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

[James Feist]

On 8/28/2020 9:43 AM, Alex Qiu wrote:
> Hi James,
> 
> Thx for the reply! So right now, one thing is that the sensor is not 
> dependent on the power state of the host solely, but also dependent on 
> the boot progress of the host.

Would the BiosPost power state not suffice?

> And the more serious issue is that 
> returning EAGAIN from the driver freezes the sensor, which is what I'm 
> debugging right now. Do we have special treatment on errno returned by 
> the driver? Thx.

I ran into a similar issue with the CPUSensor and this was my fix:
https://github.com/openbmc/dbus-sensors/commit/c22b842bfa8cfe798d83f99fa7aa9f142278c21d#diff-ccbe0562fe1d501b4c1c42d967a02ea0

I haven't hit this issue with hwmon sensor though.

> 
> - Alex Qiu
> 
> 
> On Fri, Aug 28, 2020 at 9:38 AM James Feist <james.feist at linux.intel.com 
> <mailto:james.feist at linux.intel.com>> wrote:
> 
>     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>
>      > <mailto: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
>      >
>