cxl/region: Use region data to get the root decoder

To find a region's root decoder, the endpoint's HPA range is used to
search the matching decoder by its range. With address translation the
endpoint decoder's range is in a different address space and thus
cannot be used to determine the root decoder.

The region parameters are encapsulated within struct cxl_region_context
and may include the translated Host Physical Address (HPA) range. Use
this context to identify the root decoder rather than relying on the
endpoint.

Modify cxl_find_root_decoder() and add the region context as
parameter. Rename this function to get_cxl_root_decoder() as a
counterpart to put_cxl_root_decoder(). Simplify the implementation by
removing function cxl_port_find_switch_decode(). The function is
unnecessary because it is not referenced or utilized elsewhere in the
code.

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Tested-by: Gregory Price <gourry@gourry.net>
Signed-off-by: Robert Richter <rrichter@amd.com>
Link: https://patch.msgid.link/20260114164837.1076338-8-rrichter@amd.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Author: Robert Richter

drivers/cxl/core/region.c

@@ -3469,47 +3469,44 @@ static int devm_cxl_add_dax_region(struct cxl_region *cxlr)
 	return rc;
 }
 
-static int match_decoder_by_range(struct device *dev, const void *data)
+static int match_root_decoder(struct device *dev, const void *data)
 {
 	const struct range *r1, *r2 = data;
-	struct cxl_decoder *cxld;
+	struct cxl_root_decoder *cxlrd;
 
-	if (!is_switch_decoder(dev))
+	if (!is_root_decoder(dev))
 		return 0;
 
-	cxld = to_cxl_decoder(dev);
-	r1 = &cxld->hpa_range;
-	return range_contains(r1, r2);
-}
-
-static struct cxl_decoder *
-cxl_port_find_switch_decoder(struct cxl_port *port, struct range *hpa_range)
-{
-	struct device *cxld_dev = device_find_child(&port->dev, hpa_range,
-						    match_decoder_by_range);
+	cxlrd = to_cxl_root_decoder(dev);
+	r1 = &cxlrd->cxlsd.cxld.hpa_range;
 
-	return cxld_dev ? to_cxl_decoder(cxld_dev) : NULL;
+	return range_contains(r1, r2);
 }
 
+/*
+ * Note, when finished with the device, drop the reference with
+ * put_device() or use the put_cxl_root_decoder helper.
+ */
 static struct cxl_root_decoder *
-cxl_find_root_decoder(struct cxl_endpoint_decoder *cxled)
+get_cxl_root_decoder(struct cxl_endpoint_decoder *cxled,
+		     struct cxl_region_context *ctx)
 {
 	struct cxl_memdev *cxlmd = cxled_to_memdev(cxled);
 	struct cxl_port *port = cxled_to_port(cxled);
 	struct cxl_root *cxl_root __free(put_cxl_root) = find_cxl_root(port);
-	struct cxl_decoder *root, *cxld = &cxled->cxld;
-	struct range *hpa_range = &cxld->hpa_range;
+	struct device *cxlrd_dev;
 
-	root = cxl_port_find_switch_decoder(&cxl_root->port, hpa_range);
-	if (!root) {
+	cxlrd_dev = device_find_child(&cxl_root->port.dev, &ctx->hpa_range,
+				      match_root_decoder);
+	if (!cxlrd_dev) {
 		dev_err(cxlmd->dev.parent,
 			"%s:%s no CXL window for range %#llx:%#llx\n",
-			dev_name(&cxlmd->dev), dev_name(&cxld->dev),
-			hpa_range->start, hpa_range->end);
-		return NULL;
+			dev_name(&cxlmd->dev), dev_name(&cxled->cxld.dev),
+			ctx->hpa_range.start, ctx->hpa_range.end);
+		return ERR_PTR(-ENXIO);
 	}
 
-	return to_cxl_root_decoder(&root->dev);
+	return to_cxl_root_decoder(cxlrd_dev);
 }
 
 static int match_region_by_range(struct device *dev, const void *data)
@@ -3706,9 +3703,10 @@ int cxl_add_to_region(struct cxl_endpoint_decoder *cxled)
 	};
 
 	struct cxl_root_decoder *cxlrd __free(put_cxl_root_decoder) =
-		cxl_find_root_decoder(cxled);
-	if (!cxlrd)
-		return -ENXIO;
+		get_cxl_root_decoder(cxled, &ctx);
+
+	if (IS_ERR(cxlrd))
+		return PTR_ERR(cxlrd);
 
 	/*
 	 * Ensure that, if multiple threads race to construct_region()