cxl: Add decoders_committed sysfs attribute to cxl_port

This attribute allows cxl-cli to determine whether there are decoders
committed to a memdev.  This is only a snapshot of the state, and
doesn't offer any protection or serialization against a concurrent
disable-region operation.

Reviewed-by: Jim Harris <jim.harris@samsung.com>
Suggested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/169747907439.272156.10261062080830155662.stgit@djiang5-mobl3
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Author: davejiang

Documentation/ABI/testing/sysfs-bus-cxl

@@ -178,6 +178,21 @@ Description:
 		hardware decoder target list.
 
 
+What:		/sys/bus/cxl/devices/portX/decoders_committed
+Date:		October, 2023
+KernelVersion:	v6.7
+Contact:	linux-cxl@vger.kernel.org
+Description:
+		(RO) A memory device is considered active when any of its
+		decoders are in the "committed" state (See CXL 3.0 8.2.4.19.7
+		CXL HDM Decoder n Control Register). Hotplug and destructive
+		operations like "sanitize" are blocked while device is actively
+		decoding a Host Physical Address range. Note that this number
+		may be elevated without any regionX objects active or even
+		enumerated, as this may be due to decoders established by
+		platform firwmare or a previous kernel (kexec).
+
+
 What:		/sys/bus/cxl/devices/decoderX.Y
 Date:		June, 2021
 KernelVersion:	v5.14

drivers/cxl/core/port.c

@@ -534,8 +534,33 @@ static void cxl_port_release(struct device *dev)
 	kfree(port);
 }
 
+static ssize_t decoders_committed_show(struct device *dev,
+				       struct device_attribute *attr, char *buf)
+{
+	struct cxl_port *port = to_cxl_port(dev);
+	int rc;
+
+	down_read(&cxl_region_rwsem);
+	rc = sysfs_emit(buf, "%d\n", cxl_num_decoders_committed(port));
+	up_read(&cxl_region_rwsem);
+
+	return rc;
+}
+
+static DEVICE_ATTR_RO(decoders_committed);
+
+static struct attribute *cxl_port_attrs[] = {
+	&dev_attr_decoders_committed.attr,
+	NULL,
+};
+
+static struct attribute_group cxl_port_attribute_group = {
+	.attrs = cxl_port_attrs,
+};
+
 static const struct attribute_group *cxl_port_attribute_groups[] = {
 	&cxl_base_attribute_group,
+	&cxl_port_attribute_group,
 	NULL,
 };