Date:   Mon, 05 Dec 2016 17:22:20 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Todor Tomov <todor.tomov@...aro.org>
Cc:     mchehab@...nel.org, laurent.pinchart+renesas@...asonboard.com,
        hans.verkuil@...co.com, javier@....samsung.com,
        s.nawrocki@...sung.com, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org, bjorn.andersson@...aro.org,
        srinivas.kandagatla@...aro.org
Subject: Re: [PATCH 08/10] media: camss: Add files which handle the video device nodes

Hi Todor,

Thank you for the patch.

On Friday 25 Nov 2016 16:57:20 Todor Tomov wrote:
> These files handle the video device nodes of the camss driver.

camss is a quite generic, I'm a bit concerned about claiming that acronym in 
the global kernel namespace. Would it be too long if we prefixed symbols with 
msm_camss instead ?

> Signed-off-by: Todor Tomov <todor.tomov@...aro.org>
> ---
>  drivers/media/platform/qcom/camss-8x16/video.c | 597 ++++++++++++++++++++++
>  drivers/media/platform/qcom/camss-8x16/video.h |  67 +++
>  2 files changed, 664 insertions(+)
>  create mode 100644 drivers/media/platform/qcom/camss-8x16/video.c
>  create mode 100644 drivers/media/platform/qcom/camss-8x16/video.h
> 
> diff --git a/drivers/media/platform/qcom/camss-8x16/video.c
> b/drivers/media/platform/qcom/camss-8x16/video.c new file mode 100644
> index 0000000..0bf8ea9
> --- /dev/null
> +++ b/drivers/media/platform/qcom/camss-8x16/video.c
> @@ -0,0 +1,597 @@
> +/*
> + * video.c
> + *
> + * Qualcomm MSM Camera Subsystem - V4L2 device node
> + *
> + * Copyright (c) 2013-2015, The Linux Foundation. All rights reserved.
> + * Copyright (C) 2015-2016 Linaro Ltd.
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 and
> + * only version 2 as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + */
> +#include <media/media-entity.h>
> +#include <media/v4l2-dev.h>
> +#include <media/v4l2-device.h>
> +#include <media/v4l2-ioctl.h>
> +#include <media/v4l2-mc.h>
> +#include <media/videobuf-core.h>
> +#include <media/videobuf2-dma-contig.h>
> +
> +#include "video.h"
> +#include "camss.h"
> +
> +/*
> + * struct format_info - ISP media bus format information
> + * @code: V4L2 media bus format code
> + * @pixelformat: V4L2 pixel format FCC identifier
> + * @bpp: Bits per pixel when stored in memory
> + */
> +static const struct format_info {
> +	u32 code;
> +	u32 pixelformat;
> +	unsigned int bpp;
> +} formats[] = {
> +	{ MEDIA_BUS_FMT_UYVY8_2X8, V4L2_PIX_FMT_UYVY, 16 },
> +	{ MEDIA_BUS_FMT_VYUY8_2X8, V4L2_PIX_FMT_VYUY, 16 },
> +	{ MEDIA_BUS_FMT_YUYV8_2X8, V4L2_PIX_FMT_YUYV, 16 },
> +	{ MEDIA_BUS_FMT_YVYU8_2X8, V4L2_PIX_FMT_YVYU, 16 },
> +	{ MEDIA_BUS_FMT_SBGGR8_1X8, V4L2_PIX_FMT_SBGGR8, 8 },
> +	{ MEDIA_BUS_FMT_SGBRG8_1X8, V4L2_PIX_FMT_SGBRG8, 8 },
> +	{ MEDIA_BUS_FMT_SGRBG8_1X8, V4L2_PIX_FMT_SGRBG8, 8 },
> +	{ MEDIA_BUS_FMT_SRGGB8_1X8, V4L2_PIX_FMT_SRGGB8, 8 },
> +	{ MEDIA_BUS_FMT_SBGGR10_1X10, V4L2_PIX_FMT_SBGGR10P, 10 },
> +	{ MEDIA_BUS_FMT_SGBRG10_1X10, V4L2_PIX_FMT_SGBRG10P, 10 },
> +	{ MEDIA_BUS_FMT_SGRBG10_1X10, V4L2_PIX_FMT_SGRBG10P, 10 },
> +	{ MEDIA_BUS_FMT_SRGGB10_1X10, V4L2_PIX_FMT_SRGGB10P, 10 },
> +	{ MEDIA_BUS_FMT_SBGGR12_1X12, V4L2_PIX_FMT_SRGGB12P, 12 },
> +	{ MEDIA_BUS_FMT_SGBRG12_1X12, V4L2_PIX_FMT_SGBRG12P, 12 },
> +	{ MEDIA_BUS_FMT_SGRBG12_1X12, V4L2_PIX_FMT_SGRBG12P, 12 },
> +	{ MEDIA_BUS_FMT_SRGGB12_1X12, V4L2_PIX_FMT_SRGGB12P, 12 }
> +};
> +
> +/*
> ---------------------------------------------------------------------------
> -- + * Helper functions
> + */
> +
> +/*
> + * video_mbus_to_pix - Convert v4l2_mbus_framefmt to v4l2_pix_format
> + * @mbus: v4l2_mbus_framefmt format (input)
> + * @pix: v4l2_pix_format format (output)
> + *
> + * Fill the output pix structure with information from the input mbus
> format.
> + *
> + * Return 0 on success or a negative error code otherwise
> + */
> +static unsigned int video_mbus_to_pix(const struct v4l2_mbus_framefmt
> *mbus,
> +				      struct v4l2_pix_format *pix)
> +{
> +	unsigned int i;
> +
> +	memset(pix, 0, sizeof(*pix));
> +	pix->width = mbus->width;
> +	pix->height = mbus->height;
> +
> +	for (i = 0; i < ARRAY_SIZE(formats); ++i) {
> +		if (formats[i].code == mbus->code)
> +			break;
> +	}
> +
> +	if (WARN_ON(i == ARRAY_SIZE(formats)))
> +		return -EINVAL;
> +
> +	pix->pixelformat = formats[i].pixelformat;
> +	pix->bytesperline = pix->width * formats[i].bpp / 8;
> +	pix->bytesperline = ALIGN(pix->bytesperline, 8);

