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    SubjectRe: [PATCH 08/10] media: camss: Add files which handle the video device nodes
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    On 12/05/2016 03:45 PM, Laurent Pinchart wrote:
    > Hello,
    >
    > On Monday 05 Dec 2016 14:44:55 Hans Verkuil wrote:
    >> On 11/25/2016 03:57 PM, Todor Tomov wrote:
    >>> These files handle the video device nodes of the camss driver.
    >>>
    >>> Signed-off-by: Todor Tomov <todor.tomov@linaro.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
    >
    > [snip]
    >
    >>> +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;
    >>
    >> Add modes VB2_DMABUF and VB2_READ. These are for free, so why not?
    >> Especially DMABUF is of course very desirable to have.
    >
    > I certainly agree with VB2_DMABUF, but I wouldn't expose VB2_READ. read() for
    > this kind of device is inefficient and we should encourage userspace
    > application to move away from it (and certainly make it very clear that new
    > applications should not use read() with this device).

    VB2_READ is very nice when debugging (no need to program a stream I/O application first)
    and useful when you want to pipe the video.

    Nobody uses read() in 'regular' applications since it is obviously inefficient, but
    I don't see that as a reason not to offer VB2_READ.

    I don't think this is something anyone will ever abuse, and it is a nice feature
    to have (and for free as well).

    Regards,

    Hans

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