On Sunday 09 May 2010 11:23:05 Hans Verkuil wrote:
> What is VIDIOC_QUERYSTD supposed to do when there is no signal?
>
> The spec says this:
>
> "The hardware may be able to detect the current video standard automatically.
> To do so, applications call VIDIOC_QUERYSTD with a pointer to a v4l2_std_id
> type. The driver stores here a set of candidates, this can be a single flag
> or a set of supported standards if for example the hardware can only
> distinguish between 50 and 60 Hz systems. When detection is not possible or
> fails, the set must contain all standards supported by the current video
> input or output."
>
> The last sentence is the problem. There are several possibilities:
>
> 1) The hardware is physically unable to detect the current video std. In that
> case this ioctl shouldn't be implemented at all.
>
> 2) While detecting the std an error occurs (e.g. i2c read error). In that case
> the error should be returned.
>
> 3) There is no input signal. Does that constitute 'detection is not possible
> or
> fails'? If so, then all supported standards should be returned. But that seems
> very strange. After all, I did detect the standard: i.e. there is none, so I
> would say that QUERYSTD should return V4L2_STD_UNKNOWN (0).
>
> A quick check of the current state of affairs when no signal is present
> reveals
> that:
>
> - saa7115, ks0127, saa7191 return 0 with std set to V4L2_STD_ALL
> - adv7180, vpx3220 return 0 with std set to V4L2_STD_UNKNOWN
> - saa7110 returns 0 with std set to the current std
> - bt819 and bttv do not handle this case at all, and just pick 50 Hz or 60 Hz
> - tvp514x returns -EINVAL.
>
> Lovely... :-)
>
> It is clear that applications currently have no hope in hell to use the output
> of querystd in a reliable manner. For all practical purposes the behavior of
> querystd when no signal is present is undefined.
>
> I would propose to specify that if no signal is present then QUERYSTD should
> return 0 with std V4L2_STD_UNKNOWN.
I received no comments, so I will prepare patches to implement this proposal.
Regards,
Hans
> It would also be consistent with QUERY_DV_PRESET where the preset DV_INVALID
> is returned in that case.
>
> If we decide to change it, then it is trivial to fix all drivers that
> implement
> querystd.
>
> Comments?
>
> Hans
>
>
--
Hans Verkuil - video4linux developer - sponsored by TANDBERG, part of Cisco
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