channels:

	+------------+-----------------------+
	| CS Rows    |       Channels        |
	+------------+-----------+-----------+
	|            |  ``ch0``  |  ``ch1``  |
	+============+===========+===========+
	|            |**DIMM_A0**|**DIMM_B0**|
	+------------+-----------+-----------+
	| ``csrow0`` |   rank0   |   rank0   |
	+------------+-----------+-----------+
	| ``csrow1`` |   rank1   |   rank1   |
	+------------+-----------+-----------+
	|            |**DIMM_A1**|**DIMM_B1**|
	+------------+-----------+-----------+
	| ``csrow2`` |    rank0  |  rank0    |
	+------------+-----------+-----------+
	| ``csrow3`` |    rank1  |  rank1    |
	+------------+-----------+-----------+

In the above example, 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.
In the example above 2 dual ranked DIMMs are similarly placed. Thus,
both csrow0 and csrow1 are populated. On the other hand, when 2 single
ranked DIMMs are placed in slots DIMM_A0 and DIMM_B0, then they will
have just one csrow (csrow0) and csrow1 will be empty. The pattern
repeats itself for csrow2 and csrow3. Also note that some memory
controllers don't have any logic to identify the memory module, see
``rankX`` directories below.

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
		   ....

Within each of the ``mcX`` directory 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.

For a description of the sysfs API, please see:

	Documentation/ABI/testing/sysfs-devices-edac


``dimmX`` or ``rankX`` directories
----------------------------------

The recommended way to use the EDAC subsystem is to look at the information
provided by the ``dimmX`` or ``rankX`` directories [#f5]_.

A typical EDAC system has the following structure under
``/sys/devices/system/edac/``\ [#f6]_::

	/sys/devices/system/edac/
	├── mc
	│   ├── mc0
	│   │   ├── ce_count
	│   │   ├── ce_noinfo_count
	│   │   ├── dimm0
	│   │   │   ├── dimm_ce_count
	│   │   │   ├── dimm_dev_type
	│   │   │   ├── dimm_edac_mode
	│   │   │   ├── dimm_label
	│   │   │   ├── dimm_location
	│   │   │   ├── dimm_mem_type
	│   │   │   ├── dimm_ue_count
	│   │   │   ├── size
	│   │   │   └── uevent
	│   │   ├── max_location
	│   │   ├── mc_name
	│   │   ├── reset_counters
	│   │   ├── seconds_since_reset
	│   │   ├── size_mb
	│   │   ├── ue_count
	│   │   ├── ue_noinfo_count
	│   │   └── uevent
	│   ├── mc1
	│   │   ├── ce_count
	│   │   ├── ce_noinfo_count
	│   │   ├── dimm0
	│   │   │   ├── dimm_ce_count
	│   │   │   ├── dimm_dev_type
	│   │   │   ├── dimm_edac_mode
	│   │   │   ├── dimm_label
	│   │   │   ├── dimm_location
	│   │   │   ├── dimm_mem_type
	│   │   │   ├── dimm_ue_count
	│   │   │   ├── size
	│   │   │   └── uevent
	│   │   ├── max_location
	│   │   ├── mc_name
	│   │   ├── reset_counters
	│   │   ├── seconds_since_reset
	│   │   ├── size_mb
	│   │   ├── ue_count
	│   │   ├── ue_noinfo_count
	│   │   └── uevent
	│   └── uevent
	└── uevent

In the ``dimmX`` directories are EDAC control and attribute files for
this ``X`` memory module:

- ``size`` - Total memory managed by this csrow attribute file

	This attribute file displays, in count of megabytes, the memory
	that this csrow contains.

- ``dimm_ue_count`` - Uncorrectable Errors count attribute file

	This attribute file displays the total count of uncorrectable
	errors that have occurred on this DIMM. If panic_on_ue is set
	this counter will not have a chance to increment, since EDAC
	will panic the system.

- ``dimm_ce_count`` - Correctable Errors count attribute file

	This attribute file displays the total count of correctable
	errors that have occurred on this DIMM. 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.

