cxl/region: Add support to indicate region has extended linear cache

Add a region sysfs attribute to show the size of the extended linear
cache if there is any. The attribute is invisible when the cache
size is 0, which indicates it does not exist.

Moved the cxl_region_visible() location in order to pick up the
new sysfs attribute definition.

[ dj: Fixed spelling errors noted by Benjamin ]

Reviewed-by: Alison Schofield <[e-mail]>
Reviewed-by: Ben Cheatham <[e-mail]>
Reviewed-by: Jonathan Cameron <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Dave Jiang <[e-mail]>

Documentation/ABI/testing/sysfs-bus-cxl

···
 		changed, only freed by writing 0. The kernel makes no guarantees
 		that data is maintained over an address space freeing event, and
 		there is no guarantee that a free followed by an allocate
-		results in the same address being allocated.
+		results in the same address being allocated. If extended linear
+		cache is present, the size indicates extended linear cache size
+		plus the CXL region size.
 
+What:		/sys/bus/cxl/devices/regionZ/extended_linear_cache_size
+Date:		October, 2025
+KernelVersion:	v6.19
+Contact:	[e-mail]
+Description:
+		(RO) The size of extended linear cache, if there is an extended
+		linear cache. Otherwise the attribute will not be visible.
 
 What:		/sys/bus/cxl/devices/regionZ/mode
 Date:		January, 2023

drivers/cxl/core/region.c

···
 }
 static DEVICE_ATTR_RW(commit);
 
-static umode_t cxl_region_visible(struct kobject *kobj, struct attribute *a,
-				  int n)
-{
-	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_PARTMODE_PMEM)
-		return 0444;
-	return a->mode;
-}
-
 static ssize_t interleave_ways_show(struct device *dev,
 				    struct device_attribute *attr, char *buf)
 {
···
 }
 static DEVICE_ATTR_RW(size);
 
+static ssize_t extended_linear_cache_size_show(struct device *dev,
+					       struct device_attribute *attr,
+					       char *buf)
+{
+	struct cxl_region *cxlr = to_cxl_region(dev);
+	struct cxl_region_params *p = &cxlr->params;
+	ssize_t rc;
+
+	ACQUIRE(rwsem_read_intr, rwsem)(&cxl_rwsem.region);
+	if ((rc = ACQUIRE_ERR(rwsem_read_intr, &rwsem)))
+		return rc;
+	return sysfs_emit(buf, "%#llx\n", p->cache_size);
+}
+static DEVICE_ATTR_RO(extended_linear_cache_size);
+
 static struct attribute *cxl_region_attrs[] = {
 	&dev_attr_uuid.attr,
 	&dev_attr_commit.attr,
···
 	&dev_attr_resource.attr,
 	&dev_attr_size.attr,
 	&dev_attr_mode.attr,
+	&dev_attr_extended_linear_cache_size.attr,
 	NULL,
 };
+
+static umode_t cxl_region_visible(struct kobject *kobj, struct attribute *a,
+				  int n)
+{
+	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_PARTMODE_PMEM)
+		return 0444;
+
+	/*
+	 * Don't display extended linear cache attribute if there is no
+	 * extended linear cache.
+	 */
+	if (a == &dev_attr_extended_linear_cache_size.attr &&
+	    cxlr->params.cache_size == 0)
+		return 0;
+
+	return a->mode;
+}
 
 static const struct attribute_group cxl_region_group = {
 	.attrs = cxl_region_attrs,
···
 		dev_warn(cxlmd->dev.parent,
 			 "Extended linear cache calculation failed rc:%d\n", rc);
 	}
+
+	rc = sysfs_update_group(&cxlr->dev.kobj, &cxl_region_group);
+	if (rc)
+		return rc;
 
 	rc = insert_resource(cxlrd->res, res);
 	if (rc) {