[PATCH 1/1] irqchip: irq-gic: forward SGI to itself for cortex-a7 single core

[Peter Chen]

On Mon, Aug 08, 2016 at 02:59:16PM +0100, Marc Zyngier wrote:
> On Mon, 8 Aug 2016 14:48:42 +0100
> Mark Rutland <mark.rutland at arm.com> wrote:
> 
> > On Mon, Aug 08, 2016 at 09:28:47PM +0800, Peter Chen wrote:
> > > On Mon, Aug 08, 2016 at 02:07:54PM +0100, Mark Rutland wrote:  
> > > > I see that for arm64 we have:
> > > > 
> > > > static inline bool arch_irq_work_has_interrupt(void)
> > > > {
> > > > 	return !!__smp_cross_call;
> > > > }
> > > > 
> > > > Could we do similarly for ARM, and ony register gic_raise_softirq if
> > > > we have non-zero SGI targets?
> > > > 
> > > > If I've understood correctly, that would make things behave as they do
> > > > for UP on you system.  
> > 
> > [...]
> > 
> > > > If self-IPI is necessary, then this would be up to the GIC code to
> > > > solve.
> > > > 
> > > > For that case, it would be nicer if we could detect whether this was
> > > > necessary based on the GIC registers alone. That way we handle the
> > > > various ways this can be integrated, aren't totally relient on the DT,
> > > > work in VMs, etc.  
> > > 
> > > How we can detect IPI capabilities based on GIC register?  
> > 
> > Check the mask associated with SGIs, as we do for gic_get_cpumask(). If
> > this is zero, we have a non-multiprocessor GIC (or one that's otherwise
> > broken), and can't do SGI in the usual way.
> > 
> > However, it only makes sense to do this if self-IPI is truly a
> > necessity. Given there are other interrupt controllers that can't do
> > self-IPI, avoiding self-IPI in general would be a better strategy,
> > avoiding churn in each and every driver...
> 
> Indeed. And I won't take such a patch until all other avenues have been
> explored, including fixing core code if required...
> 

Ok, it seems both you and Mark agree with disable IPI for GIC who has only
self-IPI capability (GICD_ITARGETSR0 to GICD_ITARGETSR7 are all zero), right?

But even we do that, we still have problem that the callers for
smp_cross_call don't know well if the platform has IPI capability. Eg,
IRQ work considers the SMP system has IPI capability, but it is not a
must in this case (Cortex-A7 MPcore version, but cpu number is one).
It will cause NULL pointer dereference problem as __smp_cross_call is
NULL, and we need to make below change to let it work:

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 937c892..276bd94 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -487,7 +487,8 @@ static const char *ipi_types[NR_IPI] __tracepoint_string = {
 static void smp_cross_call(const struct cpumask *target, unsigned int ipinr)
 {
 	trace_ipi_raise_rcuidle(target, ipi_types[ipinr]);
-	__smp_cross_call(target, ipinr);
+	if (__smp_cross_call)
+		__smp_cross_call(target, ipinr);
 }

-- 

Best Regards,
Peter Chen