Gstreamer v4l2src could not negotiate format

[Joel A Fernandes]

Hi Wim,

> Some snippet from the debug log (it queries the supported formats):
>
> gstv4l2object.c:1032:gst_v4l2_object_fill_format_list:<v4l2src0>  got 4
> format(s):
>> 0:00:01.588623047   898   0x324050 INFO                    v4l2
>> gstv4l2object.c:1038:gst_v4l2_object_fill_format_list:<v4l2src0>
>> YU12 (emulated)
>> 0:00:01.588684083   898   0x324050 INFO                    v4l2
>> gstv4l2object.c:1038:gst_v4l2_object_fill_format_list:<v4l2src0>
>> YV12 (emulated)
>> 0:00:01.588745118   898   0x324050 INFO                    v4l2
>> gstv4l2object.c:1038:gst_v4l2_object_fill_format_list:<v4l2src0>
>> BGR3 (emulated)
>> 0:00:01.588806153   898   0x324050 INFO                    v4l2
>> gstv4l2object.c:1038:gst_v4l2_object_fill_format_list:<v4l2src0>
>> RGB3 (emulated)
>
> So the camera supports only 4 formats: YU12, YV12, BGR3 and RGB3 all of them
> emulated.
>
> Since you are trying to make it produce UYVY with the capsfilter, this will
> fail, as the error message suggests. It does not call VIDEOC_S_FMT because
> the query of the formats revealed that UYVY is not supported.
>

Thanks for your suggestion. The camera driver supports only UYVY and
YUYV for now so I'm unusure about why (who) is returning these
formats. I dug into this further, and it seems that there are no
query/enumeration related IOCTLs being called during this enumeration
process. libv4l2 I'm guessing is returning some "emulated" formats
which seem to be unsupported.

My questions are:

1. Why does the v4l2src plugin not call the S_FMT function? I'm
guessing because of errors during v4l2s_src_get_caps which fails
because it tries to find the size for unsupported formats by making
IOCTLs to the driver (ioctl TRY_FMT)

2. In this case, how do I override the "format detection" part of v4l2src?

3. Is there something flawed in the way format detection/query is happening?

4. Why does it not use the caps filter parameters I pass in my
pipeline [1] instead of trying to query for formats?

5. Does libv4l2 conditionally make IOCTL system calls, when the
application requests it to do so by calling libv4l2_ioctl? I notice
that there are no ENUM_FMT calls at all when the code that determines
formats was executed, though v4l2src appeared to call libv4l2_ioctl.

Many Thanks,
Joel

[1] gst-launch -v v4l2src device=/dev/video2 !
'video/x-raw-yuv,width=640,height=480,format=(fourcc)UYVY' ! TIVidenc1
codecName=mpeg4enc engineName=codecServer ! filesink location=output.m4v