diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
index f332fe7af92b..c35c002b65dd 100644
--- a/drivers/cxl/core/pci.c
+++ b/drivers/cxl/core/pci.c
@@ -241,17 +241,36 @@ static void disable_hdm(void *_cxlhdm)
 	       hdm + CXL_HDM_DECODER_CTRL_OFFSET);
 }
 
-static int devm_cxl_enable_hdm(struct device *host, struct cxl_hdm *cxlhdm)
+int devm_cxl_enable_hdm(struct cxl_port *port, struct cxl_hdm *cxlhdm)
 {
-	void __iomem *hdm = cxlhdm->regs.hdm_decoder;
+	void __iomem *hdm;
 	u32 global_ctrl;
 
+	/*
+	 * If the hdm capability was not mapped there is nothing to enable and
+	 * the caller is responsible for what happens next.  For example,
+	 * emulate a passthrough decoder.
+	 */
+	if (IS_ERR(cxlhdm))
+		return 0;
+
+	hdm = cxlhdm->regs.hdm_decoder;
 	global_ctrl = readl(hdm + CXL_HDM_DECODER_CTRL_OFFSET);
+
+	/*
+	 * If the HDM decoder capability was enabled on entry, skip
+	 * registering disable_hdm() since this decode capability may be
+	 * owned by platform firmware.
+	 */
+	if (global_ctrl & CXL_HDM_DECODER_ENABLE)
+		return 0;
+
 	writel(global_ctrl | CXL_HDM_DECODER_ENABLE,
 	       hdm + CXL_HDM_DECODER_CTRL_OFFSET);
 
-	return devm_add_action_or_reset(host, disable_hdm, cxlhdm);
+	return devm_add_action_or_reset(&port->dev, disable_hdm, cxlhdm);
 }
+EXPORT_SYMBOL_NS_GPL(devm_cxl_enable_hdm, CXL);
 
 int cxl_dvsec_rr_decode(struct device *dev, int d,
 			struct cxl_endpoint_dvsec_info *info)
@@ -425,7 +444,7 @@ int cxl_hdm_decode_init(struct cxl_dev_state *cxlds, struct cxl_hdm *cxlhdm,
 	if (info->mem_enabled)
 		return 0;
 
-	rc = devm_cxl_enable_hdm(&port->dev, cxlhdm);
+	rc = devm_cxl_enable_hdm(port, cxlhdm);
 	if (rc)
 		return rc;
 
diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
index 044a92d9813e..f93a28538962 100644
--- a/drivers/cxl/cxl.h
+++ b/drivers/cxl/cxl.h
@@ -710,6 +710,7 @@ struct cxl_endpoint_dvsec_info {
 struct cxl_hdm;
 struct cxl_hdm *devm_cxl_setup_hdm(struct cxl_port *port,
 				   struct cxl_endpoint_dvsec_info *info);
+int devm_cxl_enable_hdm(struct cxl_port *port, struct cxl_hdm *cxlhdm);
 int devm_cxl_enumerate_decoders(struct cxl_hdm *cxlhdm,
 				struct cxl_endpoint_dvsec_info *info);
 int devm_cxl_add_passthrough_decoder(struct cxl_port *port);
diff --git a/drivers/cxl/port.c b/drivers/cxl/port.c
index eb57324c4ad4..17a95f469c26 100644
--- a/drivers/cxl/port.c
+++ b/drivers/cxl/port.c
@@ -60,13 +60,17 @@ static int discover_region(struct device *dev, void *root)
 static int cxl_switch_port_probe(struct cxl_port *port)
 {
 	struct cxl_hdm *cxlhdm;
-	int rc;
+	int rc, nr_dports;
 
-	rc = devm_cxl_port_enumerate_dports(port);
-	if (rc < 0)
-		return rc;
+	nr_dports = devm_cxl_port_enumerate_dports(port);
+	if (nr_dports < 0)
+		return nr_dports;
 
 	cxlhdm = devm_cxl_setup_hdm(port, NULL);
+	rc = devm_cxl_enable_hdm(port, cxlhdm);
+	if (rc)
+		return rc;
+
 	if (!IS_ERR(cxlhdm))
 		return devm_cxl_enumerate_decoders(cxlhdm, NULL);
 
@@ -75,7 +79,7 @@ static int cxl_switch_port_probe(struct cxl_port *port)
 		return PTR_ERR(cxlhdm);
 	}
 
-	if (rc == 1) {
+	if (nr_dports == 1) {
 		dev_dbg(&port->dev, "Fallback to passthrough decoder\n");
 		return devm_cxl_add_passthrough_decoder(port);
 	}
diff --git a/tools/testing/cxl/Kbuild b/tools/testing/cxl/Kbuild
index fba7bec96acd..6f9347ade82c 100644
--- a/tools/testing/cxl/Kbuild
+++ b/tools/testing/cxl/Kbuild
@@ -6,6 +6,7 @@ ldflags-y += --wrap=acpi_pci_find_root
 ldflags-y += --wrap=nvdimm_bus_register
 ldflags-y += --wrap=devm_cxl_port_enumerate_dports
 ldflags-y += --wrap=devm_cxl_setup_hdm
+ldflags-y += --wrap=devm_cxl_enable_hdm
 ldflags-y += --wrap=devm_cxl_add_passthrough_decoder
 ldflags-y += --wrap=devm_cxl_enumerate_decoders
 ldflags-y += --wrap=cxl_await_media_ready
diff --git a/tools/testing/cxl/test/mock.c b/tools/testing/cxl/test/mock.c
index de3933a776fd..284416527644 100644
--- a/tools/testing/cxl/test/mock.c
+++ b/tools/testing/cxl/test/mock.c
@@ -149,6 +149,21 @@ struct cxl_hdm *__wrap_devm_cxl_setup_hdm(struct cxl_port *port,
 }
 EXPORT_SYMBOL_NS_GPL(__wrap_devm_cxl_setup_hdm, CXL);
 
+int __wrap_devm_cxl_enable_hdm(struct cxl_port *port, struct cxl_hdm *cxlhdm)
+{
+	int index, rc;
+	struct cxl_mock_ops *ops = get_cxl_mock_ops(&index);
+
+	if (ops && ops->is_mock_port(port->uport))
+		rc = 0;
+	else
+		rc = devm_cxl_enable_hdm(port, cxlhdm);
+	put_cxl_mock_ops(index);
+
+	return rc;
+}
+EXPORT_SYMBOL_NS_GPL(__wrap_devm_cxl_enable_hdm, CXL);
+
 int __wrap_devm_cxl_add_passthrough_decoder(struct cxl_port *port)
 {
 	int rc, index;