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

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Date:   Tue, 10 Jan 2017 13:32:21 +0200
From:   Todor Tomov <todor.tomov@...aro.org>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Hans Verkuil <hverkuil@...all.nl>
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 Laurent, Hans,

On 12/05/2016 05:25 PM, Laurent Pinchart wrote:
> 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@...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
>>>
>>> [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).
> 

Thank you for the discussion over this. Both of you have reasonable arguments
about the read i/o.
I'd say that you cannot always save a person from himself. I'll add VB2_READ.

-- 
Best regards,
Todor Tomov