linux-kernel - Re: [PATCH v7 5/5] clk: dt: Introduce binding for critical clock support

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Message-ID: <20150727073149.GV3436@x1>
Date:	Mon, 27 Jul 2015 08:31:49 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kernel@...inux.com, mturquette@...aro.org, sboyd@...eaurora.org,
	devicetree@...r.kernel.org, geert@...ux-m68k.org,
	s.hauer@...gutronix.de
Subject: Re: [PATCH v7 5/5] clk: dt: Introduce binding for critical clock
 support

On Mon, 27 Jul 2015, Maxime Ripard wrote:

> Hi Lee,
> 
> On Wed, Jul 22, 2015 at 02:04:15PM +0100, Lee Jones wrote:
> > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > ---
> >  .../devicetree/bindings/clock/clock-bindings.txt   | 39 ++++++++++++++++++++++
> >  1 file changed, 39 insertions(+)
> > 
> > diff --git a/Documentation/devicetree/bindings/clock/clock-bindings.txt b/Documentation/devicetree/bindings/clock/clock-bindings.txt
> > index 06fc6d5..4137034 100644
> > --- a/Documentation/devicetree/bindings/clock/clock-bindings.txt
> > +++ b/Documentation/devicetree/bindings/clock/clock-bindings.txt
> > @@ -44,6 +44,45 @@ For example:
> >    clocks by index. The names should reflect the clock output signal
> >    names for the device.
> >  
> > +critical-clock:	Some hardware contains bunches of clocks which, in normal
> > +		circumstances, must never be turned off.  If drivers a) fail to
> > +		obtain a reference to any of these or b) give up a previously
> > +		obtained reference during suspend, it is possible that some
> > +		Operating Systems might attempt to disable them to save power.
> > +		If this happens a platform can fail irrecoverably as a result.
> > +		Usually the only way to recover from these failures is to
> > +		reboot.
> > +
> > +		To avoid either of these two scenarios from catastrophically
> > +		disabling an otherwise perfectly healthy running system,
> > +		clocks can be identified as 'critical' using this property from
> > +		inside a clocksource's node.
> > +
> > +		This property is not to be abused.  It is only to be used to
> > +		protect platforms from being crippled by gated clocks, NOT as a
> > +		convenience function to avoid using the framework correctly
> > +		inside device drivers.
> > +
> > +		Expected values are hardware clock indices.  If the
> > +		clock-indices property (see below) is used, then supplied
> > +		values must correspond to one of the listed identifiers.
> > +		Using the clock-indices example below, hardware clock <2>
> > +		is missing, therefore it is considered invalid to then
> > +		list clock <2> as a critical clock.
> 
> I think we should also consider having it simply as a boolean. Using
> indices for clocks that don't have any (for example because it only
> provides a single clock) seem to not really make much sense.

Then how would you distinguish between the clocks if the provider
provides more than a single clock?

> Also, since you can have a bunch of them, using critical-clocks seem
> more appropriate.

I can change the name to critical-clocks, no problem.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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