phosphor-pid-control: PWM not changing back from Manual to Auto mode

phosphor-pid-control: PWM not changing back from Manual to Auto mode

[Charles Boyer]

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Hello Josh and Ed,

I used the OEM IPMI command to enter Manual mode of phosphor-pid-control, and then I set the PWM value. After leaving Manual mode, I would expect swampd to bring the PWM back to the value from before entering Manual mode. However, the PWM remains the value I set in Manual mode. Although, when I induced a temperature sensor to approach its setpoint, I observed the PWM to change.

I believe the reason is because of https://github.com/openbmc/phosphor-pid-control/blob/cca9c659889d149c06e95bab4b8808db4f1e3eab/dbus/dbuswrite.cpp#L69. My system uses DBus fan sensors, so the fan controller will call upon DbusWritePercent::write, which writes the calculated output value (ovalue) if it is different than the oldValue. When I enter Manual mode and set the PWM, the oldValue does not change; when I exit Manual mode, the ovalue and oldValue are the same as before Manual mode, so it skips the PWM write.

How can the PWM "immediately" return to its original value from before Manual mode?

Thanks,
Charles


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Re: phosphor-pid-control: PWM not changing back from Manual to Auto mode

[Ed Tanous]

On Mon, Nov 15, 2021 at 3:20 PM Charles Boyer <Charles.Boyer@fii-usa.com> wrote:
>
> Hello Josh and Ed,
>
> I used the OEM IPMI command to enter Manual mode of phosphor-pid-control, and then I set the PWM value. After leaving Manual mode, I would expect swampd to bring the PWM back to the value from before entering Manual mode. However, the PWM remains the value I set in Manual mode. Although, when I induced a temperature sensor to approach its setpoint, I observed the PWM to change.
>
> I believe the reason is because of https://github.com/openbmc/phosphor-pid-control/blob/cca9c659889d149c06e95bab4b8808db4f1e3eab/dbus/dbuswrite.cpp#L69. My system uses DBus fan sensors, so the fan controller will call upon DbusWritePercent::write, which writes the calculated output value (ovalue) if it is different than the oldValue. When I enter Manual mode and set the PWM, the oldValue does not change; when I exit Manual mode, the ovalue and oldValue are the same as before Manual mode, so it skips the PWM write.
>
> How can the PWM "immediately" return to its original value from before Manual mode?

This is a known issue.  as the insecure validation feature was
defined, it required a full bmc reboot to reset all state at the end
of a validation/debug/tuning session.  This is one instance of many
where "best effort" clearly isn't good enough, although I'd be happy
to see patches to make the behavior better.

>
> Thanks,
> Charles
>