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

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

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.

Jonathan