[PATCH 3/6] irqchip: gic: use writel instead of dsb + writel_relaxed

[will.deacon@arm.com (Will Deacon)]

On Thu, Feb 06, 2014 at 11:54:30AM +0000, Catalin Marinas wrote:
> On Thu, Feb 06, 2014 at 11:51:21AM +0000, Will Deacon wrote:
> > On Thu, Feb 06, 2014 at 11:45:59AM +0000, Catalin Marinas wrote:
> > > On Thu, Feb 06, 2014 at 11:30:50AM +0000, Will Deacon wrote:
> > > > diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
> > > > index 341c6016812d..03fe5ef3f2fe 100644
> > > > --- a/drivers/irqchip/irq-gic.c
> > > > +++ b/drivers/irqchip/irq-gic.c
> > > > @@ -662,11 +662,10 @@ void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)
> > > >  	/*
> > > >  	 * Ensure that stores to Normal memory are visible to the
> > > >  	 * other CPUs before issuing the IPI.
> > > > +	 *
> > > > +	 * This always happens on GIC0.
> > > >  	 */
> > > > -	dsb();
> > > > -
> > > > -	/* this always happens on GIC0 */
> > > > -	writel_relaxed(map << 16 | irq, gic_data_dist_base(&gic_data[0]) + GIC_DIST_SOFTINT);
> > > > +	writel(map << 16 | irq, gic_data_dist_base(&gic_data[0]) + GIC_DIST_SOFTINT);
> > > 
> > > That's heavier than a dsb() since with outer caches on ARM we also get
> > > an outer_sync() call.
> > 
> > Yes, which I think we actually need in this case, since we're trying to make
> > normal writes visible to a CPU before a device write hits the GIC.
> 
> If they are all in the inner shareable domain and with the caches
> enabled, we don't need to flush the outer cache (as in the PL310 case
> which is common to all CPUs; other saner outer caches propagate the
> barrier ;). The outer_sync is needed when the memory accesses are
> non-cacheable and we need to drain both the CPU write-buffer and the
> PL310 one.
> 
> For our case here, we only need to ensure the visibility of writes on a
> CPU to another but assuming SMP and caches enabled, so DSB is enough.

Hmm, but we *do* use this for boot and need to ensure that any mailboxes are
visible. Maybe we have enough cacheflushing/barriers for that already, but
I'm really uncomfortable making this a simple dsb(ishst).

Will