Re: [PATCH v2] cxl: core/region - ignore interleave granularity when ways=1

[Davidlohr Bueso]

On Wed, 02 Apr 2025, Gregory Price wrote:

> When validating decoder IW/IG when setting up regions, the granularity
> is irrelevant when iw=1 - all accesses will always route to the only
> target anyway - so all ig values are "correct". Loosen the requirement
> that `ig = (parent_iw * parent_ig)` when iw=1.
>
> On some Zen5 platforms, the platform BIOS specifies a 256-byte
> interleave granularity window for host bridges when there is only
> one target downstream.  This leads to Linux rejecting the configuration
> of a region with a x2 root with two x1 hostbridges.
>
> Decoder Programming:
>   root - iw:2 ig:256
>   hb1  - iw:1 ig:256  (Linux expects 512)
>   hb2  - iw:1 ig:256  (Linux expects 512)
>   ep1  - iw:2 ig:256
>   ep2  - iw:2 ig:256
>
> This change allows all decoders downstream of a passthrough decoder to
> also be configured as passthrough (iw:1 ig:X), but still disallows
> downstream decoders from applying subsequent interleaves.
>
> e.g. in the above example if there was another decoder south of hb1
> attempting to interleave 2 endpoints - Linux would enforce hb1.ig=512
> because the southern decoder would have iw:2 and require ig=pig*piw.
>
> Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
> Reviewed-by: Dave Jiang <dave.jiang@xxxxxxxxx>

Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>