Re: Common clock framework API vs RT patchset

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

On Tue, Aug 04, 2015 at 10:23:31AM -0500, Nishanth Menon wrote:
> Consider clk_enable/disable/set_parent/setfreq operations. none of these
> operations are "atomic" from hardware point of view. instead, they are a
> set of steps which culminates to moving from state A to state B of the
> clock tree configuration.

There's a world of difference between clk_enable()/clk_disable() and
the rest of the clk API.

clk_enable()/clk_disable() _should_ be callable from any context, since
you may need to enable or disable a clock from any context.  The remainder
of the clk API is callable only from contexts where sleeping is permissible.

The reason we have this split is because clk_enable()/clk_disable() have
historically been used in interrupt handlers, and they're specifically
not supposed to impose big delays.

Things like waiting for a PLL to re-lock is time-consuming, so it's not
something I'd expect to see behind a clk_enable() implementation (the
fact you can't sleep in there is a big hint.)  Such waits should be in
the clk_prepare() stage instead.

Now, as for clk_enable() being interrupted - if clk_enable() is interrupted
and another clk_enable() comes along for the same clock, that second
clk_enable() should not return until the clock has actually been enabled,
and it's up to the implementation to decode how to achieve that.  If that
means a RT implementation using a raw spinlock, then that's one option
(which basically would have the side effect of blocking until the preempted
clk_enable() finishes its business.)  Alternatively, if we can preempt
inside clk_enable(), then the clk_enable() implementation should be written
to cope with that (eg, by the second clk_enable() fiddling with the hardware,
and the first thread noticing that it has nothing to do.)

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