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+### Virtual sensors
+
+Author:
+  Vijay Khemka <vijaykhemka@fb.com>; <vijay!>
+
+Primary assignee:
+  Vijay Khemka <vijaykhemka@fb.com>; <vijay!>
+
+Created:
+  2020-05-13
+
+## Problem Description
+There are some sensors in the system whose values are derived from actual
+sensor data and some other factor like fan speed for temperature
+sensors, some complex and non standard scale factors or some other algorithmic
+calculations. Some of Virtual sensor examples are:
+
+1. Airflow: Function of Fan RPM reported in units of cubic-feet per second.
+2. Voltage delta between two ADC inputs. When the actual diff is what which
+   should not cross a threshold.
+3. Power consumption when we have Energy/time.
+
+This design will allow to generate a new virtual sensors which are defined to
+be specific manipulation/function of existing sensors.
+
+## Background and References
+None.
+
+## Requirements
+This should implement a new virtual sensor for every given sensor and update
+them as per changes in existing sensors.
+This implementation should provide
+- a daemon to create and monitor each sensor defined in config file and
+  update them based on value change in each given sensor.
+- Every virtual sensor will be waiting for event for change in dbus value of
+  each sensor parameter and will update this sensor.
+- Sensor parameter can be any sensor dbus path and it also supports
+  chaining of virtual sensors.
+- a dbus interface like other sensors to allow other services to access its
+  data
+- capability to read parameter details from config file
+- capability to read a algorithm string from config file and use the same
+  algorithm in calculating value for new virtual sensor
+
+## Proposed Design
+Create a D-bus service "xyz.openbmc_project.VirtualSensor" with object paths
+for each sensor: "/xyz/openbmc_project/sensor/temperature/sensor_name"
+
+There is a JSON configuration file for name, old path and algorithm.
+For example,
+
+```json
+  {
+    "Name": "Virtual_Inlet_Temp",
+    "Algo": "P1 + P2 + 5 - P3 * 0.1",
+    "Params":
+    {
+      "P1": "/xyz/openbmc_project/sensor/temperature/"Inlet_Temp",
+      "P2": "/xyz/openbmc_project/sensor/fan/fan_0",
+      "P3": "200"
+    },
+    "Thresholds":
+    {
+	"CriticalHigh": 90,
+	"CriticalLow": 20,
+	"WarningHigh": 70,
+	"WarningLow": 30
+    }
+  }
+
+Name:       Name of virtual sensor
+Algo:       An algorithm to be used to calculate value
+Params:     It can be an existing/virtual sensor dbus path to read value or it
+            can be a value to be used in calculation
+Thresholds: It has critical and warning high/low value to monitor sensor
+            value and used as per sensor interface defined in
+            phosphor-dbus-interfaces.
+
+```
+## Alternatives Considered
+None
+
+## Impacts
+This application is monitoring existing sensors whenever they change values
+then only it will update this sensors so impact should be very minimal.
+
+## Testing
+Testing can be done by monitoring data read from dbus interface over a period
+of time and also can see these data if it changes by comparing with given
+sensors.