Re: [PATCH v3 3/5] clk: introduce the common clock framework

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Wed, Nov 30, 2011 at 06:20:50PM -0700, Paul Walmsley wrote:
> 1. When a clock user calls clk_enable() on a clock, the clock framework 
> should prevent other users of the clock from changing the clock's rate.  
> This should persist until the clock user calls clk_disable() (but see also 
> #2 below).  This will ensure that clock users can rely on the rate 
> returned by clk_get_rate(), as long as it's called between clk_enable() 
> and clk_disable().  And since the clock's rate is guaranteed to remain the 
> same during this time, code that cannot tolerate clock rate changes 
> without special handling (such as driver code for external I/O devices) 
> will work safely without further modification.

So, if you have a PLL whose parent clock is not used by anything else.
You want to program it to a certain rate.

You call clk_disable() on the PLL clock.  This walks up the tree and
disables the parent.  You then try to set the rate using clk_set_rate().
clk_set_rate() in this circumstance can't wait for the PLL to lock
because it can't - there's no reference clock for it.

You then call clk_enable().  The PLL now takes its time to lock.  You
can't sleep in clk_enable() because it might be called from atomic
contexts, so you have to spin waiting for this.

Overloading clk_disable/clk_enable in this way is a bad solution to
this problem.