Commit 384a290283fde63ba8dc671fca5420111cdac19a seems to break 11MPCore boot

[Russell King - ARM Linux]

On Wed, Jan 30, 2013 at 04:21:32PM +0000, Russell King - ARM Linux wrote:
> On Wed, Jan 30, 2013 at 11:00:50AM -0500, Nicolas Pitre wrote:
> > On Wed, 30 Jan 2013, Punit Agrawal wrote:
> > 
> > > Hi Nicolas,
> > > 
> > > I was trying to boot 3.8-rc5 on Realview EB 11MPCore using
> > > realview-smp_defconfig as a starting point but the kernel failed to progress
> > > past the log below (config attached).
> > > 
> > > Pawel suggested I try reverting 384a290283fde63ba8dc671fca5420111cdac19a -
> > > "ARM: gic: use a private mapping for CPU target interfaces" that you've
> > > authored. With this commit reverted the kernel boots.
> > > 
> > > I am not quite sure why the commit breaks 11MPCore but Pawel (cc'd) might be
> > > able to shed light on that.
> > 
> > That would be appreciated as I don't have any good answer to provide.
> > 
> > Typically, this patch highlighted problems with bad holding pen 
> > implementations where secondary CPUs would enter the kernel all at the 
> > same time. In that case the kernel was crashing even before displaying 
> > "CPU2: Booted secondary processor".
> 
> Well, the patch still looks fine to me.  It might be a good idea to
> dump out the value of GIC_DIST_TARGET + 0, just in case there's some
> version of the GIC which doesn't advertise its CPU mask via that
> register (it should, because it corresponds with SGI0..3, and every
> spec I have says that it will be implemented if these IRQs are present).
> 
> We do know already that there are some implementations out there which
> don't conform to these documents...

Right, okay.  This is the bug.  GIC_DIST_TARGET+0 can most certainly read
as zeros on MPCore platforms (it's in the MPCore engineering spec).

Only interrupts 29, 30 and 31 read as non-zero and return the corresponding
CPU mask.  Interrupts 0-28 read as zero.

However, this is further complicated: in later GIC revisions, it says that
these registers can return 0 for unimplemented interrupts.  Are interrupts
29-31 always guaranteed to be implemented?  I don't think we can rely on
that.

What we could do is scan interrupts 0-31 for a non-zero value.  If they're
all zero, we should complain.  Otherwise, we use the first non-zero value
we find and validate it for a single bit set.