Re: [PATCH v2] cxl/region: Allow 6 & 12 way regions on 3-way HB interleaves

[Alison Schofield <alison.schofield@intel.com>]

On Fri, Mar 14, 2025 at 12:00:36PM +0000, Jonathan Cameron wrote:
> On Thu,  6 Mar 2025 15:22:37 -0800
> alison.schofield@intel.com wrote:
> 
> > From: Alison Schofield <alison.schofield@intel.com>
> > 
> > The CXL driver requires the granularity of a region and its root
> > decoder to be the same. This is particularly restrictive for 3-way
> > host bridge interleaves where the only spec defined interleave
> > configurations for creating 6-way and 12-way regions on a 3-way HB
> > interleave require mixed granularities.
> > 
> > CXL 3.2 Specification 9.13.1.1:
> > Legal Interleaving Configurations: 12-way, 6-way, and 3-way
> 
> Ah this is finally a valid reason to do coarser interleave first
> (going away from host). That was subject of long discussions way back
> when original interleaving code was discussed (mostly because
> my mental model did it that way around and I couldn't follow what
> the kernel code was doing).
> 
> We 'could' revisit allowing this more generally - at least for
> already configured set ups.  Only real reason for that is
> either that someone ships a config that does it for a different
> case, or that we want to avoid special casing 3*x cases.

Hi Jonathan,

I'll bring these configs up as an OPEN at the upcoming community collab,
and hope you can elaborate more there.

--Alison


> 
> Jonathan