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Cannot use phosphor-virtual-sensor sensors as input #5

Open PatrickRudolph opened 2 months ago

PatrickRudolph commented 2 months ago

Problem description: When phosphor-virtual-sensor is used to create a sensor named SensorA, which evaluates to a finite value at construction, it sets the Value on the Dbus interface. When the calculated value doesn't change, it never emits a dbus property changed event (of course not). The value can be read using busctl get-property xyz.openbmc_project.VirtualSensor /xyz/openbmc_project/sensors/xxxx/yyyy xyz.openbmc_project.Sensor.Value Value and is not nan.

When a second sensor, named SensorB is created, that uses SensorA as DbusParam it will never be able to see a non nan value, since:

  1. initSensorValue() cannot find the sensor as the sdbusplus event loop isn't running yet
  2. the call of getDbusProperty after the event loop is running always returns nan, as getDbusProperty is blocking and syncronous and the sensor is provided by phosphor-virtual-sensor itself.
  3. SensorA doesn't change it's value, thus never generates a property changed event

Note: For most existing use cases this isn't noticeable since they do not use phosphor-virtual-sensor's sensors as input or the used sensor values change a lot after creation.

Workaround: Add micro noise to phosphor-virtual-sensor's sensors and thus cause multiple unnecessary dbus property change events.

Possible solutions:

  1. Use boost asio.
  2. Manually emit property change for all phosphor-virtual-sensors once after creation?
williamspatrick commented 2 months ago

Can you give more details here? phosphor-virtual-sensor generally works, so I'm not following what you're looking for. Is there some configuration that doesn't work?

PatrickRudolph commented 2 months ago

Updated the issue description. Please let me know if it's still unclear how to reproduce.

williamspatrick commented 2 months ago

Updated the issue description. Please let me know if it's still unclear how to reproduce.

So you want PVS to use its own sensor for additional calculation? I don't think this is possible in an easy way in the current code, even if there were a signal. A dbus daemon can't read its own stuff over dbus (you get an ELOOP). We'd have to add explicit syntax for "get this from an internal sensor object without going to dbus".

I know it isn't pleasant but you can duplicate the PVS config for sensor A as part of sensor B?