[PATCH RFC] clk: add support for automatic parent handling

[Russell King - ARM Linux]

On Thu, Apr 21, 2011 at 11:12:36AM +0200, Thomas Gleixner wrote:
> On Thu, 21 Apr 2011, Uwe Kleine-König wrote:
> > > Which is a complete failure to begin with. Why the heck do you need a
> > > callback to figure that out?
> > > 
> > > Because someone claimed, that you need a callback to make it safe from
> > > changing? Or what's the reason for this?
> > If there were a pointer tracking the parent you still needed to program
> > out the get_parent function anyhow to set the pointer initially. But I
> > agree that in other situations having a pointer is more comfortable and
> > saves ugly error handling e.g. in set_parent.
> 
> That's nonsense. Why do you want a get_parent() function at all?
> parent is set up when the clock is established in the clock tree. And
> that's not done by some random driver. That's a property of the clock.

It's *not* a fixed property of the clock.  It's a runtime property.

If you change the rate of a clock, you may need to change its parent.
If you change the operating point of the device, (eg, sleep mode where
the main system PLL gets turned off) you may need to change its parent
to a slower clock to ensure wakeup timers continue to run but at a lower
resolution.

Sure the API doesn't handle the runtime cases at all well, but that's
not a reason to enforce a fixed tree structure on something which isn't
a fixed tree structure.