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

[Will Deacon]

On Thu, Feb 06, 2014 at 12:00:35PM +0000, Catalin Marinas wrote:
> On Thu, Feb 06, 2014 at 11:57:39AM +0000, Will Deacon wrote:
> > 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:
> > > > > > -	/* 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).
> 
> For boot we explicitly flush the caches for the shared data, so we don't
> need this. The dsb() here is for standard smp_call_* etc. We didn't have
> outer_sync() before, so you are slightly changing the functionality here
> (arguably, ishst is relaxing the requirements but I'm not worried about
> this, I consider that's the standard use-case for this function).

Ok, so if we assume that a dsb(ishst) is sufficient because the CPU we're
talking to is either (a) coherent in the inner-shareable domain or (b)
incoherent, and we flushed everything to PoC, then why wouldn't a dmb(ishst)
work?

Will