sensor->inventory associations in phosphor-hwmon

[Matt Spinler]

Hi,

We have a need to know which sensor objects hosted by the various 
phosphor-hwmon
daemons belong to which inventory objects.  For example, we need to know 
that
the /xyz/openbmc_project/sensors/power/ps1_input_power sensor belongs to 
the
/xyz/openbmc_project/inventory/system/powersupply1 item.

To accomplish this, I am proposing we add a new environment variable to 
the
system specific phosphor-hwmon config files that the phosphor-hwmon code
can then use to create associations interfaces that will cause the 
mapper to
create the appropriate association D-Bus paths.

As an example, we could add to the power-supply1 conf file an 
ASSOCIATIONS
variable for each sensor (assuming ps1_input_power is represented by 
IN1):

     ASSOCIATIONS_IN1=/xyz/openbmc_project/inventory/system/powersupply1

The code would then see that and create a new org.openbmc.Associations 
interface
on the /xyz/openbmc_project/sensors/power/ps1_input_power path with an
associations property of:

     ["inventory",
      "sensors",
      "/xyz/openbmc_project/inventory/system/powersupply1"]

This leads to the mapper creating:

     /xyz/openbmc_project/sensors/power/ps1_input_power/inventory

With an xyz.openbmc_project.Association interface with an
endpoints property of:

     ["/xyz/openbmc_project/inventory/system/powersupply1"]

And also the other association path:

     /xyz/openbmc_project/inventory/system/powersupply1/sensors

With an xyz.openbmc_project.Association interface with an
endpoints property of:

     ["/xyz/openbmc_project/sensors/power/ps1_input_power"]

This now allows one to both find all sensors associated with a power 
supply, and
find which inventory item is associated with a particular sensor.  Note 
that
the endpoints property is an array, and can get entries from multiple 
places.

I suppose we could also put the forward/reverse association names, in 
this case
"inventory" and "sensors", into the hwmon config file, and also allow 
creating
multiple associations for each sensor if there is a usecase, though I 
would like
to keep it as simple as possible:

ASSOCIATIONS1_IN1=inventory,sensors,/xyz/openbmc_project/inventory/system/powersupply1
ASSOCIATIONS2_IN1=chassis,contained_sensors,/xyz/openbmc_project/inventory/system/chassis

What does everyone think?

Thanks
Matt