Re: [PATCH v2 6/6] arm64: renesas: r8a7795: Add L2 cache-controller nodes

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Dec 09, 2015 at 05:58:38PM +0100, Dirk Behme wrote:
> >>For what is the arm64 dts entry
> >>
> >>cpu@0 {
> >>	...
> >>	next-level-cache = <&L2_0>;
> >>};
> >>
> >>L2_0: l2-cache0 {
> >>	compatible = "cache";
> >>};
> >>
> >>good for at all, then?
> >
> >With the other properties from ePAPR you can acquire information on the
> >geometry of the cache, which cannot be acquired from architected
> >registers.
> 
> 
> Just for my understanding: Yes, if other properties from ePAPR like
> geometry of the cache are added to the device tree l2 cache entries
> then it makes sense to have them.
> 
> But an "empty" entry like the one given in the example above doesn't
> make much sense and could be removed without loosing any
> functionality?
> 
> It looks to me that most of the L2 entries we have in
> arch/arm64/boot/dts are such "empty" entries.
>
> Is this understanding correct?

You are mostly correct, just missing some history.

It was previously (erroneously) assumed that the cache geometry could be
derived from architected registers used for set/way maintenance.
However, we knew that this did not describe how those caches were linked
to each other (e.g. which CPU shared with level x cache). So the
description in DT was required to provide that.

Now, we all know that the geometry is necessary too. Those DTs should be
fixed.

Sudeep, do you know what's happening on that front?

Thanks,
Mark.