Does the hardware support padding at the end of lines ? If so it would be 
useful to use the maximum of the computed and requested by userspace values 
(up to the maximum size of the padding supported by the hardware).

> +	pix->sizeimage = pix->bytesperline * pix->height;

Similarly, should we support padding at the end of the image ?

> +	pix->colorspace = mbus->colorspace;
> +	pix->field = mbus->field;
> +
> +	return 0;
> +}
> +
> +static struct v4l2_subdev *video_remote_subdev(struct camss_video *video,
> +					       u32 *pad)
> +{
> +	struct media_pad *remote;
> +
> +	remote = media_entity_remote_pad(&video->pad);
> +
> +	if (!remote || !is_media_entity_v4l2_subdev(remote->entity))
> +		return NULL;
> +
> +	if (pad)
> +		*pad = remote->index;
> +
> +	return media_entity_to_v4l2_subdev(remote->entity);
> +}
> +
> +static int video_get_subdev_format(struct camss_video *video,
> +				   struct v4l2_format *format)
> +{
> +	struct v4l2_subdev_format fmt;
> +	struct v4l2_subdev *subdev;
> +	u32 pad;
> +	int ret;
> +
> +	subdev = video_remote_subdev(video, &pad);
> +	if (subdev == NULL)
> +		return -EINVAL;
> +
> +	fmt.pad = pad;
> +	fmt.which = V4L2_SUBDEV_FORMAT_ACTIVE;
> +
> +	ret = v4l2_subdev_call(subdev, pad, get_fmt, NULL, &fmt);
> +	if (ret)
> +		return ret;
> +
> +	format->type = video->type;
> +	return video_mbus_to_pix(&fmt.format, &format->fmt.pix);
> +}
> +
> +/* ------------------------------------------------------------------------
> + * Video queue operations
> + */
> +
> +static int video_queue_setup(struct vb2_queue *q,
> +	unsigned int *num_buffers, unsigned int *num_planes,
> +	unsigned int sizes[], struct device *alloc_devs[])
> +{
> +	struct camss_video *video = vb2_get_drv_priv(q);
> +
> +	if (*num_planes) {
> +		if (*num_planes != 1)
> +			return -EINVAL;
> +
> +		if (sizes[0] < video->active_fmt.fmt.pix.sizeimage)
> +			return -EINVAL;
> +
> +		return 0;
> +	}
> +
> +	*num_planes = 1;
> +
> +	sizes[0] = video->active_fmt.fmt.pix.sizeimage;
> +
> +	return 0;
> +}
> +
> +static int video_buf_prepare(struct vb2_buffer *vb)
> +{
> +	struct vb2_v4l2_buffer *vbuf = to_vb2_v4l2_buffer(vb);
> +	struct camss_video *video = vb2_get_drv_priv(vb->vb2_queue);
> +	struct camss_buffer *buffer = container_of(vbuf, struct camss_buffer,
> +						   vb);

