HID: core: store the unique system identifier in hid_device

This unique identifier is currently used only for ensuring uniqueness in
sysfs. However, this could be handful for userspace to refer to a specific
hid_device by this id.

2 use cases are in my mind: LEDs (and their naming convention), and
HID-BPF.
Reviewed-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NBenjamin Tissoires <benjamin.tissoires@redhat.com>
Link: https://lore.kernel.org/r/20220902132938.2409206-9-benjamin.tissoires@redhat.com

drivers/hid/hid-core.c

@@ -2739,10 +2739,12 @@ int hid_add_device(struct hid_device *hdev)
 			hid_warn(hdev, "bad device descriptor (%d)\n", ret);
 	}
 
+	hdev->id = atomic_inc_return(&id);
+
 	/* XXX hack, any other cleaner solution after the driver core
 	 * is converted to allow more than 20 bytes as the device name? */
 	dev_set_name(&hdev->dev, "%04X:%04X:%04X.%04X", hdev->bus,
-		     hdev->vendor, hdev->product, atomic_inc_return(&id));
+		     hdev->vendor, hdev->product, hdev->id);
 
 	hid_debug_register(hdev, dev_name(&hdev->dev));
 	ret = device_add(&hdev->dev);

include/linux/hid.h

@@ -658,6 +658,8 @@ struct hid_device {							/* device report descriptor */
 	struct list_head debug_list;
 	spinlock_t  debug_list_lock;
 	wait_queue_head_t debug_wait;
+
+	unsigned int id;						/* system unique id */
 };
 
 #define to_hid_device(pdev) \