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SubjectRe: [PATCH v1 4/7] thermal: introduce the Power Allocator governor
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
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.

Cheers,
Javi


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