You can define a static inline function to wrap this container_of() as it's 
used in multiple places in this file.

> +
> +	vb2_set_plane_payload(vb, 0, video->active_fmt.fmt.pix.sizeimage);
> +	if (vb2_get_plane_payload(vb, 0) > vb2_plane_size(vb, 0))

Wouldn't it be more efficient to compare video->active_fmt.fmt.pix.sizeimage 
instead of vb2_get_plane_payload(vb, 0) ? The two should be identical as 
you've just called vb2_set_plane_payload(). I would also move the 
vb2_set_plane_payload() call after the error check.

> +		return -EINVAL;
> +
> +	vbuf->field = V4L2_FIELD_NONE;
> +
> +	buffer->addr = vb2_dma_contig_plane_dma_addr(vb, 0);
> +
> +	return 0;
> +}
> +
> +static void video_buf_queue(struct vb2_buffer *vb)
> +{
> +	struct vb2_v4l2_buffer *vbuf = to_vb2_v4l2_buffer(vb);
> +	struct camss_video *video = vb2_get_drv_priv(vb->vb2_queue);
> +	struct camss_buffer *buffer = container_of(vbuf, struct camss_buffer,
> +						   vb);
> +
> +	camss_video_call(video, queue_buffer, buffer);

Should I assume that this abstraction means you'll add support for more than 
the RDI in the future ? ;-)

I'd remove the camss_video_call() macro and use

	video->ops->queue_buffer(video, buffer);

directly as it's easier to follow, and we don't need to test video->ops && 
video->ops->op as all operations should always be provided.

> +}
> +
> +

A single blank line is enough.

