net: usb: qmi_wwan: add qmap id sysfs file for qmimux interfaces

Add qmimux interface sysfs file qmap/mux_id to show qmap id set
during the interface creation, in order to provide a method for
userspace to associate QMI control channels to network interfaces.
Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Acked-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Aleksander Morgado <aleksander@aleksander.es>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/usb/qmi_wwan.c

@@ -218,6 +218,28 @@ static int qmimux_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
 	return 1;
 }
 
+static ssize_t mux_id_show(struct device *d, struct device_attribute *attr, char *buf)
+{
+	struct net_device *dev = to_net_dev(d);
+	struct qmimux_priv *priv;
+
+	priv = netdev_priv(dev);
+
+	return sysfs_emit(buf, "0x%02x\n", priv->mux_id);
+}
+
+static DEVICE_ATTR_RO(mux_id);
+
+static struct attribute *qmi_wwan_sysfs_qmimux_attrs[] = {
+	&dev_attr_mux_id.attr,
+	NULL,
+};
+
+static struct attribute_group qmi_wwan_sysfs_qmimux_attr_group = {
+	.name = "qmap",
+	.attrs = qmi_wwan_sysfs_qmimux_attrs,
+};
+
 static int qmimux_register_device(struct net_device *real_dev, u8 mux_id)
 {
 	struct net_device *new_dev;
@@ -240,6 +262,8 @@ static int qmimux_register_device(struct net_device *real_dev, u8 mux_id)
 		goto out_free_newdev;
 	}
 
+	new_dev->sysfs_groups[0] = &qmi_wwan_sysfs_qmimux_attr_group;
+
 	err = register_netdevice(new_dev);
 	if (err < 0)
 		goto out_free_newdev;