Re: [PATCH RFC 1/2] Documentation: arm: add cache DT bindings

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Wed, Dec 04, 2013 at 01:29:55PM +0000, Dave Martin wrote:
> On Mon, Dec 02, 2013 at 04:20:04PM +0000, Lorenzo Pieralisi wrote:
> > On ARM systems the cache topology cannot be probed at runtime, in
> > particular, it is impossible to probe which CPUs share a given cache
> > level. Power management software requires this knowledge to implement
> > optimized power down sequences, hence this patch adds a document that
> > defines the DT cache bindings for ARM systems. The bindings are compliant
> > with ePAPR (PowerPC bindings), and rely on the cache bindings already
> > standardized in the ePAPR v1.1 document; ARM required updates are underlined
> > in the binding document.
> > 
> > Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> > ---
> >  Documentation/devicetree/bindings/arm/cache.txt | 25 +++++++++++++++++++++++++
> >  1 file changed, 25 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/arm/cache.txt
> > 
> > diff --git a/Documentation/devicetree/bindings/arm/cache.txt b/Documentation/devicetree/bindings/arm/cache.txt
> > new file mode 100644
> > index 0000000..009cddb
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/arm/cache.txt
> > @@ -0,0 +1,25 @@
> > +==========================================
> > +ARM processors cache binding description
> > +==========================================
> > +
> > +Device tree bindings for ARM processor caches adhere to the cache bindings
> > +described in [3], in section 3.8 for multi-level and shared caches.
> 
> For architected levels of cache, most of the cache properties can be
> probed through the system register interface.
> 
> Should we be explicit about prohibiting properties in the DT describing
> cache characteristics that can be probed?

I think so, yes.

> > +On ARM, internal caches cannot be described in the cpu node but require
> > +specific nodes marked with compatible string set to "cache" (see [3],
> > +section 3.8).
> > +
> > +Furthermore the cache bindings in [3] require the following property update:
> > +
> > +- [Table 3.9] cache-level: This property of cache nodes must match the cache
> > +			   level encoded in the processors CLIDR (v7) and
> > +			   CLIDR_EL1 (v8) registers, as described in [1][2].
> 
> I still don't understand why a given cache definitely has the same level
> number assigned to it with respect to each CPU.  Is that an architectural
> requirement?
> 
> All those LoC, LoU, LoUIS properties etc. are probeable per-CPU not
> globally...  I don't see why they couldn't be different.

That's a good point, that definitely requires investigation.

Thanks,
Lorenzo