> +static int video_check_format(struct camss_video *video)
> +{
> +	struct v4l2_pix_format *pix = &video->active_fmt.fmt.pix;
> +	struct v4l2_format format;
> +	int ret;
> +
> +	ret = video_get_subdev_format(video, &format);
> +	if (ret < 0)
> +		return ret;
> +
> +	if (pix->pixelformat != format.fmt.pix.pixelformat ||
> +	    pix->height != format.fmt.pix.height ||
> +	    pix->width != format.fmt.pix.width ||
> +	    pix->bytesperline != format.fmt.pix.bytesperline ||
> +	    pix->sizeimage != format.fmt.pix.sizeimage ||
> +	    pix->field != format.fmt.pix.field)
> +		return -EINVAL;
> +
> +	return 0;
> +}
> +
> +static int video_start_streaming(struct vb2_queue *q, unsigned int count)
> +{
> +	struct camss_video *video = vb2_get_drv_priv(q);
> +	struct video_device *vdev = video->vdev;
> +	struct media_entity *entity;
> +	struct media_pad *pad;
> +	struct v4l2_subdev *subdev;
> +	int ret;
> +
> +	ret = media_entity_pipeline_start(&vdev->entity, &video->pipe);
> +	if (ret < 0)
> +		return ret;
> +
> +	ret = video_check_format(video);
> +	if (ret < 0)
> +		goto error;
> +
> +	entity = &vdev->entity;
> +	while (1) {
> +		pad = &entity->pads[0];
> +		if (!(pad->flags & MEDIA_PAD_FL_SINK))
> +			break;
> +
> +		pad = media_entity_remote_pad(pad);
> +		if (!pad || !is_media_entity_v4l2_subdev(pad->entity))
> +			break;
> +
> +		entity = pad->entity;
> +		subdev = media_entity_to_v4l2_subdev(entity);
> +
> +		ret = v4l2_subdev_call(subdev, video, s_stream, 1);
> +		if (ret < 0 && ret != -ENOIOCTLCMD)
> +			goto error;
> +	}
> +
> +	return 0;
> +
> +error:
> +	media_entity_pipeline_stop(&vdev->entity);
> +
> +	return ret;
> +}
> +
> +static void video_stop_streaming(struct vb2_queue *q)
> +{
> +	struct camss_video *video = vb2_get_drv_priv(q);
> +	struct video_device *vdev = video->vdev;
> +	struct media_entity *entity;
> +	struct media_pad *pad;
> +	struct v4l2_subdev *subdev;
> +	struct v4l2_subdev *subdev_vfe;
> +
> +	entity = &vdev->entity;
> +	while (1) {
> +		pad = &entity->pads[0];
> +		if (!(pad->flags & MEDIA_PAD_FL_SINK))
> +			break;
> +
> +		pad = media_entity_remote_pad(pad);
> +		if (!pad || !is_media_entity_v4l2_subdev(pad->entity))
> +			break;
> +
> +		entity = pad->entity;
> +		subdev = media_entity_to_v4l2_subdev(entity);
> +
> +		if (strstr(subdev->name, "vfe")) {
> +			subdev_vfe = subdev;
> +		} else if (strstr(subdev->name, "ispif")) {
> +			v4l2_subdev_call(subdev, video, s_stream, 0);
> +			v4l2_subdev_call(subdev_vfe, video, s_stream, 0);
> +		} else {
> +			v4l2_subdev_call(subdev, video, s_stream, 0);
> +		}
> +	}
> +
> +	media_entity_pipeline_stop(&vdev->entity);
> +
> +	camss_video_call(video, flush_buffers);
> +}
> +
> +static struct vb2_ops msm_video_vb2_q_ops = {

You can make this static const.

> +	.queue_setup     = video_queue_setup,
> +	.buf_prepare     = video_buf_prepare,
> +	.buf_queue       = video_buf_queue,
> +	.start_streaming = video_start_streaming,
> +	.stop_streaming  = video_stop_streaming,
> +};
> +
> +/* ------------------------------------------------------------------------
> + * V4L2 ioctls
> + */
> +
> +static int video_querycap(struct file *file, void *fh,
> +			  struct v4l2_capability *cap)
> +{
> +	strlcpy(cap->driver, "qcom-camss", sizeof(cap->driver));
> +	strlcpy(cap->card, "Qualcomm Camera Subsystem", sizeof(cap->card));
> +	strlcpy(cap->bus_info, "platform:qcom-camss", sizeof(cap->bus_info));

	snprintf(cap->bus_info, sizeof(cap->bus_info), "platform:%s",
		 dev_name(...));

would be better.

