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

[Michael Tretter <m.tretter@pengutronix.de>]

On Tue, 01 Dec 2020 20:27:30 +0100, Thomas Haemmerle wrote:
> 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@wolfvision.net>

While I agree that this driver overall could really use some love, this patch
at least fixes the driver for user space applications that scan for v4l2
devices.

Reviewed-By: Michael Tretter <m.tretter@pengutronix.de>

> ---
> 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
> 
>