Re: [PATCH V4 5/5] arm: mvebu: Added SMP support for Armada XP

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

On Tue, Nov 20, 2012 at 05:07:59PM +0000, Thomas Petazzoni wrote:
> Dear Will Deacon,
> 
> On Tue, 20 Nov 2012 17:01:27 +0000, Will Deacon wrote:
> 
> > > I prefer to keep them separate, as we can use coherency without SMP.
> > > That's why I create the coherency_ll.S file instead of putting
> > > everything in this file.
> > 
> > Hmm, can you elaborate a bit more on coherency without SMP please? Do
> > these controls also affect IO coherency?
> 
> The coherency unit does coherency between CPUs, but also between the
> CPU and the devices for DMA. So for example the MV78230 is a single-core
> variant of the Armada XP, so having CONFIG_SMP for it is useless, but
> you still want the CPU to join the coherency fabric in order to provide
> coherency with I/O.
> 
> Thanks to the coherency unit, we have an almost DMA coherent
> architecture (see the HW I/O coherency patches from Grégory for
> details).

Ok, gotcha -- I didn't realise it was all controlled by the same register on
the CPU side.

Thanks for the explanation,

Will