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From: Patrick Venture <venture@google.com>
Date: Mon, 13 May 2019 09:32:12 -0700
Message-ID: <CAO=notzQc2CX9Dx4gjX6HwnuCKnjmD_3ek+tv-XAfO+LxdQ3WA@mail.gmail.com>
Subject: Re: pid control configuration
To: Vijay Khemka <vijaykhemka@fb.com>
Cc: Ed Tanous <ed.tanous@intel.com>, 
 "openbmc@lists.ozlabs.org" <openbmc@lists.ozlabs.org>
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On Fri, May 10, 2019 at 2:21 PM Vijay Khemka <vijaykhemka@fb.com> wrote:
>
>
>
> =EF=BB=BFOn 5/6/19, 9:20 AM, "Ed Tanous" <ed.tanous@intel.com> wrote:
>
>     On 5/2/19 4:24 PM, Vijay Khemka wrote:
>     >  My understanding here is pid data are only defined for temp sensor=
s which will generate pwm values for fan.
>     >
>
>     The above is not how phosphor pid control operates.  The temperature
>     controllers "request" a given fan RPM, then the fan PID controller
>     (which is scanned much more quickly than the temperature controllers)
>     attempts to drive the fan to that speed.  This ends up being a two st=
age
>     feedback loop that can perform better than a single PID loop.
>
> Ed, we are not using cascade control here in facebook. We only use outer =
loop which gives data for fan to drive.
> Is it possible to still run Phosphor pid controller with single loop?

Your cascade PID could be a pass-through -- so that it receives the
set-point from the sensor PID and then just sets it directly within
that loop..  It may require a new PID controller that is just a
pass-through to explicitly exist -- i haven't dove into
phosphor-pid-control in a while, but you probably need to write a
basically empty PID controller object type.

>
> Regards
> -Vijay
>