> +	cap->capabilities = V4L2_CAP_VIDEO_CAPTURE | V4L2_CAP_STREAMING |
> +							V4L2_CAP_DEVICE_CAPS;
> +	cap->device_caps = V4L2_CAP_VIDEO_CAPTURE | V4L2_CAP_STREAMING;
> +
> +	return 0;
> +}
> +
> +static int video_enum_fmt(struct file *file, void *fh, struct v4l2_fmtdesc
> *f)
> +{
> +	struct camss_video *video = video_drvdata(file);
> +	struct v4l2_format format;
> +	int ret;
> +
> +	if (f->type != video->type)
> +		return -EINVAL;
> +
> +	if (f->index)
> +		return -EINVAL;
> +
> +	ret = video_get_subdev_format(video, &format);
> +	if (ret < 0)
> +		return ret;
> +
> +	f->pixelformat = format.fmt.pix.pixelformat;
> +
> +	return 0;
> +}
> +
> +static int video_g_fmt(struct file *file, void *fh, struct v4l2_format *f)
> +{
> +	struct camss_video *video = video_drvdata(file);
> +
> +	if (f->type != video->type)
> +		return -EINVAL;
> +
> +	*f = video->active_fmt;
> +
> +	return 0;
> +}
> +
> +static int video_s_fmt(struct file *file, void *fh, struct v4l2_format *f)
> +{
> +	struct camss_video *video = video_drvdata(file);
> +	int ret;
> +
> +	if (f->type != video->type)
> +		return -EINVAL;
> +
> +	ret = video_get_subdev_format(video, f);
> +	if (ret < 0)
> +		return ret;
> +
> +	video->active_fmt = *f;
> +
> +	return 0;
> +}
> +
> +static int video_try_fmt(struct file *file, void *fh, struct v4l2_format
> *f) +{
> +	struct camss_video *video = video_drvdata(file);
> +
> +	if (f->type != video->type)
> +		return -EINVAL;
> +
> +	return video_get_subdev_format(video, f);
> +}
> +
> +static int video_enum_input(struct file *file, void *fh,
> +			    struct v4l2_input *input)
> +{
> +	if (input->index > 0)
> +		return -EINVAL;
> +
> +	strlcpy(input->name, "camera", sizeof(input->name));
> +	input->type = V4L2_INPUT_TYPE_CAMERA;
> +
> +	return 0;
> +}
> +
> +static int video_g_input(struct file *file, void *fh, unsigned int *input)
> +{
> +	*input = 0;
> +
> +	return 0;
> +}
> +
> +static int video_s_input(struct file *file, void *fh, unsigned int input)
> +{
> +	return input == 0 ? 0 : -EINVAL;
> +}
> +
> +static const struct v4l2_ioctl_ops msm_vid_ioctl_ops = {
> +	.vidioc_querycap          = video_querycap,
> +	.vidioc_enum_fmt_vid_cap  = video_enum_fmt,
> +	.vidioc_g_fmt_vid_cap     = video_g_fmt,
> +	.vidioc_s_fmt_vid_cap     = video_s_fmt,
> +	.vidioc_try_fmt_vid_cap   = video_try_fmt,
> +	.vidioc_reqbufs           = vb2_ioctl_reqbufs,
> +	.vidioc_querybuf          = vb2_ioctl_querybuf,
> +	.vidioc_qbuf              = vb2_ioctl_qbuf,
> +	.vidioc_dqbuf             = vb2_ioctl_dqbuf,
> +	.vidioc_create_bufs       = vb2_ioctl_create_bufs,
> +	.vidioc_streamon          = vb2_ioctl_streamon,
> +	.vidioc_streamoff         = vb2_ioctl_streamoff,
> +	.vidioc_enum_input        = video_enum_input,
> +	.vidioc_g_input           = video_g_input,
> +	.vidioc_s_input           = video_s_input,
> +};
> +
> +/* ------------------------------------------------------------------------
> + * V4L2 file operations
> + */
> +
> +/*
> + * video_init_format - Helper function to initialize format
> + *
> + * Initialize all pad formats with default values.
> + */
> +static int video_init_format(struct file *file, void *fh)
> +{
> +	struct v4l2_format format;
> +
> +	memset(&format, 0, sizeof(format));
> +	format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
> +
> +	return video_s_fmt(file, fh, &format);

This will set the active format every time you open the device node, I don't 
think that's what you want.

