cxl/region/extent: Expose region extent information in sysfs

Extent information can be helpful to the user to coordinate memory usage
with the external orchestrator and FM.

Expose the details of region extents by creating the following
sysfs entries.

        /sys/bus/cxl/devices/dax_regionX/extentX.Y
        /sys/bus/cxl/devices/dax_regionX/extentX.Y/offset
        /sys/bus/cxl/devices/dax_regionX/extentX.Y/length
        /sys/bus/cxl/devices/dax_regionX/extentX.Y/tag

Based on an original patch by Navneet Singh.

Reviewed-by: Jonathan Cameron <[email protected]>
Reviewed-by: Fan Ni <[email protected]>
Tested-by: Fan Ni <[email protected]>
Signed-off-by: Ira Weiny <[email protected]>

---
Changes:
[iweiny: rebase]
[iweiny: s/tag/uuid/ throughout the code]
[iweiny: update sysfs docs to 2025]

Author: weiny2

Documentation/ABI/testing/sysfs-bus-cxl

@@ -639,3 +639,39 @@ Description:
 		The count is persistent across power loss and wraps back to 0
 		upon overflow. If this file is not present, the device does not
 		have the necessary support for dirty tracking.
+
+
+What:		/sys/bus/cxl/devices/dax_regionX/extentX.Y/offset
+Date:		May, 2025
+KernelVersion:	v6.16
+Contact:	[email protected]
+Description:
+		(RO) [For Dynamic Capacity regions only] Users can use the
+		extent information to create DAX devices on specific extents.
+		This is done by creating and destroying DAX devices in specific
+		sequences and looking at the mappings created.  Extent offset
+		within the region.
+
+
+What:		/sys/bus/cxl/devices/dax_regionX/extentX.Y/length
+Date:		May, 2025
+KernelVersion:	v6.16
+Contact:	[email protected]
+Description:
+		(RO) [For Dynamic Capacity regions only] Users can use the
+		extent information to create DAX devices on specific extents.
+		This is done by creating and destroying DAX devices in specific
+		sequences and looking at the mappings created.  Extent length
+		within the region.
+
+
+What:		/sys/bus/cxl/devices/dax_regionX/extentX.Y/uuid
+Date:		May, 2025
+KernelVersion:	v6.16
+Contact:	[email protected]
+Description:
+		(RO) [For Dynamic Capacity regions only] Users can use the
+		extent information to create DAX devices on specific extents.
+		This is done by creating and destroying DAX devices in specific
+		sequences and looking at the mappings created.  UUID of this
+		extent.

drivers/cxl/core/extent.c

@@ -6,6 +6,63 @@
 
 #include "core.h"
 
+static ssize_t offset_show(struct device *dev, struct device_attribute *attr,
+			   char *buf)
+{
+	struct region_extent *region_extent = to_region_extent(dev);
+
+	return sysfs_emit(buf, "%#llx\n", region_extent->hpa_range.start);
+}
+static DEVICE_ATTR_RO(offset);
+
+static ssize_t length_show(struct device *dev, struct device_attribute *attr,
+			   char *buf)
+{
+	struct region_extent *region_extent = to_region_extent(dev);
+	u64 length = range_len(&region_extent->hpa_range);
+
+	return sysfs_emit(buf, "%#llx\n", length);
+}
+static DEVICE_ATTR_RO(length);
+
+static ssize_t uuid_show(struct device *dev, struct device_attribute *attr,
+			 char *buf)
+{
+	struct region_extent *region_extent = to_region_extent(dev);
+
+	return sysfs_emit(buf, "%pUb\n", &region_extent->uuid);
+}
+static DEVICE_ATTR_RO(uuid);
+
+static struct attribute *region_extent_attrs[] = {
+	&dev_attr_offset.attr,
+	&dev_attr_length.attr,
+	&dev_attr_uuid.attr,
+	NULL
+};
+
+static uuid_t empty_uuid = { 0 };
+
+static umode_t region_extent_visible(struct kobject *kobj,
+				     struct attribute *a, int n)
+{
+	struct device *dev = kobj_to_dev(kobj);
+	struct region_extent *region_extent = to_region_extent(dev);
+
+	if (a == &dev_attr_uuid.attr &&
+	    uuid_equal(&region_extent->uuid, &empty_uuid))
+		return 0;
+
+	return a->mode;
+}
+
+static const struct attribute_group region_extent_attribute_group = {
+	.attrs = region_extent_attrs,
+	.is_visible = region_extent_visible,
+};
+
+__ATTRIBUTE_GROUPS(region_extent_attribute);
+
 static void cxled_release_extent(struct cxl_endpoint_decoder *cxled,
 				 struct cxled_extent *ed_extent)
 {
@@ -44,6 +101,7 @@ static void region_extent_release(struct device *dev)
 static const struct device_type region_extent_type = {
 	.name = "extent",
 	.release = region_extent_release,
+	.groups = region_extent_attribute_groups,
 };
 
 bool is_region_extent(struct device *dev)