linux-kernel - Re: [PATCH v1 4/7] thermal: introduce the Power Allocator governor

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Date:	Wed, 25 Feb 2015 15:00:38 -0400
From:	Eduardo Valentin <edubezval@...il.com>
To:	Javi Merino <javi.merino@....com>
Cc:	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Punit Agrawal <Punit.Agrawal@....com>,
	"broonie@...nel.org" <broonie@...nel.org>,
	Zhang Rui <rui.zhang@...el.com>
Subject: Re: [PATCH v1 4/7] thermal: introduce the Power Allocator governor

On Wed, Feb 25, 2015 at 02:48:49PM +0000, Javi Merino wrote:
> Hi Eduardo,
> 
> On Tue, Feb 24, 2015 at 06:21:26PM +0000, Eduardo Valentin wrote:
> > On Wed, Jan 28, 2015 at 05:00:35PM +0000, Javi Merino wrote:
> > > +
> > > +k_d
> > > +---
> > > +
> > > +`k_d` configures the PID loop's derivative term constant.  It's
> > > +recommended to leave it as the default: 0.
> > > +
> > 
> > I know we are considering K_d = 0. However, ...
> > 
> > <yet another big cut>
> > 
> > > +	/*
> > > +	 * Calculate the derivative term
> > > +	 *
> > > +	 * We do err - prev_err, so with a positive k_d, a decreasing
> > > +	 * error (i.e. driving closer to the line) results in less
> > > +	 * power being applied, slowing down the controller)
> > > +	 */
> > > +	d = mul_frac(tz->tzp->k_d, err - params->prev_err);
> > 
> > 
> > ... Shouldn't the above d component consider the rate of changes over time of the error?
> > 
> > I would expect you should do:
> > d = k_d * (dE / dt)
> > 
> > or
> > 
> > d = K_d * ((err - params->prev_err) / sampling_period)
> > 
> > in plain C:
> > 
> > +	d = mul_frac(tz->tzp->k_d, err - params->prev_err);
> > +	d /= tz->passive_polling; /* might require fixed point division */
> 
> Could do.  To be honest, both k_d and passive_polling are constants so

Yes, I agree that they are constants. But if you deploy the thermal zone
with different sampling period on different devices, then the behavior
will change.

> I don't think you get anything by doing this other than the added
> complexity of the fixed point division.  As you said, the default k_d
> is 0, so I'm not strongly against it.

OK. Then I would prefer to add the division, as it makes the code aligned
to the concept.

> 
> Cheers,
> Javi

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