greybus: connection: Add sysfs 'ap_cport_id' file for connections

Its a very useful piece of information, i.e. the cport id of the AP to
which the cport of the module is connected, and is required lots of
times. It isn't known in advance as it is allocated at runtime.

This patch creates another file 'ap_cport_id', only for the connection
directories, which will give the cport id of the AP.
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/Documentation/sysfs-bus-greybus

@@ -126,6 +126,14 @@ Description:
 		3 - error
 		4 - destroying
 
+What:		/sys/bus/greybus/device/.../ap_cport_id
+Date:		October 2015
+KernelVersion:	4.XX
+Contact:	Greg Kroah-Hartman <greg@kroah.com>
+Description:
+		The cport ID of the AP, to which cport of the module is
+		connected.
+
 What:		/sys/bus/greybus/device/.../protocol_id
 Date:		October 2015
 KernelVersion:	4.XX

drivers/staging/greybus/connection.c

@@ -116,9 +116,18 @@ protocol_id_show(struct device *dev, struct device_attribute *attr, char *buf)
 }
 static DEVICE_ATTR_RO(protocol_id);
 
+static ssize_t
+ap_cport_id_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	struct gb_connection *connection = to_gb_connection(dev);
+	return sprintf(buf, "%hu\n", connection->hd_cport_id);
+}
+static DEVICE_ATTR_RO(ap_cport_id);
+
 static struct attribute *connection_attrs[] = {
 	&dev_attr_state.attr,
 	&dev_attr_protocol_id.attr,
+	&dev_attr_ap_cport_id.attr,
 	NULL,
 };