Date: Thu, 4 Jan 2018 19:25:02 +0100 (CET)
From: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
To: Kieran Bingham <kbingham@kernel.org>
cc: linux-media@vger.kernel.org, linux-kernel@vger.kernel.org,
        laurent.pinchart@ideasonboard.com,
        Olivier BRAUN <olivier.braun@stereolabs.com>,
        kieran.bingham@ideasonboard.com,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Hans Verkuil <hans.verkuil@cisco.com>,
        Jaejoong Kim <climbbb.kim@gmail.com>,
        Baoyou Xie <baoyou.xie@linaro.org>,
        Nicolas Dufresne <nicolas.dufresne@collabora.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jim Lin <jilin@nvidia.com>,
        Daniel Patrick Johnson <teknotus@teknot.us>
Subject: Re: [RFC/RFT PATCH 3/6] uvcvideo: Protect queue internals with
 helper
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Sender: linux-kernel-owner@vger.kernel.org

Hi Kieran,

On Wed, 3 Jan 2018, Kieran Bingham wrote:

> From: Kieran Bingham <kieran.bingham@ideasonboard.com>
> 
> The URB completion operation obtains the current buffer by reading
> directly into the queue internal interface.
> 
> Protect this queue abstraction by providing a helper
> uvc_queue_get_current_buffer() which can be used by both the decode
> task, and the uvc_queue_next_buffer() functions.
> 
> Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> ---
>  drivers/media/usb/uvc/uvc_queue.c | 34 +++++++++++++++++++++++++++-----
>  drivers/media/usb/uvc/uvc_video.c |  7 +------
>  drivers/media/usb/uvc/uvcvideo.h  |  2 ++-
>  3 files changed, 32 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_queue.c b/drivers/media/usb/uvc/uvc_queue.c
> index c8d78b2f3de4..0711e3d9ff76 100644
> --- a/drivers/media/usb/uvc/uvc_queue.c
> +++ b/drivers/media/usb/uvc/uvc_queue.c
> @@ -399,6 +399,34 @@ void uvc_queue_cancel(struct uvc_video_queue *queue, int disconnect)
>  	spin_unlock_irqrestore(&queue->irqlock, flags);
>  }
>  
> +/*
> + * uvc_queue_get_current_buffer: Obtain the current working output buffer
> + *
> + * Buffers may span multiple packets, and even URBs, therefore the active buffer
> + * remains on the queue until the EOF marker.
> + */
> +static struct uvc_buffer *
> +__uvc_queue_get_current_buffer(struct uvc_video_queue *queue)
> +{
> +	if (!list_empty(&queue->irqqueue))
> +		return list_first_entry(&queue->irqqueue, struct uvc_buffer,
> +					queue);
> +	else
> +		return NULL;

I think the preferred style is not to use "else" in such cases. It might 
even be prettier to write

	if (list_empty(...))
		return NULL;

	return list_first_entry(...);

Thanks
Guennadi