How to Configure Phosphor-pid-control

A system needs two groups of configurations: zones and sensors.

Json Configuration

The json object should be a dictionary with two keys, sensors and zones. sensors is a list of the sensor dictionaries, whereas zones is a list of zones.

Sensors

"sensors" : [
    {
        "name": "fan1",
        "type": "fan",
        "readPath": "/xyz/openbmc_project/sensors/fan_tach/fan1",
        "writePath": "/sys/devices/platform/ahb/ahb:apb/1e786000.pwm-tacho-controller/hwmon/**/pwm1",
        "min": 0,
        "max": 255
    },
    {
        "name": "fan2",
        "type": "fan",
        "readPath": "/xyz/openbmc_project/sensors/fan_tach/fan2",
        "writePath": "/sys/devices/platform/ahb/ahb:apb/1e786000.pwm-tacho-controller/hwmon/**/pwm2",
        "min": 0,
        "max": 255,
        "timeout": 4,
    },
...

A sensor has a name, a type, a readPath, a writePath, a minimum value, a maximum value, and a timeout.

The name is used to reference the sensor in the zone portion of the configuration.

The type is the type of sensor it is. This influences how its value is treated. Supports values are: fan, temp, and margin.

TODO: Add further details on what the types mean.

The readPath is the path that tells the daemon how to read the value from this sensor. It is optional, allowing for write-only sensors. If the value is absent or None it'll be treated as a write-only sensor.

If the readPath value contains: /xyz/openbmc_project/extsensors/ it'll be treated as a sensor hosted by the daemon itself whose value is provided externally. The daemon will own the sensor and publish it to dbus. This is currently only supported for temp and margin sensor types.

If the readPath value contains: /xyz/openbmc_project/ (this is checked after external), then it's treated as a passive dbus sensor. A passive dbus sensor is one that listens for property updates to receive its value instead of actively reading the Value property.

If the readPath value contains: /sys/ this is treated as a directly read sysfs path. There are two supported paths:

/sys/class/hwmon/hwmon0/pwm1
/sys/devices/platform/ahb/1e786000.pwm-tacho-controller/hwmon/<asterisk asterisk>/pwm1

The writePath is the path to set the value for the sensor. This is only valid for a sensor of type fan. The path is optional. If can be empty or None. It then only supports two options.

If the writePath value contains: /sys/ this is treated as a directory written sysfs path. There are two support paths:

/sys/class/hwmon/hwmon0/pwm1
/sys/devices/platform/ahb/1e786000.pwm-tacho-controller/hwmon/<asterisk asterisk>/pwm1

If the writePath value contains: /xyz/openbmc_project/sensors/fan_tach/fan{N} it sets of a sensor object that writes over dbus to the xyz.openbmc_project.Control.FanPwm interface. The writePath should be the full object path.

busctl introspect xyz.openbmc_project.Hwmon-1644477290.Hwmon1 /xyz/openbmc_project/sensors/fan_tach/fan1 --no-pager
NAME                                TYPE      SIGNATURE RESULT/VALUE                             FLAGS
org.freedesktop.DBus.Introspectable interface -         -                                        -
.Introspect                         method    -         s                                        -
org.freedesktop.DBus.Peer           interface -         -                                        -
.GetMachineId                       method    -         s                                        -
.Ping                               method    -         -                                        -
org.freedesktop.DBus.Properties     interface -         -                                        -
.Get                                method    ss        v                                        -
.GetAll                             method    s         a{sv}                                    -
.Set                                method    ssv       -                                        -
.PropertiesChanged                  signal    sa{sv}as  -                                        -
xyz.openbmc_project.Control.FanPwm  interface -         -                                        -
.Target                             property  t         255                                      emits-change writable
xyz.openbmc_project.Sensor.Value    interface -         -                                        -
.MaxValue                           property  x         0                                        emits-change writable
.MinValue                           property  x         0                                        emits-change writable
.Scale                              property  x         0                                        emits-change writable
.Unit                               property  s         "xyz.openbmc_project.Sensor.Value.Uni... emits-change writable
.Value                              property  x         2823                                     emits-change writable

The minimum and maximum values are optional. When maximum is non-zero it expects to write a percentage value converted to a value between the minimum and maximum.