- ``dimm_dev_type``  - Device type attribute file

	This attribute file will display what type of DRAM device is
	being utilized on this DIMM.
	Examples:

		- x1
		- x2
		- x4
		- x8

- ``dimm_edac_mode`` - EDAC Mode of operation attribute file

	This attribute file will display what type of Error detection
	and correction is being utilized.

- ``dimm_label`` - memory module label control file

	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.

- ``dimm_location`` - location of the memory module

	The location can have up to 3 levels, and describe how the
	memory controller identifies the location of a memory module.
	Depending on the type of memory and memory controller, it
	can be:

		- *csrow* and *channel* - used when the memory controller
		  doesn't identify a single DIMM - e. g. in ``rankX`` dir;
		- *branch*, *channel*, *slot* - typically used on FB-DIMM memory
		  controllers;
		- *channel*, *slot* - used on Nehalem and newer Intel drivers.

- ``dimm_mem_type`` - Memory Type attribute file

	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

.. [#f5] On some systems, the memory controller doesn't have any logic
  to identify the memory module. On such systems, the directory is called ``rankX``.
  On modern Intel memory controllers, the memory controller identifies the
  memory modules directly. On such systems, the directory is called ``dimmX``.

.. [#f6] There are also some ``power`` directories and ``subsystem``
  symlinks inside the sysfs mapping that are automatically created by
  the sysfs subsystem. Currently, they serve no purpose.


System Logging
--------------

If logging for UEs and CEs is enabled, then system logs will contain
information indicating that errors 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:

	+---------------------------------------+-------------+
	| Content                               | Example     |
	+=======================================+=============+
	| 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 or
not. (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:


- ``check_pci_parity`` - Enable/Disable PCI Parity checking control file

	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


- ``pci_parity_count`` - Parity Count

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


Module parameters
-----------------

- ``edac_mc_panic_on_ue`` - Panic on UE control file

	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


- ``edac_mc_log_ue`` - Log UE control file


	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


- ``edac_mc_log_ce`` - Log CE control file


	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


- ``edac_mc_poll_msec`` - Polling period control file


	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`` - Panic on PCI PARITY Error


	This control file 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
----------------

In the header file, edac_pci.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 http://bluesmoke.sourceforget.net
website) installs itself as::

	/sys/devices/system/edac/test-instance

in this directory are various controls, a symlink and one or more ``instance``
directories.

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 enable any others to create their own
unique drivers for their hardware systems.

The ``test_device_edac`` sample driver is located at the
http://bluesmoke.sourceforge.net project site for EDAC.


Usage of EDAC APIs on Nehalem and newer Intel CPUs
--------------------------------------------------

On older Intel architectures, the memory controller was part of the North
Bridge chipset. Nehalem, Sandy Bridge, Ivy Bridge, Haswell, Sky Lake and
newer Intel architectures integrated an enhanced version of the memory
controller (MC) inside the CPUs.

This chapter will cover the differences of the enhanced memory controllers
found on newer Intel CPUs, such as ``i7core_edac``, ``sb_edac`` and
``sbx_edac`` drivers.

.. note::

   The Xeon E7 processor families use a separate chip for the memory
   controller, called Intel Scalable Memory Buffer. This section doesn't
   apply for such families.

1) There is one Memory Controller per Quick Patch Interconnect
   (QPI). At the driver, the term "socket" means one QPI. This is
   associated with a physical CPU socket.

   Each MC have 3 physical read channels, 3 physical write channels and
   3 logic channels. The driver currently sees it as just 3 channels.
   Each channel can have up to 3 DIMMs.

   The minimum known unity is DIMMs. There are no information about csrows.
   As EDAC API maps the minimum unity is csrows, the driver sequentially
   maps channel/DIMM into different csrows.

