cxl: core/region - ignore interleave granularity when ways=1

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.

[DJ: Fixed up against 6.15-rc1]

Signed-off-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Tested-by: Li Zhijian <lizhijian@fujitsu.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20250402232552.999634-1-gourry@gourry.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Author: Gregory Price

drivers/cxl/core/region.c

@@ -1446,7 +1446,7 @@ static int cxl_port_setup_targets(struct cxl_port *port,
 
 	if (test_bit(CXL_REGION_F_AUTO, &cxlr->flags)) {
 		if (cxld->interleave_ways != iw ||
-		    cxld->interleave_granularity != ig ||
+		    (iw > 1 && cxld->interleave_granularity != ig) ||
 		    !region_res_match_cxl_range(p, &cxld->hpa_range) ||
 		    ((cxld->flags & CXL_DECODER_F_ENABLE) == 0)) {
 			dev_err(&cxlr->dev,