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 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.
Service xyz.openbmc_project.Hwmon-<hash>.Hwmon1 Path /xyz/openbmc_project/sensors/<type>/<label> Interfaces xyz.openbmc_project.Sensor.[*], others (see below) Signals: All properties for an interface will broadcast signal changed
Path definitions
<type> : The HWMon class name in lower case.
temperature, fan_tach, voltage.<label> : User defined name of the sensor.
ambient, cpu0, fan5Note: The label shall comply with "Valid Object Paths" of D-Bus Spec, that shall only contain the ASCII characters "[A-Z][a-z][0-9]_".
Hash definition
The hash value in the service name is used to give the service a unique and stable name. It is a decimal number that is obtained by hashing characteristics of the device it is monitoring using std::hash().
The BMCWeb Redfish support returns information about sensors. The support depends on two types of ObjectMapper associations to find the necessary sensor information on D-Bus.
Sensors are grouped by chassis in Redfish. An ObjectMapper association is used to link a chassis to all the sensors within the chassis. This includes the sensors for all hardware that is considered to be within the chassis. For example, a chassis might contain two fan sensors, an ambient temperature sensor, and a VRM output voltage sensor.
The association links the following D-Bus object paths together:
A sensor is usually related to a low-level hardware item, such as a fan, power supply, VRM, or CPU. The Redfish sensor support can obtain the following information from the related hardware item:
For this reason, an ObjectMapper association is used to link a low-level hardware item to its sensors. For example, a processor VRM could have temperature and output voltage sensors, or a dual-rotor fan could have two tach sensors.
The association links the following D-Bus object paths together:
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 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 parent directory.
example: openbmc/phosphor-dbus-interfaces/xyz/openbmc_project/Sensor/Threshold/Warning.yaml
Maps to D-Bus interface xyz.openbmc_project.Sensor.Threshold.Warning
Each 'name' property in the YAML file maps directly to D-Bus properties.
example:
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-3301914901.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
"/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
}
Aside from the xyz.openbmc_project.Sensor interfaces, the sensor D-Bus objects may also expose the following interfaces:
xyz.openbmc_project.Control.FanSpeedxyz.openbmc_project.Control.FanPwmxyz.openbmc_project.State.Decorator.OperationalStatusAny 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.openbmc_project.Sensor.Value" : value = 5 |
| 1 | 6 | "xyz.openbmc_project.Sensor.Value" : value = 6 , "xyz.openbmc_project.Sensor.Threshold.Warning" : WarningAlarmLow = 0 |
| 5 | 6 | "xyz.openbmc_project.Sensor.Value" : value = 6 |
| 6 | 1 | "xyz.openbmc_project.Sensor.Value" : value = 1 , "xyz.openbmc_project.Sensor.Threshold.Warning" : WarningAlarmLow = 1 |
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.
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.
HWMon sensors are defined in the recipes-phosphor/sensor/phosphor-hwmon% path within the machine configuration. The children of the obmc/hwmon directory should follow the path of either:
The children of the devicetree/base directory path on the system, as defined by the kernel. The code obtains this from the OF_FULLNAME udev environment variable.
If the device isn't in the device tree, then the device path can be used.
As an example, the Palmetto configuration 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
This next example shows using the device path as opposed to the devicetree path for the OCC device on an OpenPOWER system. Note how a '--' replaces a ':' in the directory names for the conf file.
recipes-phosphor/sensors/phosphor-hwmon%/obmc/hwmon/devices/platform/gpio-fsi/fsi0/slave@00--00/00--00--00--06/sbefifo1-dev0/occ-hwmon.1.conf
which maps to this specific sensor and conf file on the system...
