How and why v4l2src able to take YV12 format?

[jeyp4]

I modified GST_V4L2_USE_LIBV4L2=1 in a shell. But still v4l2src can't take
YV12 format?

nvidia at jetson:~$ echo $GST_V4L2_USE_LIBV4L2

nvidia at jetson:~$ GST_V4L2_USE_LIBV4L2=1
nvidia at jetson:~$ echo $GST_V4L2_USE_LIBV4L2
1
nvidia at jetson:~$ gst-launch-1.0 -v v4l2src device=/dev/video0 !
'video/x-raw,format=(string)YV12,width=1280,height=720' ! xvimagesink
Setting pipeline to PAUSED ...
Pipeline is live and does not need PREROLL ...
Setting pipeline to PLAYING ...
ERROR: from element /GstPipeline:pipeline0/GstV4l2Src:v4l2src0: Internal
data stream error.
Additional debug info:
gstbasesrc.c(3055): gst_base_src_loop ():
/GstPipeline:pipeline0/GstV4l2Src:v4l2src0:
streaming stopped, reason not-negotiated (-4)
Execution ended after 0:00:00.000127687
Setting pipeline to PAUSED ...
Setting pipeline to READY ...
Setting pipeline to NULL ...
Freeing pipeline ...
nvidia at jetson:~$ gst-inspect-1.0 --version
gst-inspect-1.0 version 1.14.5
GStreamer 1.14.5
https://launchpad.net/distros/ubuntu/+source/gstreamer1.0




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