[PATCH 09/13] SPEAr: clk: Add Auxiliary Synthesizer clock

[Sascha Hauer]

On Tue, Apr 17, 2012 at 08:30:24PM +0000, Arnd Bergmann wrote:
> On Tuesday 17 April 2012, Sascha Hauer wrote:
> > > +{
> > > +     struct clk_aux *aux = to_clk_aux(hw);
> > > +     unsigned int num = 1, den = 1, val, eqn;
> > > +     unsigned long flags = 0;
> > > +
> > > +     if (aux->lock)
> > > +             spin_lock_irqsave(aux->lock, flags);
> > > +
> > > +     val = readl_relaxed(aux->reg);
> > > +
> > > +     if (aux->lock)
> > > +             spin_unlock_irqrestore(aux->lock, flags);
> > > +
> > 
> > A single read is atomic and needs no lock.
> 
> That depends on whether the read has any side-effects. There
> is another function that does a read-modify-write on the
> same register, and you might need to prevent this one from
> reading the register between the other read and the following
> write.
> 
> The spinlock also has the benefit of enforcing the ordering
> of the read with regard to your instruction stream, otherwise
> it can be delayed on out-of-order CPUs until the value
> is actually used.

Ah, I learned something. Sorry for the noise.

Sascha


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