Comments on Sensor design.

[Xo Wang]

Hi Patrick,

On Tue, Nov 1, 2016 at 12:24 PM, Patrick Williams <patrick at stwcx.xyz> wrote:
> On Tue, Nov 01, 2016 at 10:10:08AM -0400, Brad Bishop wrote:
>> I can see a path here to extending what a sensor is (additional properties)
>> without impacting the existing API.  Do we lose that capability with option #1?
>> What about com.ibm.Sensor or org.open_power.Sensor?
>
> I don't think we lose that.  Someone could always extend it with a
> 'com.ibm.Sensor.Temperature' interface if they wanted.
>
> I fully expect that we will have at least one extension which is related
> to thresholds.
>     xyz.openbmc_project.Sensor.Threshold
>         property max
>         property min
>
> I suspect what I just wrote isn't correct and may require multiple
> interfaces to handle all the different threshold types.
>
>> >
>> > 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’?
>>
>> Leaning towards the latter, for the reason stated above.
>>
>
> The big trade-off to me between them, besides minor impact of multiple
> implementations of effectively the same content, has to do with signals
> and filtering.  I could see it interesting to listen for signals on all
> *.Sensor instead of having to listen for them each individually.  Maybe
> the path is sufficient for this as well.
>

I don't think that listening to multiple interfaces or even all
interfaces of an object path is an excessive burden to the
application, since that's only a matter of writing the appropriate
sd_bus_add_match match rule.

The benefit is that a signal handler would be able to differentiate
sensor types based on the originating interface, removing the need to
parse its contents for purposes of dispatching or filtering. That also
means signals can be slightly lighter weight (as in payload).

> --
> Patrick Williams
>
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cheers
xo