   For example, supposing the following layout::

	Ch0 phy rd0, wr0 (0x063f4031): 2 ranks, UDIMMs
	  dimm 0 1024 Mb offset: 0, bank: 8, rank: 1, row: 0x4000, col: 0x400
	  dimm 1 1024 Mb offset: 4, bank: 8, rank: 1, row: 0x4000, col: 0x400
        Ch1 phy rd1, wr1 (0x063f4031): 2 ranks, UDIMMs
	  dimm 0 1024 Mb offset: 0, bank: 8, rank: 1, row: 0x4000, col: 0x400
	Ch2 phy rd3, wr3 (0x063f4031): 2 ranks, UDIMMs
	  dimm 0 1024 Mb offset: 0, bank: 8, rank: 1, row: 0x4000, col: 0x400

   The driver will map it as::

	csrow0: channel 0, dimm0
	csrow1: channel 0, dimm1
	csrow2: channel 1, dimm0
	csrow3: channel 2, dimm0

   exports one DIMM per csrow.

   Each QPI is exported as a different memory controller.

2) The MC has the ability to inject errors to test drivers. The drivers
   implement this functionality via some error injection nodes:

   For injecting a memory error, there are some sysfs nodes, under
   ``/sys/devices/system/edac/mc/mc?/``:

   - ``inject_addrmatch/*``:
      Controls the error injection mask register. It is possible to specify
      several characteristics of the address to match an error code::

         dimm = the affected dimm. Numbers are relative to a channel;
         rank = the memory rank;
         channel = the channel that will generate an error;
         bank = the affected bank;
         page = the page address;
         column (or col) = the address column.

      each of the above values can be set to "any" to match any valid value.

      At driver init, all values are set to any.

      For example, to generate an error at rank 1 of dimm 2, for any channel,
      any bank, any page, any column::

		echo 2 >/sys/devices/system/edac/mc/mc0/inject_addrmatch/dimm
		echo 1 >/sys/devices/system/edac/mc/mc0/inject_addrmatch/rank

	To return to the default behaviour of matching any, you can do::

		echo any >/sys/devices/system/edac/mc/mc0/inject_addrmatch/dimm
		echo any >/sys/devices/system/edac/mc/mc0/inject_addrmatch/rank

   - ``inject_eccmask``:
          specifies what bits will have troubles,

   - ``inject_section``:
       specifies what ECC cache section will get the error::

		3 for both
		2 for the highest
		1 for the lowest

   - ``inject_type``:
       specifies the type of error, being a combination of the following bits::

		bit 0 - repeat
		bit 1 - ecc
		bit 2 - parity

   - ``inject_enable``:
       starts the error generation when something different than 0 is written.

   All inject vars can be read. root permission is needed for write.

   Datasheet states that the error will only be generated after a write on an
   address that matches inject_addrmatch. It seems, however, that reading will
   also produce an error.

   For example, the following code will generate an error for any write access
   at socket 0, on any DIMM/address on channel 2::

	echo 2 >/sys/devices/system/edac/mc/mc0/inject_addrmatch/channel
	echo 2 >/sys/devices/system/edac/mc/mc0/inject_type
	echo 64 >/sys/devices/system/edac/mc/mc0/inject_eccmask
	echo 3 >/sys/devices/system/edac/mc/mc0/inject_section
	echo 1 >/sys/devices/system/edac/mc/mc0/inject_enable
	dd if=/dev/mem of=/dev/null seek=16k bs=4k count=1 >& /dev/null

   For socket 1, it is needed to replace "mc0" by "mc1" at the above
   commands.

   The generated error message will look like::

	EDAC MC0: UE row 0, channel-a= 0 channel-b= 0 labels "-": NON_FATAL (addr = 0x0075b980, socket=0, Dimm=0, Channel=2, syndrome=0x00000040, count=1, Err=8c0000400001009f:4000080482 (read error: read ECC error))

3) Corrected Error memory register counters

   Those newer MCs have some registers to count memory errors. The driver
   uses those registers to report Corrected Errors on devices with Registered
   DIMMs.

   However, those counters don't work with Unregistered DIMM. As the chipset
   offers some counters that also work with UDIMMs (but with a worse level of
   granularity than the default ones), the driver exposes those registers for
   UDIMM memories.

