Locking in the clk API

[Russell King - ARM Linux]

On Fri, Jan 21, 2011 at 04:53:44PM -0500, Nicolas Pitre wrote:
> So I think that the API must be augmented with more methods, such as:
> 
> clk_slow_enable():
>   - may sleep
>   - may be a no-op if the clk_fast_enable() is supported
> 
> clk_fast_enable():
>   - may not sleep, used in atomic context
>   - may be a no-op if controlling the clock takes time, in which case 
>     clk_slow_enable() must have set the clock up entirely
> 
> ... and similar for clk_slow_disable() and clk_fast_disable().

Isn't this along the same lines as my clk_prepare() vs clk_enable()
suggestion?

I suggested that clk_prepare() be callable only from non-atomic contexts,
and do whatever's required to ensure that the clock is available.  That
may end up enabling the clock as a result.

clk_enable() callable from atomic contexts, and turns the clock on if
the hardware supports such an operation.

So, if you have something like:

Xtal--->PLL--->Routing/Masking--->Device

clk = clk_get() returns the clock for the device.

clk_prepare(clk) would walk up the clock tree, selecting the routing and
preparing each clock.  Clocks prior to _and_ including the PLL would need
to be enabled.

clk_enable(clk) would walk up the tree if the clock isn't already enabled,
calling clk_enable() on the parent clock.  As we require prepared clocks
to already be enabled, this automatically stops at the PLL.

To encourage correct usage, we just need to make sure that clk_prepare()
has a might_sleep() thing, and clk_enable() throws a fit if it's used
on a clk without prepare being used first.  The second point is not easy
to do in a foolproof manner though, but doing _something_ is better than
nothing.