linux-kernel - Re: [PATCH 5/8] thermal:cpu cooling:tegra: Provide deferred probing for tegra driver

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Date:	Mon, 17 Nov 2014 12:57:44 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Lukasz Majewski <l.majewski@...sung.com>
Cc:	Mikko Perttunen <mikko.perttunen@...si.fi>,
	Eduardo Valentin <edubezval@...il.com>,
	Zhang Rui <rui.zhang@...el.com>,
	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>,
	Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Vincenzo Frascino <vincenzo.frascino@...com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Lukasz Majewski <l.majewski@...ess.pl>,
	Nobuhiro Iwamatsu <iwamatsu@...auri.org>,
	Mikko Perttunen <mperttunen@...dia.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Alexandre Courbot <gnurou@...il.com>,
	linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/8] thermal:cpu cooling:tegra: Provide deferred probing
 for tegra driver

On Fri, Nov 14, 2014 at 12:24:37PM +0100, Lukasz Majewski wrote:
> Hi Mikko,
> 
> > Tested-by: Mikko Perttunen <mikko.perttunen@...si.fi>
> 
> Thanks for testing.
> 
> > 
> > One potential issue I can see is that if the cpufreq driver fails to 
> > probe then you'll never get the thermal driver either. For example, 
> > Tegra124 currently has no cpufreq driver, so if CONFIG_CPU_THERMAL
> > was enabled, then the soctherm driver would never be able to probe.
> 
> Yes, this is a potential issue. However, this option in tegra_defconfig
> is by default disabled when thermal is enabled.

Not everybody uses tegra_defconfig for their kernel builds. In fact I'd
imagine that the majority of kernels use a variant of multi_v7_defconfig
and therefore CPU_THERMAL might get enabled irrespective of any Tegra
support.

I think a better solution would be to add this check only when proper
support for CPU frequency based cooling is added. That is, when a call
to cpufreq_cooling_register() (or a variant thereof) is added.

But while at it, why not make it so that cpufreq_cooling_register()
detects if a cpufreq driver has been registered yet and propagate
-EPROBE_DEFER if necessary?

Thierry

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