xref: /openbmc/docs/designs/virtual-sensors.md (revision 8f6e989f) 
1### Virtual sensors
2
3Author:
4  Vijay Khemka <vijaykhemka@fb.com>; <vijay!>
5
6Primary assignee:
7  Vijay Khemka <vijaykhemka@fb.com>; <vijay!>
8
9Created:
10  2020-05-13
11
12## Problem Description
13There are some sensors in the system whose values are derived from actual
14sensor data and some other factor like fan speed for temperature
15sensors, some complex and non standard scale factors or some other algorithmic
16calculations. Some of Virtual sensor examples are:
17
181. Airflow: Function of Fan RPM reported in units of cubic-feet per second.
192. Voltage delta between two ADC inputs. When the actual diff is what which
20   should not cross a threshold.
213. Power consumption when we have Energy/time.
22
23This design will allow to generate a new virtual sensors which are defined to
24be specific manipulation/function of existing sensors.
25
26## Background and References
27None.
28
29## Requirements
30This should implement a new virtual sensor for every given sensor and update
31them as per changes in existing sensors.
32This implementation should provide
33- a daemon to create and monitor each sensor defined in config file and
34  update them based on value change in each given sensor.
35- Every virtual sensor will be waiting for event for change in dbus value of
36  each sensor parameter and will update this sensor.
37- Sensor parameter can be any sensor dbus path and it also supports
38  chaining of virtual sensors.
39- a dbus interface like other sensors to allow other services to access its
40  data
41- capability to read parameter details from config file
42- capability to read a algorithm string from config file and use the same
43  algorithm in calculating value for new virtual sensor
44
45## Proposed Design
46Create a D-bus service "xyz.openbmc_project.VirtualSensor" with object paths
47for each sensor: "/xyz/openbmc_project/sensor/temperature/sensor_name"
48
49There is a JSON configuration file for name, old path and algorithm.
50For example,
51
52```json
53  {
54    "Desc" :
55    {
56        "Name" : "Virtual_Inlet_Temp",
57        "SensorType" : "temperature"
58    },
59    "Params":
60    {
61      "ConstParam" :
62      [
63        {
64          "ParamName" : "P1",
65          "Value" : 1.1
66        }
67      ],
68      "DbusParam" :
69      [
70        {
71          "ParamName" : "P2",
72          "Desc" :
73          {
74              "Name" : "MB_INLET_TEMP",
75              "SensorType" : "temperature"
76          }
77        },
78        {
79          "ParamName" : "P3",
80          "Desc" :
81          {
82            "Name" : "MB_FAN0_TACH",
83            "SensorType" : "fan_tach"
84          }
85        }
86      ]
87    },
88    "Expression": "P1 + P2 + 5 - P3 * 0.1",
89    "Thresholds":
90    {
91      "CriticalHigh": 90,
92      "CriticalLow": 20,
93      "WarningHigh": 70,
94      "WarningLow": 30
95    }
96  }
97
98Desc:       It defines name and unit of a sensor. Main Desc object defines
99            details about new virtual sensor and inside DbusParam object,
100            it defines details about existing dbus sensors.
101Name:       Name of virtual sensor
102SensorType: Unit type of sensors and supported
103Expression: An algorithm to be used to calculate sensor value
104Params:     There are currently 2 types of parameter supported ConstParam
105            and DbusParam. Every pamas should have a name to be reffered
106            in expression
107ConstParam: This is a parameter which has a constant value defined here
108DbusParam:  This is an existing/virtual sensor which is listed in dbus and
109            it's value can be read from dbus path
110Thresholds: It has critical and warning high/low value to monitor sensor
111            value and used as per sensor interface defined in
112            phosphor-dbus-interfaces.
113
114```
115## Alternatives Considered
116None
117
118## Impacts
119This application is monitoring existing sensors whenever they change values
120then only it will update this sensors so impact should be very minimal.
121
122## Testing
123Testing can be done by monitoring data read from dbus interface over a period
124of time and also can see these data if it changes by comparing with given
125sensors.
126