thunderbolt: Add 'generation' attribute for devices

The Thunderbolt standard went through several major iterations, here
called generation. USB4, which will be based on Thunderbolt, will be
generation 4. Let userspace know the generation of the controller in
the devices in order to distinguish between Thunderbolt and USB4, so
it can be shown in various user interfaces.
Signed-off-by: Christian Kellner <christian@kellner.me>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>

Documentation/ABI/testing/sysfs-bus-thunderbolt

@@ -80,6 +80,14 @@ Contact:	thunderbolt-software@lists.01.org
 Description:	This attribute contains 1 if Thunderbolt device was already
 		authorized on boot and 0 otherwise.
 
+What: /sys/bus/thunderbolt/devices/.../generation
+Date:		Jan 2020
+KernelVersion:	5.5
+Contact:	Christian Kellner <christian@kellner.me>
+Description:	This attribute contains the generation of the Thunderbolt
+		controller associated with the device. It will contain 4
+		for USB4.
+
 What: /sys/bus/thunderbolt/devices/.../key
 Date:		Sep 2017
 KernelVersion:	4.13

drivers/thunderbolt/switch.c

@@ -1120,6 +1120,15 @@ device_name_show(struct device *dev, struct device_attribute *attr, char *buf)
 }
 static DEVICE_ATTR_RO(device_name);
 
+static ssize_t
+generation_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	struct tb_switch *sw = tb_to_switch(dev);
+
+	return sprintf(buf, "%u\n", sw->generation);
+}
+static DEVICE_ATTR_RO(generation);
+
 static ssize_t key_show(struct device *dev, struct device_attribute *attr,
 			char *buf)
 {
@@ -1325,6 +1334,7 @@ static struct attribute *switch_attrs[] = {
 	&dev_attr_boot.attr,
 	&dev_attr_device.attr,
 	&dev_attr_device_name.attr,
+	&dev_attr_generation.attr,
 	&dev_attr_key.attr,
 	&dev_attr_nvm_authenticate.attr,
 	&dev_attr_nvm_version.attr,