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/cxl.h b/drivers/cxl/cxl.h
index c0e5308e4d1b..d683ae5e0f7d 100644
--- a/drivers/cxl/cxl.h
+++ b/drivers/cxl/cxl.h
@@ -503,7 +503,8 @@ 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);