Re: [PATCH v5 1/8] Documentation: arm: define DT idle states bindings

Lorenzo Pieralisi
2014-06-27T10:53:53Z

On Wed, Jun 25, 2014 at 03:58:49PM +0100, Mark Rutland wrote:

[...]

> > +===========================================
> > +4 - state node
> > +===========================================
> > +
> > +A state node represents an idle state description and must be defined as
> > +follows:
> > +
> > +- state node
> > +
> > + Description: must be child of the idle-states node
> > +
> > + The state node name shall follow standard device tree naming
> > + rules ([5], 2.2.1 "Node names"), in particular state nodes which
> > + are siblings within a single common parent must be given a unique name.
> > +
> > + The idle state entered by executing the wfi instruction (idle_standby
> > + SBSA,[3][4]) is considered standard on all ARM platforms and therefore
> > + must not be listed.
> > +
> > + With the definitions provided above, the following list represents
> > + the valid properties for a state node:
> > +
> > + - compatible
> > + Usage: Required
> > + Value type: <stringlist>
> > + Definition: Must be "arm,idle-state".
> > +
> > + - logic-state-retained
> > + Usage: See definition
> > + Value type: <none>
> > + Definition: if present logic is retained on state entry,
> > + otherwise it is lost.
>
> What logic state is retained? All system registers?
>
> > + - cache-state-retained
> > + Usage: See definition
> > + Value type: <none>
> > + Definition: if present cache memory is retained on state entry,
> > + otherwise it is lost.
>
> Likewise, how much of the cache hierarchy is affected? Any of it? All of
> it?
>
> > + - timer-state-retained
> > + Usage: See definition
> > + Value type: <none>
> > + Definition: if present the timer control logic is retained on
> > + state entry, otherwise it is lost.
>
> The architected generic timers? Any CPU-local timers? Or any timers
> whatsoever?

Ok, as I mentioned this timer property is a blocking point for the
entire set. I gave it more thought, and it is a very hard nut to crack,
even if we resort to power domains (tick devices do not even contain
struct device or device node pointers, even if I added a list of
phandles to timers that are lost on idle state entry I would not be able
to figure out if the tick device is lost on idle state entry).

I am reasoning in kernel terms, I know it is bad but I can't help it
in this case.

Would a boolean property like the following one be deemed acceptable, eg:

- local-timer-stop

I want to be 100% honest here, this might turn out a Linux specific
thing, or might be not, but I still think it is representative of how HW
works.

Comments welcome and would be very appreciated on this specific detail.

Thanks,
Lorenzo