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

Maxime Ripard
2015-07-28T09:35:10Z

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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@linaro.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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