linux-kernel - Re: [RFT PATCH v3 4/6] uvcvideo: queue: Simplify spin-lock usage

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Message-ID: <1566744.FFqUaHsn7F@avalon>
Date:   Tue, 16 Jan 2018 19:23:27 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Kieran Bingham <kieran.bingham@...asonboard.com>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Guennadi Liakhovetski <g.liakhovetski@....de>,
        Olivier BRAUN <olivier.braun@...reolabs.com>,
        Troy Kisky <troy.kisky@...ndarydevices.com>
Subject: Re: [RFT PATCH v3 4/6] uvcvideo: queue: Simplify spin-lock usage

Hi Kieran,

Thank you for the patch.

On Friday, 12 January 2018 11:19:26 EET Kieran Bingham wrote:
> Both uvc_start_streaming(), and uvc_stop_streaming() are called from
> userspace context. As such, they do not need to save the IRQ state, and
> can use spin_lock_irq() and spin_unlock_irq() respectively.

Note that userspace context isn't enough, the key part is that they're called 
with interrupts enabled. If they were called from userspace context but under 
a spin_lock_irq() you couldn't use spin_unlock_irq() in those functions.

> Signed-off-by: Kieran Bingham <kieran.bingham@...asonboard.com>
> ---
>  drivers/media/usb/uvc/uvc_queue.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_queue.c
> b/drivers/media/usb/uvc/uvc_queue.c index 4a581d631525..ddac4d89a291 100644
> --- a/drivers/media/usb/uvc/uvc_queue.c
> +++ b/drivers/media/usb/uvc/uvc_queue.c
> @@ -158,7 +158,6 @@ static int uvc_start_streaming(struct vb2_queue *vq,
> unsigned int count) {
>  	struct uvc_video_queue *queue = vb2_get_drv_priv(vq);
>  	struct uvc_streaming *stream = uvc_queue_to_stream(queue);
> -	unsigned long flags;
>  	int ret;
> 
>  	queue->buf_used = 0;
> @@ -167,9 +166,9 @@ static int uvc_start_streaming(struct vb2_queue *vq,
> unsigned int count) if (ret == 0)
>  		return 0;
> 
> -	spin_lock_irqsave(&queue->irqlock, flags);
> +	spin_lock_irq(&queue->irqlock);
>  	uvc_queue_return_buffers(queue, UVC_BUF_STATE_QUEUED);
> -	spin_unlock_irqrestore(&queue->irqlock, flags);
> +	spin_unlock_irq(&queue->irqlock);
> 
>  	return ret;
>  }
> @@ -178,13 +177,12 @@ static void uvc_stop_streaming(struct vb2_queue *vq)
>  {
>  	struct uvc_video_queue *queue = vb2_get_drv_priv(vq);
>  	struct uvc_streaming *stream = uvc_queue_to_stream(queue);
> -	unsigned long flags;
> 
>  	uvc_video_enable(stream, 0);
> 
> -	spin_lock_irqsave(&queue->irqlock, flags);
> +	spin_lock_irq(&queue->irqlock);
>  	uvc_queue_return_buffers(queue, UVC_BUF_STATE_ERROR);
> -	spin_unlock_irqrestore(&queue->irqlock, flags);
> +	spin_unlock_irq(&queue->irqlock);
>  }

Please add a one-line comment above both functions to state

/* Must be called with interrupts enabled. */

With this and the commit message updated,

Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>

>  static const struct vb2_ops uvc_queue_qops = {

-- 
Regards,

Laurent Pinchart