> +}
> +
> +static int video_open(struct file *file)
> +{
> +	struct video_device *vdev = video_devdata(file);
> +	struct camss_video *video = video_drvdata(file);
> +	struct camss_video_fh *handle;
> +	int ret;
> +
> +	handle = kzalloc(sizeof(*handle), GFP_KERNEL);
> +	if (handle == NULL)
> +		return -ENOMEM;
> +
> +	v4l2_fh_init(&handle->vfh, video->vdev);
> +	v4l2_fh_add(&handle->vfh);
> +
> +	handle->video = video;
> +	file->private_data = &handle->vfh;
> +
> +	ret = v4l2_pipeline_pm_use(&vdev->entity, 1);
> +	if (ret < 0) {
> +		dev_err(video->camss->dev, "Failed to power up pipeline\n");
> +		goto error_pm_use;
> +	}
> +
> +	ret = video_init_format(file, &handle->vfh);
> +	if (ret < 0) {
> +		dev_err(video->camss->dev, "Failed to init format\n");
> +		goto error_init_format;
> +	}
> +
> +	return 0;
> +
> +error_init_format:
> +	v4l2_pipeline_pm_use(&vdev->entity, 0);
> +
> +error_pm_use:
> +	v4l2_fh_del(&handle->vfh);
> +	kfree(handle);
> +
> +	return ret;
> +}
> +
> +static int video_release(struct file *file)
> +{
> +	struct video_device *vdev = video_devdata(file);
> +	struct camss_video *video = video_drvdata(file);
> +	struct v4l2_fh *vfh = file->private_data;
> +	struct camss_video_fh *handle = container_of(vfh, struct 
camss_video_fh,
> +						     vfh);
> +
> +	vb2_ioctl_streamoff(file, vfh, video->type);
> +
> +	v4l2_pipeline_pm_use(&vdev->entity, 0);
> +
> +	v4l2_fh_del(vfh);
> +	kfree(handle);
> +	file->private_data = NULL;
> +
> +	return 0;
> +}
> +
> +static unsigned int video_poll(struct file *file,
> +				   struct poll_table_struct *wait)
> +{
> +	struct camss_video *video = video_drvdata(file);
> +
> +	return vb2_poll(&video->vb2_q, file, wait);
> +}
> +
> +static int video_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +	struct camss_video *video = video_drvdata(file);
> +
> +	return vb2_mmap(&video->vb2_q, vma);
> +}
> +
> +static const struct v4l2_file_operations msm_vid_fops = {
> +	.owner          = THIS_MODULE,
> +	.unlocked_ioctl = video_ioctl2,
> +	.open           = video_open,
> +	.release        = video_release,
> +	.poll           = video_poll,
> +	.mmap		= video_mmap,
> +};
> +
> +/* ------------------------------------------------------------------------
> + * CAMSS video core
> + */
> +
> +int msm_video_register(struct camss_video *video, struct v4l2_device
> *v4l2_dev,
> +		       const char *name)
> +{
> +	struct media_pad *pad = &video->pad;
> +	struct video_device *vdev;
> +	struct vb2_queue *q;
> +	int ret;
> +
> +	vdev = video_device_alloc();
> +	if (vdev == NULL) {
> +		dev_err(v4l2_dev->dev, "Failed to allocate video device\n");
> +		return -ENOMEM;
> +	}
> +
> +	video->vdev = vdev;
> +
> +	q = &video->vb2_q;
> +	q->drv_priv = video;
> +	q->mem_ops = &vb2_dma_contig_memops;
> +	q->ops = &msm_video_vb2_q_ops;
> +	q->type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
> +	q->io_modes = VB2_MMAP;
> +	q->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC;
> +	q->buf_struct_size = sizeof(struct camss_buffer);
> +	q->dev = video->camss->dev;
> +	ret = vb2_queue_init(q);
> +	if (ret < 0) {
> +		dev_err(v4l2_dev->dev, "Failed to init vb2 queue\n");
> +		return ret;
> +	}
> +
> +	pad->flags = MEDIA_PAD_FL_SINK;
> +	ret = media_entity_pads_init(&vdev->entity, 1, pad);
> +	if (ret < 0) {
> +		dev_err(v4l2_dev->dev, "Failed to init video entity\n");
> +		goto error_media_init;
> +	}
> +
> +	vdev->fops = &msm_vid_fops;
> +	vdev->ioctl_ops = &msm_vid_ioctl_ops;
> +	vdev->release = video_device_release;

This will result in a race condition between device unbind and userspace 
access to the video node. You should instead refcount the top-level camss data 
structure, and provide a custom release function here that decrements the 
refcount. If you do that you can embed your struct video_device in struct 
camss_video instead of allocating it dynamically. The media_entity_cleanup() 
call will have to move from msm_video_unregister() to the release operation 
for your top-level structure.

To test this race condition, perform the following steps:

- Configure the pipeline
- Open the video device node, configure it and start streaming
- Unbind the device through the sysfs unbind attribute of the driver
- Stop streaming

You'll likely get an oops with you current version.

