diff --git a/Documentation/ABI/testing/sysfs-bus-nvdimm b/Documentation/ABI/testing/sysfs-bus-nvdimm
index 64eb8f4c6a41..46dafd8482b9 100644
--- a/Documentation/ABI/testing/sysfs-bus-nvdimm
+++ b/Documentation/ABI/testing/sysfs-bus-nvdimm
@@ -48,7 +48,8 @@ What:		/sys/bus/nd/devices/nmemX/cxl/id
 Date:		November 2022
 KernelVersion:	6.2
 Contact:	Dave Jiang <dave.jiang@intel.com>
-Description:	(RO) Show the id (serial) of the device. This is CXL specific.
+Description:	(RO) Show the id (serial) of the device, formatted as an
+		unsigned 64-bit decimal value. This is CXL specific.
 
 What:		/sys/bus/nd/devices/nmemX/cxl/provider
 Date:		November 2022
diff --git a/drivers/cxl/core/pmem.c b/drivers/cxl/core/pmem.c
index 68462e38a977..5a3bb7e8a1f1 100644
--- a/drivers/cxl/core/pmem.c
+++ b/drivers/cxl/core/pmem.c
@@ -219,12 +219,14 @@ static struct cxl_nvdimm *cxl_nvdimm_alloc(struct cxl_nvdimm_bridge *cxl_nvb,
 	dev->bus = &cxl_bus_type;
 	dev->type = &cxl_nvdimm_type;
 	/*
-	 * A "%llx" string is 17-bytes vs dimm_id that is max
-	 * NVDIMM_KEY_DESC_LEN
+	 * dev_id is the nvdimm dimm_id used for security key lookup.
+	 * It must match id_show(), which emits the CXL serial as an
+	 * unsigned decimal. A u64 decimal string is at most 20 digits
+	 * plus NUL.
 	 */
-	BUILD_BUG_ON(sizeof(cxl_nvd->dev_id) < 17 ||
+	BUILD_BUG_ON(sizeof(cxl_nvd->dev_id) < 21 ||
 		     sizeof(cxl_nvd->dev_id) > NVDIMM_KEY_DESC_LEN);
-	sprintf(cxl_nvd->dev_id, "%llx", cxlmd->cxlds->serial);
+	sprintf(cxl_nvd->dev_id, "%llu", cxlmd->cxlds->serial);
 
 	return cxl_nvd;
 }
diff --git a/drivers/cxl/core/trace.h b/drivers/cxl/core/trace.h
index d37876096dd7..c379d60047fc 100644
--- a/drivers/cxl/core/trace.h
+++ b/drivers/cxl/core/trace.h
@@ -107,7 +107,7 @@ TRACE_EVENT(cxl_aer_uncorrectable_error,
 		memcpy(__entry->header_log, hl,
 			CXL_HEADERLOG_TRACE_SIZE_U32 * sizeof(u32));
 	),
-	TP_printk("memdev=%s host=%s serial=%lld: status: '%s' first_error: '%s'",
+	TP_printk("memdev=%s host=%s serial=%llu: status: '%s' first_error: '%s'",
 		  __get_str(memdev), __get_str(host), __entry->serial,
 		  show_uc_errs(__entry->status),
 		  show_uc_errs(__entry->first_error)
@@ -166,7 +166,7 @@ TRACE_EVENT(cxl_aer_correctable_error,
 		__entry->serial = cxlmd->cxlds->serial;
 		__entry->status = status;
 	),
-	TP_printk("memdev=%s host=%s serial=%lld: status: '%s'",
+	TP_printk("memdev=%s host=%s serial=%llu: status: '%s'",
 		  __get_str(memdev), __get_str(host), __entry->serial,
 		  show_ce_errs(__entry->status)
 	)
@@ -206,7 +206,7 @@ TRACE_EVENT(cxl_overflow,
 		__entry->last_ts = le64_to_cpu(payload->last_overflow_timestamp);
 	),
 
-	TP_printk("memdev=%s host=%s serial=%lld: log=%s : %u records from %llu to %llu",
+	TP_printk("memdev=%s host=%s serial=%llu: log=%s : %u records from %llu to %llu",
 		__get_str(memdev), __get_str(host), __entry->serial,
 		cxl_event_log_type_str(__entry->log), __entry->count,
 		__entry->first_ts, __entry->last_ts)
@@ -279,7 +279,7 @@ TRACE_EVENT(cxl_overflow,
 	__entry->hdr_head_id = (hdr).head_id
 
