Re: [PATCH 08/10] media: camss: Add files which handle the video device nodes

Laurent Pinchart
2016-12-05T15:24:42Z

Hi Hans,

On Monday 05 Dec 2016 16:02:55 Hans Verkuil wrote:
> On 12/05/2016 03:45 PM, Laurent Pinchart wrote:
> > 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)

There's at least v4l2-ctl and yavta that can (and should) be used for
debugging ;-)

> and useful when you want to pipe the video.

Except that you lose frame boundaries. It's really a poor man's solution that
should never be used in any "real" application. I'd rather add an option to
v4l2-ctl to output the video stream to stdout (and/or stderr).

> 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,

Famous last words ;-)

> and it is a nice feature to have (and for free as well).

--
Regards,

Laurent Pinchart