L1 & L2 cache flush sequence on CortexA5 MPcore w.r.t low power modes

[Russell King - ARM Linux]

On Mon, May 14, 2012 at 05:21:50PM +0100, Lorenzo Pieralisi wrote:
> On Mon, May 14, 2012 at 04:58:59PM +0100, Russell King - ARM Linux wrote:
> > On Mon, May 14, 2012 at 04:50:22PM +0100, Lorenzo Pieralisi wrote:
> > > > 2. L2 disable
> > > > 3. L1 clean & invalidate
> > > 
> > > This is wrong again since while cleaning and invalidating the cache (L1 here)
> > > can still allocate and this must not happen.
> > 
> > No it isn't.  There is never anything wrong with allocating new caches lines
> > into a cache which is going to (eventually) be powered down.  Ever.
> 
> What if the cache allocates a dirty cache line moved from L1 of another
> processor ?
> 
> > What would be wrong is if we end up with dirty cache lines in the cache
> > to be powered down for data which we _care_ about preserving when power
> > is lost.
> > 
> > That's a _very_ _very_ important difference.
> 
> That's exactly the point I am making. dirty cache lines can be migrated across
> processors caches. If we want to shut down a single core we have to be 100%
> sure that dirty cache lines (if we care about that data, we might be not as you
> pointed out) must not be present in L1 when we shut the core down. If the C
> bit in SCTLR is not cleared before cleaning and invalidating this is not
> guaranteed from an architectural point of view.
> 
> Occurences might be rare, but it is still not safe to clean the cache with the
> C bit set.

It's not safe to disable the C bit without first pushing the dirty data out
to RAM either.  It's a catch-22 situation - because turning the C bit off
not only stops the caches allocating new lines but also prevents them being
searched.

That means your view of cacheable memory suddenly changes beneath you when
the C bit is turned off.

>From what you're saying - and from my understanding of your cache behaviours,
even the sequence:
- clean cache
- disable C bit
- clean cache
is buggy.

I think what you're effectively saying is that it is not possible to safely
power down a cache on an ARM SMP CPU...