On Wednesday 11 January 2012 11:05:36 Rupert Eibauer wrote:
> Hello,
>
> I am working on the linux driver for our new video capture board and
> facing some compatibility problem which seems to be related to unclear
> language in the V4L specification.
>
> The device supports a larger number of inputs, but with a different set of
> standards on each of them.
>
> Now, the problem is that ffmpeg is using VIDIOC_ENUMSTD/VIDIOC_S_STD
> before VIDIOC_S_INPUT.
> So the driver has to give a list of supported standards before knowing
> which input the device will be taking.
>
> Currently I am experimenting with a workaround to remember the set
> standard and then failing
> on VIDIOC_S_INPUT when not compatible, but this seems to create more
> problems than it solves. I have
> the feeling that this behaviour is not covered by the specification, and
> it makes the driver unnecesarily complex.
>
> In my opinion, the standard is broken: on page
> http://v4l2spec.bytesex.org/spec/r11217.htm, it says:
> "It is good practice to select an input before querying or negotiating any
> other parameters."
> It should be changed to "Aplications must...", and ffmpeg needs fixing to
> call VIDIOC_S_INPUT before VIDIOC_ENUMSTD.
>
> I hope to get some opinion from you, if this is the right approach or not.
You are correct, it should be 'must' rather than 'good practice'.
I've made a patch that updates the text accordingly. I'll post it soon
together with some other spec updates.
Regards,
Hans
>
> Best regards,
> Rupert Eibauer
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