Re: [PATCH V2 1/5] arm: mvebu: Added support for coherency fabric in mach-mvebu

[Will Deacon <will.deacon@arm.com>]

Hi Gregory,

On Thu, Nov 15, 2012 at 03:54:39PM +0000, Gregory CLEMENT wrote:
> On 11/15/2012 11:17 AM, Will Deacon wrote:
> > Interesting, thanks for asking them about this. Does this mean that:
> 
> Here come the answers to your new questions

Great, thanks for the quick turn-around!

> > 	1. When not running coherently (i.e. before initialising the
> > 	   coherency fabric), memory is treated as non-shareable,
> > 	   non-cacheable?
> 
> It can be cacheable. The shared memory (as defined on the page table)
> will NOT be coherent by HW.

Ok, so we really are incoherent before enabling the fabric.

> > 	2. If (1), then are exclusive accesses the only way to achieve
> > 	   coherent memory accesses in this scenario?
> 
> I quote: "I suspect there is terminology miss-use: exclusive accesses
> are NOT used to achieve memory coherency - they are used to achieve
> atomicity. To achieve memory coherency while fabric is configured to
> be non-coherent, SW should use maintenance operations over the L1
> caches."

Ok, so if I'm understanding correctly then I don't really see the usefulness
of having working exclusives that are incoherent. Surely it means that you
can guarantee mutual exclusion on a lock variable, but the value you actually
end up reading from the lock is junk unless you litter the accessors with cache
clean operations?

Anyway, that's by-the-by as this is all called early enough that we
shouldn't care. The thing I don't like now is that the fabric initialisation
is done entirely differently on the primary CPU than the secondaries. The
primary probes the device-tree (well, it's also now hard-coded for v2) and
accesses the registers from a C function(armada_370_xp_set_cpu_coherent) whilst
the secondaries have hardcoded addresses and access via asm
(armada_xp_secondary_startup).

Will