Dealing with a sensor which doesn't have valid reading until host is powered up

[James Feist]

On 8/27/2020 2:49 PM, Alex Qiu wrote:
> Hi James,
> 
> After some debugging, I realized that the code I pointed out earlier 
> wasn't the root cause. Update is that, the HwmonTempSensor stops 
> updating after the hwmon driver returns EAGAIN as errno. I'll keep 
> debugging...
> 
> - Alex Qiu
> 
> 
> On Tue, Aug 25, 2020 at 5:49 PM Alex Qiu <xqiu at google.com 
> <mailto:xqiu at google.com>> wrote:
> 
>     Hi James and OpenBMC community,
> 
>     We have a sensor for HwmonTempSensor which doesn't have a valid
>     reading until the host is fully booted. Before it's becoming alive
>     and useful, it's getting disabled in code
>     (https://github.com/openbmc/dbus-sensors/blob/master/include/sensor.hpp#L266)
>     because of errors thrown up by the hwmon driver. Ideally, the
>     thermal control loop should kick the fan to fail safe mode until no
>     more errors are observed.
> 
>     Any suggestions on how we should handle this kind of sensor properly?

For what its worth we use the PowerState property that has options of 
power on or BiosPost to disable scanning when the state is invalid: 
https://github.com/openbmc/dbus-sensors/blob/f27a55c775383a3fb1ac655f3eda785f6845f214/src/HwmonTempMain.cpp#L208


> 
>     Thank you!
> 
>     - Alex Qiu
>