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+# Sensor Support for OpenBMC #
+OpenBMC makes it easy to add sensors for your hardware and is compliant with
+the traditional Linux HWMon sensor format.  The architecture of OpenBMC
+sensors is to map sensors to [D-Bus](https://dbus.freedesktop.org/doc/dbus-tutorial.html)
+objects.  The D-Bus object will broadcast the `PropertiesChanged` signal when either
+the sensor or threshold value changes. It is the responsibility of other
+applications to determine the effect of the signal on the system.
+
+## D-Bus ##
+
+```
+Service     xyz.openbmc_project.Hwmon.Hwmon[x]
+Path        /xyz/openbmc_project/Sensors/<type>/<label>
+Interfaces  xyz.openbmc_project.Sensor.[*]
+
+Signals: All properties for an interface will broadcast signal changed
+```
+
+**Path definitions**
+
+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;**\<type>** : The [HWMon class]
+(https://www.kernel.org/doc/Documentation/hwmon/sysfs-interface) name in lower
+case. Examples include `temperature, fan, voltage`.
+
+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;**\<label>** : User defined
+name of the sensor.  Examples include `ambient, cpu0, fan5`
+
+
+## Development Details ##
+
+Sensor properties are standardized based on the type of sensor.  A Threshold
+sensor contains specific properties associated with the rise and fall of a
+sensor value.  The [Sensor Interfaces](https://github.com/openbmc/phosphor-dbus-interfaces/tree/master/xyz/openbmc_project/Sensor)
+are described in their respective YAML files.  The path location in the source
+tree is identical to the interface being described below the
+[phosphor-dbus-interfaces](https://github.com/openbmc/phosphor-dbus-interfaces)
+parent directory.
+
+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;example:
+[openbmc/phosphor-dbus-interfaces/xyz/openbmc_project/Sensor/Threshold/Warning.yaml]
+(https://github.com/openbmc/phosphor-dbus-interfaces/blob/master/xyz/openbmc_project/Sensor/Threshold/Warning.interface.yaml)
+
+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Maps to D-Bus interface
+`xyz.openbmc_project.Sensor.Threshold.Warning`
+
+Each 'name' property in the YAML file maps directly to D-Bus properties.
+
+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;example:
+
+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[Warning.interface.yaml]
+(https://github.com/openbmc/phosphor-dbus-interfaces/blob/master/xyz/openbmc_project/Sensor/Threshold/Warning.interface.yaml)
+
+```
+properties:
+    - name: WarningHigh
+      type: int64
+    - name: WarningLow
+      type: int64
+    - name: WarningAlarmHigh
+      type: boolean
+    - name: WarningAlarmLow
+      type: boolean
+```
+
+Maps to
+
+```
+busctl --system introspect xyz.openbmc_project.Hwmon.hwmon1 \
+ /xyz/openbmc_project/Sensors/temperature/ambient \
+ xyz.openbmc_project.Sensor.Threshold.Warning | grep property
+
+.WarningAlarmHigh                            property  b         false        emits-change writable
+.WarningAlarmLow                             property  b         false        emits-change writable
+.WarningHigh                                 property  x         40000        emits-change writable
+.WarningLow                                  property  x         10000        emits-change writable
+
+```
+
+### REST ###
+
+```
+"/xyz/openbmc_project/Sensors/temperature/ambient": {
+      "Scale": -3,
+      "Unit": "xyz.openbmc_project.Sensor.Value.Unit.DegreesC",
+      "Value": 30125,
+      "WarningAlarmHigh": 0,
+      "WarningAlarmLow": 0,
+      "WarningHigh": 40000,
+      "WarningLow": 10000
+}
+```
+
+### Signals ###
+
+Any property value change broadcasts a signal on D-Bus.  When a value trips
+past a threshold, an additional D-Bus signal is sent.
+
+Example, if the value of WarningLow is 5...
+
+| From | To | propertyChanged Signals |
+|---|---|---|
+| 1 | 5 | "xyz.org.openbmc_project.Sensor.Value" : value = 5 |
+| 1 | 6 | "xyz.org.openbmc\_project.Sensor.Value" : value = 6 ,<br>"xyz.org.openbmc\_project.Sensor.Threshold.Warning" : WarningAlarmLow = 0    |
+| 5 | 6 | "xyz.org.openbmc\_project.Sensor.Value" : value = 6  |
+| 6 | 1 | "xyz.org.openbmc\_project.Sensor.Value" : value = 1 ,<br>"xyz.org.openbmc\_project.Sensor.Threshold.Warning" : WarningAlarmLow = 1 |
+
+
+### System Configuration ###
+
+On the BMC each sensor's configuration is located in a file.  These files
+can be found as a child of the `/etc/default/obmc/hwmon` path.
+
+
+## Creating a Sensor ##
+
+There are two techniques to add a sensor to your system and which to use
+depends on if your system defines sensors via an MRW (Machine Readable
+Workbook) or not.
