linux-kernel - Re: [PATCH] thermal: avoid division by zero in power allocator

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Message-Id: <20150929133330.809e7058004395e6db79dec1@linux-foundation.org>
Date:	Tue, 29 Sep 2015 13:33:30 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Javi Merino <javi.merino@....com>,
	Zhang Rui <rui.zhang@...el.com>,
	Eduardo Valentin <edubezval@...il.com>,
	Daniel Kurtz <djkurtz@...omium.org>
Subject: Re: [PATCH] thermal: avoid division by zero in power allocator

On Mon, 28 Sep 2015 23:28:34 +0200 Andrea Arcangeli <aarcange@...hat.com> wrote:

> During boot I get a div by zero Oops regression starting in v4.3-rc3.
> 
> ...
>
> --- a/drivers/thermal/power_allocator.c
> +++ b/drivers/thermal/power_allocator.c
> @@ -144,6 +144,16 @@ static void estimate_pid_constants(struct thermal_zone_device *tz,
>  		switch_on_temp = 0;
>  
>  	temperature_threshold = control_temp - switch_on_temp;
> +	/*
> +	 * estimate_pid_constants() tries to find appropriate default
> +	 * values for thermal zones that don't provide them. If a
> +	 * system integrator has configured a thermal zone with two
> +	 * passive trip points at the same temperature, that person
> +	 * hasn't put any effort to set up the thermal zone properly
> +	 * so just give up.
> +	 */
> +	if (!temperature_threshold)
> +		return;
>  
>  	if (!tz->tzp->k_po || force)
>  		tz->tzp->k_po = int_to_frac(sustainable_power) /

a) Are we sure this won't leave tz->tzp fields uninitialized?

b) I'm not understanding that code at all.  The "proportional" term
   in a PID controller is supposed to be proportional to the (desired -
   actual) difference (aka "the error").

   But estimate_pid_constants() appears to be setting the
   "proportional" term to be proportional to 1/error!

   Maybe a description of local `temperature_threshold' would help
   clue me in.

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