Comments on Sensor design.

[Patrick Williams]

On Tue, Nov 01, 2016 at 09:58:07PM -0700, Rick Altherr wrote:
> I see Type and Unit being conflated.  I find it perfectly reasonable to
> report temperatures (called a Type in the example, in math/physics circles
> it would be a measurement dimension) in any of ºF, ºC, or K (units).  In
> the monitoring systems I've designed, I've used something like:

I would strongly suggest that we pick a base unit for all sensors of
that type to be in.  It is kind of pointless to have everyone consuming
temperatures within the BMC having to know the F->C conversion.

> Interface xyz.openbmc_project.Sensor
>                 Properties:
>                      - Instance Identifier (map<string, string>)
>                      - Value (integer)
>                      - Unit (string [enumeration])
>                      - Scale (integer, n where real_value = value*10^n)
> 
> Where the path conveys a broader grouping of device type and the
> measurement dimension:
> /xyz/openbmc_project/sensors/cpu/temperature
> /xyz/openbmc_project/sensors/fan/current_speed
> 
> That allows for querying the data in a variety of ways: all data about a
> given element in the system (/xyz/openbmc_project/sensors/cpu with
> instance_id <core=1>), one type of measurement across all instances of a
> device type (/xyz/openbmc_project/sensors/cpu/temperature), etc.

We tend to use associations to represent the relationship between
entities.  So, /.../inventory/.../core0/temperature is an association to
/.../sensor/temperature/core0temp and
/.../sensor/temperature/core0temp/measured_element would point back to
the core0 inventory item.

Is that good enough for the 'instance identifier' you are suggesting?

> 
> On Mon, Oct 31, 2016 at 2:08 PM, Patrick Williams <patrick at stwcx.xyz> wrote:
> 
> > There was a request for us to start on the openbmc interfaces for
> > Sensors in the near future.  Before we get started on it I wanted to
> > solicit feedback on two divergent approaches.
> >
> > Before I start, keep in mind that any proposals for DBus designs will
> > show up here:
> >     https://gerrit.openbmc-project.xyz/#/q/project:openbmc/phosphor-dbus-
> > interfaces+status:open
> >
> > The two divergent approaches are:
> >
> >     1. Have a single interface for all sensor readings.  Example:
> >             Interface xyz.openbmc_project.Sensor
> >                 Properties:
> >                      - Value (integer)
> >                      - Unit (string [enumeration])
> >                      - Scale (integer, n where real_value = value*10^n)
> >
> >     2. Have unique interfaces for different kinds of sensor readings.
> >             Interface xyz.openbmc_project.Sensor.Temperature
> >             Interface xyz.openbmc_project.Sensor.Tach
> >                 ( Same value and scale properties )
> >
> > In both cases the dbus path could contain the 'type':
> >     /xyz/openbmc_project/sensors/temperature/ambient
> >     /xyz/openbmc_project/sensors/fan_tach/fan0
> >
> > The question is essentially should the "Unit" property be used to
> > resolve the 'type' or should we have distinct interfaces for each
> > 'type'?
> >
> > For some comparison with other standards, CIM had a single Sensor class
> > while Redfish seems to have multiple classes (Fan, Temperature) for each
> > type.
> >
> > --
> > Patrick Williams
> >
> > _______________________________________________
> > openbmc mailing list
> > openbmc at lists.ozlabs.org
> > https://lists.ozlabs.org/listinfo/openbmc
> >
> >

-- 
Patrick Williams
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