[PATCH] Documentation: clk: Add locking documentation

[Mike Turquette]

Quoting Russell King - ARM Linux (2014-02-26 14:29:26)
> On Wed, Feb 26, 2014 at 10:52:55PM +0100, Laurent Pinchart wrote:
> > Briefly documentation the common clock framework locking scheme from a
> > clock driver point of view.
> > 
> > Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas at ideasonboard.com>
> > ---
> >  Documentation/clk.txt | 26 ++++++++++++++++++++++++++
> >  1 file changed, 26 insertions(+)
> > 
> > diff --git a/Documentation/clk.txt b/Documentation/clk.txt
> > index 699ef2a..4bd6fd7 100644
> > --- a/Documentation/clk.txt
> > +++ b/Documentation/clk.txt
> > @@ -255,3 +255,29 @@ are sorted out.
> >  
> >  To bypass this disabling, include "clk_ignore_unused" in the bootargs to the
> >  kernel.
> > +
> > +     Part 7 - Locking
> > +
> > +The common clock framework uses two global locks. One of them (the enable
> > +lock) is held across calls to the .enable, .disable and .is_enabled
> > +operations, while the other (the prepare lock) is held across calls to all other
> > +operations. This effectively divides operations in two groups from a locking
> > +perspective.
> 
> It would be a good idea to mention the types of these locks - the fact
> that the enable lock is a spinlock, and the prepare lock is a mutex.
> That's a very important distinction as there's limitations on the
> contexts that the prepare-lock can be called from.

I agree with Russell and would take it a step further: this
documentation on locking should reiterate the relationship between
clk_prepare and clk_enable, namely that clk_prepare MUST be called and
MUST complete before calling clk_enable.

This contract helps us deal with the difference in lock types and keep
things sane despite the lack of lock synchronization.

Regards,
Mike

> 
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