cxl/region: Support empty uuids for non-pmem regions

Shipping versions of the cxl-cli utility expect all regions to have a
'uuid' attribute. In preparation for 'ram' regions, update the 'uuid'
attribute to return an empty string which satisfies the current
expectations of 'cxl list -R'. Otherwise, 'cxl list -R' fails in the
presence of regions with the 'uuid' attribute missing. Force the
attribute to be read-only as there is no facility or expectation for a
'ram' region to recall its uuid from one boot to the next.
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Tested-by: Fan Ni <fan.ni@samsung.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/167601994558.1924368.12612811533724694444.stgit@dwillia2-xfh.jf.intel.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

Documentation/ABI/testing/sysfs-bus-cxl

@@ -317,7 +317,8 @@ Contact:	linux-cxl@vger.kernel.org
 Description:
 		(RW) Write a unique identifier for the region. This field must
 		be set for persistent regions and it must not conflict with the
-		UUID of another region.
+		UUID of another region. For volatile ram regions this
+		attribute is a read-only empty string.
 
 
 What:		/sys/bus/cxl/devices/regionZ/interleave_granularity

drivers/cxl/core/region.c

@@ -45,7 +45,10 @@ static ssize_t uuid_show(struct device *dev, struct device_attribute *attr,
 	rc = down_read_interruptible(&cxl_region_rwsem);
 	if (rc)
 		return rc;
-	rc = sysfs_emit(buf, "%pUb\n", &p->uuid);
+	if (cxlr->mode != CXL_DECODER_PMEM)
+		rc = sysfs_emit(buf, "\n");
+	else
+		rc = sysfs_emit(buf, "%pUb\n", &p->uuid);
 	up_read(&cxl_region_rwsem);
 
 	return rc;
@@ -300,8 +303,12 @@ static umode_t cxl_region_visible(struct kobject *kobj, struct attribute *a,
 	struct device *dev = kobj_to_dev(kobj);
 	struct cxl_region *cxlr = to_cxl_region(dev);
 
+	/*
+	 * Support tooling that expects to find a 'uuid' attribute for all
+	 * regions regardless of mode.
+	 */
 	if (a == &dev_attr_uuid.attr && cxlr->mode != CXL_DECODER_PMEM)
-		return 0;
+		return 0444;
 	return a->mode;
 }