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

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Date:	Tue, 28 Jul 2015 11:32:22 +0200
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Lee Jones <lee.jones@...aro.org>
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, Jul 27, 2015 at 08:31:49AM +0100, Lee Jones wrote:
> 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?

What I had in mind was that, you would have three cases:

  - critical-clocks is not there: no clocks are made critical

  - critical-clocks is there, but doesn't have any values: all the
    clocks provided are marked critical

  - critical-clocks is there and it has a list of values: only the
    clocks listed are marked critical.

Does that make sense to you?

Thanks,
Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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