/sys/devices/platform/gpio-fsi/fsi0/slave@00:00/00:00:00:06/sbefifo1-dev0/occ-hwmon.1 /etc/default/obmc/hwmon/devices/platform/gpio-fsi/fsi0/slave@00--00/00--00--00--06/sbefifo1-dev0/occ-hwmon.1.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.openbmc_project.Sensor.Value |
| WARNHI, WARNLO | xyz.openbmc_project.Threshold.Warning |
| CRITHI, CRITLO | xyz.openbmc_project.Threshold.Critical |
The HWMon sensor type
| HWMon sensor type | type |
|---|---|
| temp | temperature |
| in | voltage |
| * | All other names map directly |
See the HWMon 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 10-3 degrees Celsius.
LABEL_temp1=ambient WARNLO_temp1=10000 WARNHI_temp1=40000 LABEL_temp2=cpu WARNLO_temp2=10000 WARNHI_temp2=80000
The phosphor-hwmon code supports these additional config file entries:
INTERVAL
The interval, in microseconds, at which sensors should be read. If not specified the interval is 1000000us (1 second).
INTERVAL=1000000
GAIN, OFFSET
Used to support scaled sensor readings, where value = (raw sensor reading) * gain + offset
GAIN_in3 = 5.0 #GAIN is a double OFFSET_in3 = 6 #OFFSET is an integer
MINVALUE, MAXVALUE
If found, will be used to set the MinValue/MaxValue properties on the xyz.openbmc_project.Sensor.Value interface.
MINVALUE_temp1 = 1
MODE
Needed for certain device drivers, specifically the OpenPOWER OCC driver, where the instance number (the N in tempN_input) is dynamic and instead another file contains the known ID.
For example
temp26_input:29000 temp26_label:171
Where the 26 is just what hwmon assigns, but the 171 corresponds to something like an IPMI sensor value for a DIMM temperature.
The config file would then have:
MODE_temp26 = "label" #Tells the code to look in temp26_label LABEL_temp171 = "dimm3_temp" #Says that temp26_input holds dimm3_temp
REMOVERCS
Contains a list of device driver errno values where if these are obtained when reading the hardware, the corresponding sensor object will be removed from D-Bus until it is successfully read again.
REMOVERCS = "5,6" #If any sensor on the device returns a 5 or 6, remove it. REMOVERCS_temp1 = "42" #If reading temp1_input returns a 42, remove it.
TARGET_MODE
Allows one to choose the fan target mode, either RPM or PWM, if the device driver exposes both methods.
TARGET_MODE = "RPM"
PWM_TARGET
For fans that are PWM controlled, can be used to map the pwmN file to a fan M.
PWM_TARGET_fan0 = 1 #Use the pwm1 file to control fan 0
ENABLE
Will write a value to a pwmN_enable file on startup if present.
ENABLE_fan1 = 2 #Write a 2 to pwm1_enable
For an example of how sensors entries are defined, consult the example YAML
Sensor reading, according to IPMI spec, is calculated as:
y = L[(Mx + B * 10^(bExp)) * 10^(rExp)]
multiplierM in YAMLoffsetB in YAMLbExp in YAMLrExp in YAMLIn addition, phosphor-ipmi-host configuration also supports scale property, which applies for analog sensors, meaning the value read over DBus should be scaled by 10^S.
As you can tell, one should choose the coefficients based on possible sensor reading range and desired resolution. Commonly, B=0, we would have
Supported range: [0, 255 * M * 10^(scale - rExp)] Resolution: M * 10^(scale - rExp)
For a concrete example, let's say a voltage sensor reports between 0 to 5.0V. hwmon sysfs scales the value by 1000, so the sensor value read over DBus is between 0 and 5000. A possible configuration for this is:
multiplierM: 20 offsetB: 0 bExp: 0 rExp: -3 scale: -3
so for a DBus sensor value of 4986 meaning 4.986V, phosphor-ipmi-host would encode it as
x: 4986 / 20 = 249 M: 20 rExp: -3
When ipmitool sensor list is called, the tool fetches sensor factors and computes value as:
y = 20 * 249 * 10^-3 = 4.98 (V)
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 10-3 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.openbmc_project.Threshold.Warning |
| CRIT_LOW, CRIT_HIGH | N | xyz.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>
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