There are unfortunately very few drivers implementing this correctly 
(including drivers I wrote :-/). A good (even if slightly complex) example is 
uvcvideo.

> +	vdev->v4l2_dev = v4l2_dev;
> +	vdev->vfl_dir = VFL_DIR_RX;
> +	vdev->queue = &video->vb2_q;
> +	strlcpy(vdev->name, name, sizeof(vdev->name));
> +
> +	ret = video_register_device(vdev, VFL_TYPE_GRABBER, -1);
> +	if (ret < 0) {
> +		dev_err(v4l2_dev->dev, "Failed to register video device\n");
> +		goto error_video_register;
> +	}
> +
> +	video_set_drvdata(vdev, video);
> +
> +	return 0;
> +
> +error_video_register:
> +	media_entity_cleanup(&vdev->entity);
> +error_media_init:
> +	vb2_queue_release(&video->vb2_q);
> +
> +	return ret;
> +}
> +
> +void msm_video_unregister(struct camss_video *video)
> +{
> +	video_unregister_device(video->vdev);
> +	media_entity_cleanup(&video->vdev->entity);
> +	vb2_queue_release(&video->vb2_q);

If you use vb2_fop_release() as Hans proposed you can remove the 
vb2_queue_release() call here.

> +}
> diff --git a/drivers/media/platform/qcom/camss-8x16/video.h
> b/drivers/media/platform/qcom/camss-8x16/video.h new file mode 100644
> index 0000000..5ab5929d
> --- /dev/null
> +++ b/drivers/media/platform/qcom/camss-8x16/video.h
> @@ -0,0 +1,67 @@
> +/*
> + * video.h
> + *
> + * Qualcomm MSM Camera Subsystem - V4L2 device node
> + *
> + * Copyright (c) 2013-2015, The Linux Foundation. All rights reserved.
> + * Copyright (C) 2015-2016 Linaro Ltd.
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 and
> + * only version 2 as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + */
> +#ifndef QC_MSM_CAMSS_VIDEO_H
> +#define QC_MSM_CAMSS_VIDEO_H
> +
> +#include <linux/videodev2.h>
> +#include <media/media-entity.h>
> +#include <media/v4l2-dev.h>
> +#include <media/v4l2-device.h>
> +#include <media/v4l2-fh.h>
> +#include <media/v4l2-mediabus.h>
> +#include <media/videobuf2-v4l2.h>
> +
> +#define camss_video_call(f, op, args...)			\
> +	(!(f) ? -ENODEV : (((f)->ops && (f)->ops->op) ? \
> +			    (f)->ops->op((f), ##args) : -ENOIOCTLCMD))
> +
> +struct camss_buffer {
> +	struct vb2_v4l2_buffer vb;
> +	dma_addr_t addr;
> +	struct list_head queue;
> +};
> +
> +struct camss_video;
> +
> +struct camss_video_ops {
> +	int (*queue_buffer)(struct camss_video *vid, struct camss_buffer 
*buf);
> +	int (*flush_buffers)(struct camss_video *vid);
> +};
> +
> +struct camss_video {
> +	struct camss *camss;
> +	struct vb2_queue vb2_q;
> +	struct video_device *vdev;
> +	struct media_pad pad;
> +	struct v4l2_format active_fmt;
> +	enum v4l2_buf_type type;
> +	struct media_pipeline pipe;
> +	struct camss_video_ops *ops;

You can make this const.

> +};
> +
> +struct camss_video_fh {
> +	struct v4l2_fh vfh;
> +	struct camss_video *video;
> +};

If there's nothing else to be stored here you don't need a custom struct 
camss_video_fh. You can use struct v4l2_fh and retrieve the camss_video 
instance with a container_of on vfh->vdev (provided you embed the video_device 
instance in struct camss_video as I proposed above).

> +
> +int msm_video_register(struct camss_video *video, struct v4l2_device
> *v4l2_dev,
> +		       const char *name);
> +
> +void msm_video_unregister(struct camss_video *video);
> +
> +#endif /* QC_MSM_CAMSS_VIDEO_H */

-- 
Regards,

Laurent Pinchart

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