+
+
+### My sensors are not defined in an MRW ###
+
+HWMon sensors are defined in the `recipes-phosphor/sensor/phosphor-hwmon%`
+path within the [machine configuration](https://github.com/openbmc/openbmc/tree/master/meta-openbmc-machines).
+The children of the `obmc/hwmon` directory should follow the children of the
+`devicetree/base` directory path on the system as defined by the kernel.
+
+As an example, the Palmetto [configuration](https://github.com/openbmc/openbmc/blob/master/meta-openbmc-machines/meta-openpower/meta-ibm/meta-palmetto/recipes-phosphor/sensors/phosphor-hwmon%25/obmc/hwmon/ahb/apb/i2c%401e78a000/i2c-bus%40c0/tmp423%404c.conf)
+file for the ambient temperature sensor.
+
+```
+recipes-phosphor/sensors/phosphor-hwmon%/obmc/hwmon/ahb/apb/i2c@1e78a000/i2c-bus@c0/tmp423@4c.conf
+```
+
+which maps to this specific sensor and conf file on the system...
+
+```
+/sys/firmware/devicetree/base/ahb/apb/i2c@1e78a000/i2c-bus@c0/tmp423@4c
+/etc/default/obmc/hwmon/ahb/apb/i2c@1e78a000/i2c-bus@c0/tmp423@4c.conf
+```
+
+In order for the sensor to be exposed to D-Bus, the configuration file must
+describe the sensor attributes.  Attributes follow a format.
+
+```
+xxx_yyy#=value
+
+xxx = Attribute
+#   = Association number (i.e. 1-n)
+yyy = HWMon sensor type (i.e. temp, pwm)
+```
+
+| Attribute | Interfaces Added |
+|---|---|
+|LABEL | xyz.org.openbmc_project.Sensor.Value |
+| WARNHI, WARNLO | xyz.org.openbmc_project.Threshold.Warning |
+| CRITHI, CRITLO | xyz.org.openbmc_project.Threshold.Critical |
+
+
+The HWMon sensor type
+
+| [HWMon sensor type](https://www.kernel.org/doc/Documentation/hwmon/sysfs-interface) | type |
+|---|---|
+| temp | temperature |
+| in | voltage |
+| * | All other names map directly |
+
+
+
+See the [HWMon interface](https://www.kernel.org/doc/Documentation/hwmon/sysfs-interface)
+definitions for more definitions and keyword details
+
+
+
+In this conf example the tmp423 chip is wired to two temperature sensors.
+The values must be described in 10<sup>-3</sup> degrees Celsius.
+
+
+```
+LABEL_temp1=ambient
+WARNLO_temp1=10000
+WARNHI_temp1=40000
+
+LABEL_temp2=cpu
+WARNLO_temp2=10000
+WARNHI_temp2=80000
+```
+
+With that level of system information, the sensor infrastructure code can
+provide all needed D-Bus properties.
+
+
+#### My sensors are defined in an MRW ####
+
+Setting up sensor support with an MRW is done by adding a unit-hwmon-feature
+unit, for each hwmon feature needing to be monitored and then filling in the
+HWMON_FEATURE attribute.  The XML field is required however a D-Bus interface
+will only be generated when the value property is not null. Values for
+Thresholds follow the HWMon format of no decimals.  Temperature values must
+be described in 10<sup>-3</sup> degrees Celsius. The HWMON\_NAME will be used
+to derive the \<type> while the DESCRIPTIVE\_NAME creates the \<label> for the
+instance path.
+
+| Field id | Value Required | Interfaces Added |
+|---|:---:|---|
+| HWMON\_NAME | Y | xyz.org.openbmc\_project.Sensor.Value |
+| WARN\_LOW, WARN\_HIGH | N | xyz.org.openbmc\_project.Threshold.Warning |
+| CRIT\_LOW, CRIT\_HIGH | N | xyz.org.openbmc\_project.Threshold.Critical |
+
+
+Here is an example of a Fan sensor.  If the RPMs go above 80000 or below 1000
+addition signals will be sent over D-Bus.  Note that neither CRIT\_LOW or
+CRIT\_HIGH is set so `xyz.org.openbmc_project.Threshold.Critical` will not be
+added.  The instance path will be `/xyz/openbmc_project/Sensors/fan/fan0`.
+
+```
+<targetInstance>
+	<id>MAX31785.hwmon2</id>
+	<type>unit-hwmon-feature</type>
+	...
+	<attribute>
+		<id>HWMON_FEATURE</id>
+		<default>
+				<field><id>HWMON_NAME</id><value>fan1</value></field>
+				<field><id>DESCRIPTIVE_NAME</id><value>fan0</value></field>
+				<field><id>WARN_LOW</id><value>1000</value></field>
+				<field><id>WARN_HIGH</id><value>80000</value></field>
+				<field><id>CRIT_LOW</id><value></value></field>
+				<field><id>CRIT_HIGH</id><value></value></field>
+		</default>
+	</attribute>
+```
+
+
+## Additional Reading ##
+Mailing List [Comments on Sensor design](https://lists.ozlabs.org/pipermail/openbmc/2016-November/005309.html)