 #define CXL_EVT_TP_printk(fmt, ...) \
-	TP_printk("memdev=%s host=%s serial=%lld log=%s : time=%llu uuid=%pUb "	\
+	TP_printk("memdev=%s host=%s serial=%llu log=%s : time=%llu uuid=%pUb "	\
 		"len=%d flags='%s' handle=%x related_handle=%x "		\
 		"maint_op_class=%u maint_op_sub_class=%u "			\
 		"ld_id=%x head_id=%x : " fmt,					\
@@ -1088,7 +1088,7 @@ TRACE_EVENT(cxl_poison,
 		}
 	    ),
 
-	TP_printk("memdev=%s host=%s serial=%lld trace_type=%s region=%s "  \
+	TP_printk("memdev=%s host=%s serial=%llu trace_type=%s region=%s "  \
 		"region_uuid=%pU hpa=0x%llx hpa_alias0=0x%llx dpa=0x%llx " \
 		"dpa_length=0x%x source=%s flags=%s overflow_time=%llu",
 		__get_str(memdev),
diff --git a/drivers/cxl/cxl.h b/drivers/cxl/cxl.h
index bdd0132c999d..cab8ce39f465 100644
--- a/drivers/cxl/cxl.h
+++ b/drivers/cxl/cxl.h
@@ -505,7 +505,8 @@ struct cxl_nvdimm_bridge {
 	struct nvdimm_bus_descriptor nd_desc;
 };
 
-#define CXL_DEV_ID_LEN 19
+/* Holds a u64 serial as a decimal string: up to 20 digits + NUL */
+#define CXL_DEV_ID_LEN 21
 
 enum {
 	CXL_NVD_F_INVALIDATED = 0,
diff --git a/drivers/cxl/pmem.c b/drivers/cxl/pmem.c
index 261dff7ced9f..a9f50281875d 100644
--- a/drivers/cxl/pmem.c
+++ b/drivers/cxl/pmem.c
@@ -52,7 +52,7 @@ static ssize_t id_show(struct device *dev, struct device_attribute *attr, char *
 	struct cxl_nvdimm *cxl_nvd = nvdimm_provider_data(nvdimm);
 	struct cxl_dev_state *cxlds = cxl_nvd->cxlmd->cxlds;
 
-	return sysfs_emit(buf, "%lld\n", cxlds->serial);
+	return sysfs_emit(buf, "%llu\n", cxlds->serial);
 }
 static DEVICE_ATTR_RO(id);
 
diff --git a/tools/testing/cxl/test/mem.c b/tools/testing/cxl/test/mem.c
index a7da279aa3ef..7b756000a1a6 100644
--- a/tools/testing/cxl/test/mem.c
+++ b/tools/testing/cxl/test/mem.c
@@ -1713,6 +1713,7 @@ static int cxl_mock_mem_probe(struct platform_device *pdev)
 	struct cxl_mockmem_data *mdata;
 	struct cxl_mailbox *cxl_mbox;
 	struct cxl_dpa_info range_info = { 0 };
+	u64 serial;
 	int rc;
 
 	/* Increase async probe race window */
@@ -1739,7 +1740,19 @@ static int cxl_mock_mem_probe(struct platform_device *pdev)
 	if (rc)
 		return rc;
 
-	mds = cxl_memdev_state_create(dev, pdev->id + 1, 0);
+	/*
+	 * Mock serials have historically been derived from pdev->id and stayed
+	 * single-digit, so they never exercised either decimal-vs-hex key
+	 * lookup or unsigned formatting. Give one mock device a full-width
+	 * serial with bit 63 set, matching real hardware such as Montage CXL
+	 * devices. pdev->id 7 is unused by the auto-region topology.
+	 */
+	if (pdev->id == 7)
+		serial = 0x8a34567890abcdef;
+	else
+		serial = pdev->id + 1;
+
+	mds = cxl_memdev_state_create(dev, serial, 0);
 	if (IS_ERR(mds))
 		return PTR_ERR(mds);