path: root/sensor-architecture.md

# 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][1]
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-<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][2] name in lower case.
    - Examples include `temperature, fan_tach, voltage`.

* **<label\>** : User defined name of the sensor.
    - Examples include `ambient, cpu0, fan5`

**Note**: The label shall comply with "Valid Object Paths" of [D-Bus Spec][3],
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().

## 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][4]
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][5] parent directory.

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;example:
[openbmc/phosphor-dbus-interfaces/xyz/openbmc_project/Sensor/Threshold/Warning.yaml][6]


&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][6]

```
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

```

### 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
}
```

### Other Interfaces

Aside from the `xyz.openbmc_project.Sensor` interfaces, the sensor D-Bus objects
may also expose the following interfaces:

1.  `xyz.openbmc_project.Control.FanSpeed`
    - Provides a Target property to set a fan RPM value
2.  `xyz.openbmc_project.Control.FanPwm`
    - Provides a Target property to set a fan PWM value
3.  `xyz.openbmc_project.State.Decorator.OperationalStatus`
    - Provides a Functional property that tracks the state of any fault files

### 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.openbmc_project.Sensor.Value" : value = 5 |
| 1 | 6 | "xyz.openbmc\_project.Sensor.Value" : value = 6 ,<br>"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 ,<br>"xyz.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][7].
The children of the `obmc/hwmon` directory should follow the path of either:

1.  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.

2.  If the device isn't in the device tree, then the device path can be used.

As an example, the Palmetto [configuration][8]
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][2] | type |
|---|---|
| temp | temperature |
| in | voltage |
| * | All other names map directly |



See the [HWMon interface][2]
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
```

#### Additional Config File Entries
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
```

### Defining sensors in an IPMI YAML configuration file
For an example of how sensors entries are defined, consult the [example
YAML](https://github.com/openbmc/phosphor-host-ipmid/blob/master/scripts/sensor-example.yaml)

#### How to best choose coefficients
Sensor reading, according to IPMI spec, is calculated as:

```none
y = L[(Mx + B * 10^(bExp)) * 10^(rExp)]
```

- y: the 'final value' as reported by IPMItool
- x: 8 bits, unsigned, reading data encoded in IPMI response packet
- M: 10 bits, signed integer multiplier, `multiplierM` in YAML
- B: 10 bits, signed additive offset, `offsetB` in YAML
- bExp: 4 bits, signed, `bExp` in YAML
- rExp: 4 bits, signed, `rExp` in YAML

In 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:

```none
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:

```none
y = 20 * 249 * 10^-3 = 4.98 (V)
```

### 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.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>
```


## Additional Reading
Mailing List [Comments on Sensor design][9]


[1]: https://dbus.freedesktop.org/doc/dbus-tutorial.html
[2]: https://www.kernel.org/doc/Documentation/hwmon/sysfs-interface
[3]: https://dbus.freedesktop.org/doc/dbus-specification.html#basic-types
[4]: https://github.com/openbmc/phosphor-dbus-interfaces/tree/master/xyz/openbmc_project/Sensor
[5]: https://github.com/openbmc/phosphor-dbus-interfaces
[6]: https://github.com/openbmc/phosphor-dbus-interfaces/blob/master/xyz/openbmc_project/Sensor/Threshold/Warning.interface.yaml
[7]: https://github.com/openbmc/openbmc/tree/master/meta-openbmc-machines
[8]: 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
[9]: https://lists.ozlabs.org/pipermail/openbmc/2016-November/005309.html