[PATCH v4] usb: gadget: uvc: fix multiple opens

[Thomas Haemmerle <thomas.haemmerle@xxxxxxxxxxxxxx>]

Currently, the UVC function is activated when open on the corresponding
v4l2 device is called.
On another open the activation of the function fails since the
deactivation counter in `usb_function_activate` equals 0. However the
error is not returned to userspace since the open of the v4l2 device is
successful.

On a close the function is deactivated (since deactivation counter still
equals 0) and the video is disabled in `uvc_v4l2_release`, although the
UVC application potentially is streaming.

Move activation of UVC function to subscription on UVC_EVENT_SETUP
because there we can guarantee for a userspace application utilizing
UVC.
Block subscription on UVC_EVENT_SETUP while another application already
is subscribed to it, indicated by `bool func_connected` in
`struct uvc_device`.
Extend the `struct uvc_file_handle` with member `bool is_uvc_app_handle`
to tag it as the handle used by the userspace UVC application.

With this a process is able to check capabilities of the v4l2 device
without deactivating the function for the actual UVC application.

Signed-off-by: Thomas Haemmerle <thomas.haemmerle@xxxxxxxxxxxxxx>
---
v4:
  - remove unnecessary inner parentheses
  - keep and use the local video variable in `uvc_v4l2_release()`

v3:
  - replace `unsigned int connections` with `bool func_connected`
  - rename `bool connected` to `bool is_uvc_app_handle`

v2:
  - fix deadlock in `uvc_v4l2_unsubscribe_event()` (mutex is already
    locked in v4l2-core) introduced in v1
  - lock mutex in `uvc_v4l2_release()` to suppress ioctls and protect
    connected

 drivers/usb/gadget/function/uvc.h      |  2 ++
 drivers/usb/gadget/function/uvc_v4l2.c | 49 ++++++++++++++++++++++----
 2 files changed, 44 insertions(+), 7 deletions(-)

diff --git a/drivers/usb/gadget/function/uvc.h b/drivers/usb/gadget/function/uvc.h
index 23ee25383c1f..893aaa70f81a 100644
--- a/drivers/usb/gadget/function/uvc.h
+++ b/drivers/usb/gadget/function/uvc.h
@@ -117,6 +117,7 @@ struct uvc_device {
 	enum uvc_state state;
 	struct usb_function func;
 	struct uvc_video video;
+	bool func_connected;
 
 	/* Descriptors */
 	struct {
@@ -147,6 +148,7 @@ static inline struct uvc_device *to_uvc(struct usb_function *f)
 struct uvc_file_handle {
 	struct v4l2_fh vfh;
 	struct uvc_video *device;
+	bool is_uvc_app_handle;
 };
 
 #define to_uvc_file_handle(handle) \
diff --git a/drivers/usb/gadget/function/uvc_v4l2.c b/drivers/usb/gadget/function/uvc_v4l2.c
index 4ca89eab6159..197c26f7aec6 100644
--- a/drivers/usb/gadget/function/uvc_v4l2.c
+++ b/drivers/usb/gadget/function/uvc_v4l2.c
@@ -227,17 +227,55 @@ static int
 uvc_v4l2_subscribe_event(struct v4l2_fh *fh,
 			 const struct v4l2_event_subscription *sub)
 {
+	struct uvc_device *uvc = video_get_drvdata(fh->vdev);
+	struct uvc_file_handle *handle = to_uvc_file_handle(fh);
+	int ret;
+
 	if (sub->type < UVC_EVENT_FIRST || sub->type > UVC_EVENT_LAST)
 		return -EINVAL;
 
-	return v4l2_event_subscribe(fh, sub, 2, NULL);
+	if (sub->type == UVC_EVENT_SETUP && uvc->func_connected)
+		return -EBUSY;
+
+	ret = v4l2_event_subscribe(fh, sub, 2, NULL);
+	if (ret < 0)
+		return ret;
+
+	if (sub->type == UVC_EVENT_SETUP) {
+		uvc->func_connected = true;
+		handle->is_uvc_app_handle = true;
+		uvc_function_connect(uvc);
+	}
+
+	return 0;
+}
+
+static void uvc_v4l2_disable(struct uvc_device *uvc)
+{
+	uvc->func_connected = false;
+	uvc_function_disconnect(uvc);
+	uvcg_video_enable(&uvc->video, 0);
+	uvcg_free_buffers(&uvc->video.queue);
 }
 
 static int
 uvc_v4l2_unsubscribe_event(struct v4l2_fh *fh,
 			   const struct v4l2_event_subscription *sub)
 {
-	return v4l2_event_unsubscribe(fh, sub);
+	struct uvc_device *uvc = video_get_drvdata(fh->vdev);
+	struct uvc_file_handle *handle = to_uvc_file_handle(fh);
+	int ret;
+
+	ret = v4l2_event_unsubscribe(fh, sub);
+	if (ret < 0)
+		return ret;
+
+	if (sub->type == UVC_EVENT_SETUP && handle->is_uvc_app_handle) {
+		uvc_v4l2_disable(uvc);
+		handle->is_uvc_app_handle = false;
+	}
+
+	return 0;
 }
 
 static long
@@ -292,7 +330,6 @@ uvc_v4l2_open(struct file *file)
 	handle->device = &uvc->video;
 	file->private_data = &handle->vfh;
 
-	uvc_function_connect(uvc);
 	return 0;
 }
 
@@ -304,11 +341,9 @@ uvc_v4l2_release(struct file *file)
 	struct uvc_file_handle *handle = to_uvc_file_handle(file->private_data);
 	struct uvc_video *video = handle->device;
 
-	uvc_function_disconnect(uvc);
-
 	mutex_lock(&video->mutex);
-	uvcg_video_enable(video, 0);
-	uvcg_free_buffers(&video->queue);
+	if (handle->is_uvc_app_handle)
+		uvc_v4l2_disable(uvc);
 	mutex_unlock(&video->mutex);
 
 	file->private_data = NULL;
-- 
2.25.1