The timeout value is optional and controls the sensor failure behavior. If a sensor is a fan the default value is 2 seconds, otherwise it's 0. When a sensor's timeout is 0 it isn't checked against a read timeout failure case. If a sensor fails to be read within the timeout period, the zone goes into failsafe to handle the case where it doesn't know what to do -- as it doesn't have all its inputs.

Zones

"zones" : [
        {
            "id": 1,
            "minThermalOutput": 3000.0,
            "failsafePercent": 75.0,
            "pids": [],
...

Each zone has its own fields, and a list of PIDs.

field	type	meaning
`id`	`int64_t`	This is a unique identifier for the zone.
`minThermalOutput`	`double`	This is the minimum value that should be considered from the thermal outputs. Commonly used as the minimum fan RPM.
`failsafePercent`	`double`	If there is a fan PID, it will use this value if the zone goes into fail-safe as the output value written to the fan's sensors.

The id field here is used in the d-bus path to talk to the xyz.openbmc_project.Control.Mode interface.

TODO: Examine how the fan controller always treating its output as a percentage works for future cases.

PIDs

There are a few PID types: fan, temp, margin, and stepwise.

The fan PID is meant to drive fans. It's expecting to get the maximum RPM setpoint value from the owning zone and then drive the fans to that value.

A temp PID is meant to drive the RPM setpoint given an absolute temperature value (higher value indicates a warmer temperature).

A margin PID is meant to drive the RPM setpoint given a margin value (lower value indicates a warmer temperature).

The setpoint output from the thermal controllers is called RPMSetpoint() However, it doesn't need to be an RPM value.

TODO: Rename this method and others to not say necessarily RPM.

Some PID configurations have fields in common, but may be interpreted differently.

PID Field

If the PID type is not stepwise then the PID field is defined as follows:

field	type	meaning
`samplePeriod`	`double`	How frequently the value is sampled. 0.1 for fans, 1.0 for temperatures.
`proportionalCoeff`	`double`	The proportional coefficient.
`integralCoeff`	`double`	The integral coefficient.
`feedFwdOffsetCoeff`	`double`	The feed forward offset coefficient.
`feedFwdGainCoeff`	`double`	The feed forward gain coefficient.
`integralLimit_min`	`double`	The integral minimum clamp value.
`integralLimit_max`	`double`	The integral maximum clamp value.
`outLim_min`	`double`	The output minimum clamp value.
`outLim_max`	`double`	The output maximum clamp value.
`slewNeg`	`double`	Negative slew value to dampen output.
`slewPos`	`double`	Positive slew value to accelerate output.

The units for the coefficients depend on the configuration of the PIDs.

If the PID is a margin controller and its setpoint is in centigrade and output in RPM: proportionalCoeff is your p value in units: RPM/C and integral coefficient: RPM/C sec

If the PID is a fan controller whose output is pwm: proportionalCoeff is %/RPM and integralCoeff is %/RPM sec.

NOTE: The sample periods are specified in the configuration as they are used in the PID computations, however, they are not truly configurable as they are used for the update periods for the fan and thermal sensors.

type == "fan"

"name": "fan1-5",
"type": "fan",
"inputs": ["fan1", "fan5"],
"setpoint": 90.0,
"pid": {
...
}

The type fan builds a FanController PID.

field	type	meaning
`name`	`string`	The name of the PID. This is just for humans and logging.
`type`	`string`	`fan`
`inputs`	`list of strings`	The names of the sensor(s) that are used as input and output for the PID loop.
`setpoint`	`double`	Presently UNUSED
`pid`	`dictionary`	A PID dictionary detailed above.

type == "temp"

TODO: Add notes for temperature configuration.

type == "margin"

"name": "fleetingpid0",
"type": "margin",
"inputs": ["fleeting0"],
"setpoint": 10,
"pid": {
...
}

The type margin builds a ThermalController PID.

field	type	meaning
`name`	`string`	The name of the PID. This is just for humans and logging.
`type`	`string`	`margin`
`inputs`	`list of strings`	The names of the sensor(s) that are used as input for the PID loop.
`setpoint`	`double`	The setpoint value for the thermal PID. The setpoint for the margin sensors.
`pid`	`dictionary`	A PID dictionary detailed above.

The output of a margin PID loop is that it sets the setpoint value for the zone. It does this by adding the value to a list of values. The value chosen by the fan PIDs (in this cascade configuration) is the maximum value.

type == "stepwise"

TODO: Write up stepwise details.