Locking in the clk API

[Jeremy Kerr]

Hi Paul,

> This looks like a complete disaster, and is also completely inconsistent
> with how the API is being used by the vast majority of users today.

I've been basing this on the mxc clock code, which acquires a mutex for all 
clk_enable()s. This may not be representative of the majority of clock usage.

From a quick search there are a few other cases of non-atomic clock usage:

tcc:		clk_enable() acquires a global clocks_mutex
tegra:	has a clk_enable_cansleep()
davinci: clk_set_parent() aquires a global clocks_mutex

Excluding the davinci code (we won't worry about set_parent for now...), if we 
can port mxc and tcc to a sleepable clk_enable, perhaps we could just go with 
purely atomic operations.

We'd still need some method of using sleeping clocks though. How about making 
clk_enable() BUG if the clock is not atomic, and add clk_enable_cansleep() for 
the cases where clk->ops.enable may sleep.

Do we need something similar for other parts of the API? (clk_set_rate?)

Cheers,


Jeremy