   They can be read by looking at the contents of ``all_channel_counts/``::

     $ for i in /sys/devices/system/edac/mc/mc0/all_channel_counts/*; do echo $i; cat $i; done
	/sys/devices/system/edac/mc/mc0/all_channel_counts/udimm0
	0
	/sys/devices/system/edac/mc/mc0/all_channel_counts/udimm1
	0
	/sys/devices/system/edac/mc/mc0/all_channel_counts/udimm2
	0

   What happens here is that errors on different csrows, but at the same
   dimm number will increment the same counter.
   So, in this memory mapping::

	csrow0: channel 0, dimm0
	csrow1: channel 0, dimm1
	csrow2: channel 1, dimm0
	csrow3: channel 2, dimm0

   The hardware will increment udimm0 for an error at the first dimm at either
   csrow0, csrow2  or csrow3;

   The hardware will increment udimm1 for an error at the second dimm at either
   csrow0, csrow2  or csrow3;

   The hardware will increment udimm2 for an error at the third dimm at either
   csrow0, csrow2  or csrow3;

4) Standard error counters

   The standard error counters are generated when an mcelog error is received
   by the driver. Since, with UDIMM, this is counted by software, it is
   possible that some errors could be lost. With RDIMM's, they display the
   contents of the registers

Reference documents used on ``amd64_edac``
------------------------------------------

``amd64_edac`` module is based on the following documents
(available from http://support.amd.com/en-us/search/tech-docs):

1. :Title:  BIOS and Kernel Developer's Guide for AMD Athlon 64 and AMD
	   Opteron Processors
   :AMD publication #: 26094
   :Revision: 3.26
   :Link: http://support.amd.com/TechDocs/26094.PDF

2. :Title:  BIOS and Kernel Developer's Guide for AMD NPT Family 0Fh
	   Processors
   :AMD publication #: 32559
   :Revision: 3.00
   :Issue Date: May 2006
   :Link: http://support.amd.com/TechDocs/32559.pdf

3. :Title:  BIOS and Kernel Developer's Guide (BKDG) For AMD Family 10h
	   Processors
   :AMD publication #: 31116
   :Revision: 3.00
   :Issue Date: September 07, 2007
   :Link: http://support.amd.com/TechDocs/31116.pdf

4. :Title: BIOS and Kernel Developer's Guide (BKDG) for AMD Family 15h
	  Models 30h-3Fh Processors
   :AMD publication #: 49125
   :Revision: 3.06
   :Issue Date: 2/12/2015 (latest release)
   :Link: http://support.amd.com/TechDocs/49125_15h_Models_30h-3Fh_BKDG.pdf

5. :Title: BIOS and Kernel Developer's Guide (BKDG) for AMD Family 15h
	  Models 60h-6Fh Processors
   :AMD publication #: 50742
   :Revision: 3.01
   :Issue Date: 7/23/2015 (latest release)
   :Link: http://support.amd.com/TechDocs/50742_15h_Models_60h-6Fh_BKDG.pdf

6. :Title: BIOS and Kernel Developer's Guide (BKDG) for AMD Family 16h
	  Models 00h-0Fh Processors
   :AMD publication #: 48751
   :Revision: 3.03
   :Issue Date: 2/23/2015 (latest release)
   :Link: http://support.amd.com/TechDocs/48751_16h_bkdg.pdf

Credits
=======

* Written by Doug Thompson <dougthompson@xmission.com>

  - 7 Dec 2005
  - 17 Jul 2007	Updated

* |copy| Mauro Carvalho Chehab

  - 05 Aug 2009	Nehalem interface
  - 26 Oct 2016 Converted to ReST and cleanups at the Nehalem section

* EDAC authors/maintainers:

  - Doug Thompson, Dave Jiang, Dave Peterson et al,
  - Mauro Carvalho Chehab
  - Borislav Petkov
  - original author: Thayne Harbaugh