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

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Message-ID: <CAKohponfKzuK+TnQvWcvaT8hRX8XZJWXWGMQw138DCwP=qcx+A@mail.gmail.com>
Date:	Wed, 23 Jul 2014 13:55:57 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Thierry Reding <thierry.reding@...il.com>
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 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.

> But we're using cpu_dev->of_node, so we need to make sure cpu_dev
> doesn't go away suddenly. Simply keeping a reference to ->of_node
> won't ensure that.

Oh, yeah I completely agree, but don't see that as a normal code style
people follow. Probably they take cpu for granted, which doesn't look
right :)

> I guess technically it would be better if get_cpu_device() already
> incremented the reference count on the returned struct device. Currently
> it would theoretically still be possible for the device to disappear
> between the call to get_cpu_device() and a call to get_device().

I agree again.
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