linux-kernel - Re: [PATCH v2 14/16] cpufreq: Add cpufreq driver for Tegra124

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Date:	Wed, 23 Jul 2014 15:51:28 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Tuomas Tynkkynen <ttynkkynen@...dia.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	Prashant Gaikwad <pgaikwad@...dia.com>,
	Mike Turquette <mturquette@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Paul Walmsley <pwalmsley@...dia.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 14/16] cpufreq: Add cpufreq driver for Tegra124

On Wed, Jul 23, 2014 at 01:55:57PM +0530, Viresh Kumar wrote:
> On 23 July 2014 12:54, Thierry Reding <thierry.reding@...il.com> wrote:
> > ARM_TEGRA_CPUFREQ is still optional, so the select only applies when the
> > Tegra cpufreq driver is enabled. This is mostly just out of convenience,
> > though. The Tegra cpufreq driver uses the generic CPU0 cpufreq driver so
> > a select will automatically pull in the necessary dependency. With a
> 
> Not necessarily. cpufreq-cpu0 can have few unmet dependency. And so
> there are chances that tegra driver is compiled but cpufreq-cpu0 isn't as
> we didn't mention it as a *hard* dependency.
> 
> And so at boot, there wouldn't be any cpufreq support even when tegra's
> cpufreq driver is available.
> 
> Though, menuconfig may give some warnings no such situations.
> 
> > "depends on" the Tegra cpufreq driver only becomes available after
> > you've selected GENERIC_CPUFREQ_CPU0, which is somewhat unintuitive.
> >
> > To illustrate with an example: as a user, I want to enable CPU frequency
> > scaling on Tegra. So I use menuconfig to navigate to the "CPU Frequency
> > scaling" menu (enable it if not available yet) and look for an entry
> > that says "Tegra". But I can't find it because it's hidden due to the
> > lack of GENERIC_CPUFREQ_CPU0. That the Tegra CPU frequency driver uses a
> > generic driver is an implementation detail that users shouldn't have to
> > be aware of.
> 
> Don't know, the guy compiling out stuff should be knowledgeable enough to
> have a look why tegra cpufreq entry isn't shown in menu. As, probably the
> above problem I mentioned looks to be of more significance than this one,
> atleast to me :)
> 
> And, another thing to mention is that CONFIG_TEGRA_CPUFREQ is valid
> for earlier platforms as well and so a select/depends wouldn't be valid for
> earlier platforms. We probably need another Kconfig entry here.

Yes, sounds like a new Kconfig entry for this specific driver would be a
better approach and should remove all the above concerns.

Thierry

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