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

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Message-ID: <20150224182124.GB3448@developer.amazonguestwifi.org>
Date:	Tue, 24 Feb 2015 14:21:26 -0400
From:	Eduardo Valentin <edubezval@...il.com>
To:	Javi Merino <javi.merino@....com>
Cc:	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	punit.agrawal@....com, broonie@...nel.org,
	Zhang Rui <rui.zhang@...el.com>
Subject: Re: [PATCH v1 4/7] thermal: introduce the Power Allocator governor

Javi,

One minor clarification as follows:

On Wed, Jan 28, 2015 at 05:00:35PM +0000, Javi Merino wrote:

<big cut>

> +
> +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 */

---
Eduardo Valentin

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