linux-kernel - Re: [PATCH] media: uvcvideo: Read support

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Date:   Mon, 4 Mar 2019 15:36:32 +0000
From:   Kieran Bingham <kieran.bingham@...asonboard.com>
To:     Hugues Fruchet <hugues.fruchet@...com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: uvcvideo: Read support

Hi Hughes,

On 04/03/2019 12:35, Hugues Fruchet wrote:
> Add support of read() call from userspace by implementing
> uvc_v4l2_read() with vb2_read() helper.

Just thinking out loud,

This opens up UVC devices to read raw full frame images through this
interface as well.

Due to the UVC protocol, there is /already/ a full memcpy to get these
images out of the URB packets, so using a read() interface would be
another full frame copy.

I can perhaps see the usecase for reading compressed data through this
interface - but full frames don't seem appropriate. (not impossible of
course, just is it reasonable?)

If this is to be enabled, should it be enabled for compressed formats
only? or would that complicate matters?

--
Regards

Kieran


> Signed-off-by: Hugues Fruchet <hugues.fruchet@...com>
> ---
>  drivers/media/usb/uvc/uvc_queue.c | 15 ++++++++++++++-
>  drivers/media/usb/uvc/uvc_v4l2.c  | 11 ++++++++---
>  drivers/media/usb/uvc/uvcvideo.h  |  2 ++
>  3 files changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_queue.c b/drivers/media/usb/uvc/uvc_queue.c
> index 682698e..0c8a0a8 100644
> --- a/drivers/media/usb/uvc/uvc_queue.c
> +++ b/drivers/media/usb/uvc/uvc_queue.c
> @@ -227,7 +227,7 @@ int uvc_queue_init(struct uvc_video_queue *queue, enum v4l2_buf_type type,
>  	int ret;
>  
>  	queue->queue.type = type;
> -	queue->queue.io_modes = VB2_MMAP | VB2_USERPTR;
> +	queue->queue.io_modes = VB2_MMAP | VB2_USERPTR | VB2_READ;
>  	queue->queue.drv_priv = queue;
>  	queue->queue.buf_struct_size = sizeof(struct uvc_buffer);
>  	queue->queue.mem_ops = &vb2_vmalloc_memops;
> @@ -361,6 +361,19 @@ int uvc_queue_streamoff(struct uvc_video_queue *queue, enum v4l2_buf_type type)
>  	return ret;
>  }
>  
> +ssize_t uvc_queue_read(struct uvc_video_queue *queue, struct file *file,
> +		       char __user *buf, size_t count, loff_t *ppos)
> +{
> +	ssize_t ret;
> +
> +	mutex_lock(&queue->mutex);
> +	ret = vb2_read(&queue->queue, buf, count, ppos,
> +		       file->f_flags & O_NONBLOCK);
> +	mutex_unlock(&queue->mutex);
> +
> +	return ret;
> +}
> +
>  int uvc_queue_mmap(struct uvc_video_queue *queue, struct vm_area_struct *vma)
>  {
>  	return vb2_mmap(&queue->queue, vma);
> diff --git a/drivers/media/usb/uvc/uvc_v4l2.c b/drivers/media/usb/uvc/uvc_v4l2.c
> index 84be596..3866832 100644
> --- a/drivers/media/usb/uvc/uvc_v4l2.c
> +++ b/drivers/media/usb/uvc/uvc_v4l2.c
> @@ -594,7 +594,8 @@ static int uvc_ioctl_querycap(struct file *file, void *fh,
>  	strscpy(cap->driver, "uvcvideo", sizeof(cap->driver));
>  	strscpy(cap->card, vdev->name, sizeof(cap->card));
>  	usb_make_path(stream->dev->udev, cap->bus_info, sizeof(cap->bus_info));
> -	cap->capabilities = V4L2_CAP_DEVICE_CAPS | V4L2_CAP_STREAMING
> +	cap->capabilities = V4L2_CAP_DEVICE_CAPS | V4L2_CAP_STREAMING |
> +			    V4L2_CAP_READWRITE
>  			  | chain->caps;
>  
>  	return 0;
> @@ -1434,8 +1435,12 @@ static long uvc_v4l2_compat_ioctl32(struct file *file,
>  static ssize_t uvc_v4l2_read(struct file *file, char __user *data,
>  		    size_t count, loff_t *ppos)
>  {
> -	uvc_trace(UVC_TRACE_CALLS, "uvc_v4l2_read: not implemented.\n");
> -	return -EINVAL;
> +	struct uvc_fh *handle = file->private_data;
> +	struct uvc_streaming *stream = handle->stream;
> +
> +	uvc_trace(UVC_TRACE_CALLS, "uvc_v4l2_read\n");
> +
> +	return uvc_queue_read(&stream->queue, file, data, count, ppos);
>  }
>  
>  static int uvc_v4l2_mmap(struct file *file, struct vm_area_struct *vma)
> diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
> index c7c1baa..5d0515c 100644
> --- a/drivers/media/usb/uvc/uvcvideo.h
> +++ b/drivers/media/usb/uvc/uvcvideo.h
> @@ -766,6 +766,8 @@ struct uvc_buffer *uvc_queue_next_buffer(struct uvc_video_queue *queue,
>  					 struct uvc_buffer *buf);
>  struct uvc_buffer *uvc_queue_get_current_buffer(struct uvc_video_queue *queue);
>  void uvc_queue_buffer_release(struct uvc_buffer *buf);
> +ssize_t uvc_queue_read(struct uvc_video_queue *queue, struct file *file,
> +		       char __user *buf, size_t count, loff_t *ppos);
>  int uvc_queue_mmap(struct uvc_video_queue *queue,
>  		   struct vm_area_struct *vma);
>  __poll_t uvc_queue_poll(struct uvc_video_queue *queue, struct file *file,
> 


-- 
Regards
--
Kieran