linux-kernel - Re: [PATCH v4 1/2] ARM: keystone: pm: switch to use generic pm domains

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Message-ID: <20141125140440.GF3836@n2100.arm.linux.org.uk>
Date:	Tue, 25 Nov 2014 14:04:41 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Grygorii Strashko <grygorii.strashko@...com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Mike Turquette <mturquette@...aro.org>,
	Kevin Hilman <khilman@...nel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <robh+dt@...nel.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>, ssantosh@...nel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 1/2] ARM: keystone: pm: switch to use generic pm
 domains

On Tue, Nov 25, 2014 at 03:30:20PM +0200, Grygorii Strashko wrote:
> On 11/25/2014 02:09 PM, Arnd Bergmann wrote:
> > It might be possible to do this implicitly if the driver calls clk_get(),
> > basically doing clk_get() (or another call if necessary) would prevent the
> > simple pmdomain from turning it off during suspend.
> 
> Unfortunately, clk_get() will not work, because drivers still need to use it
> to get functional clocks even if they are not going to control them explicitly.
> For example, if it need to know clock's rate.

If you don't want a clock to be turned off, then clk_get() it, then
clk_prepare() it, and finally clk_enable() it.

Even if someone else gets it, the only time that a clock is turned off
is when _all_ users of it mutually agree that it can be turned off - by
every user disabling (and possibly unpreparing) it.

So, if the PM domain code gets a clock, prepares and enables it, then
a driver gets the same clock, prepares and enables it also, it won't
be disabled until _both_ the PM domain code _and_ the driver disable
and unprepare the clock.

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