[PATCH v4] GPIO PL061: Adding Clk framework support

[Russell King - ARM Linux]

On Sat, Jul 10, 2010 at 01:55:26AM +0200, Linus Walleij wrote:
> This looks mostly good to me (comments below) but will have as
> a side effect that all clock frameworks on all platforms using PrimeCells
> will need to be initialized *before* the PrimeCell devices are added.

No it doesn't - it needs the clock API to be initialized before the
first driver is probed.  As drivers are registered during the initcalls,
later than postcore level

> > -       id = amba_lookup(pcdrv->id_table, pcdev);
> > +               ret = pcdrv->probe(pcdev, id);
> > +               if (ret == 0)
> > +                       break;
> 
> So this means you always turn on the block clock at
> probe, and then leave it on if the probe was successful.

Correct, which is what drivers currently expect.  This is also why
two additional amba_* callbacks are supplied - if it is appropriate
for a driver to disable their bus clock after being probed, they can
do that but they have to participate in that.

Basically, they have to call amba_bus_clk_disable() on successful
probe, and then amba_bus_clk_enable() each time that they want to
access their registers, followed by an amba_bus_clk_disable() when
they've finished.  Finally, they must call amba_bus_clk_enable()
in their remove method and avoid amba_bus_clk_disable() before
returning.

> For drivers that need to conserve power, also the block
> clock has to be gated when the device is not in use,
> sometimes since that affects some hardware that keep
> track of the usecount of the clock tree for a larger region of
> silicon, so the power savings can be drastic.

You can't have the core code doing that.  If you unconditionally turn
the bus clock off after probe, what happens when a driver receives an
interrupt and tries to access its registers?

Hint: the core code can't know that the driver has registered an IRQ
handler.