decodebin may use nvv4l2decoder that would output into NVMM memory in NV12 format. If your appsink can’t process buffers from NVMM memory, you may just use nvvidconv such as:
... ! decodebin ! nvvidconv ! video/x-raw,format=NV12 ! appsink
If your appsink expects another video format, nvvidconv can also convert into various formats. See SRC caps from:
gst-inspect-1.0 nvvidconv
for supported formats. If your expected format is not supported, you would trying adding further videoconvert such as (here converting into BGR):
... ! decodebin ! nvvidconv ! video/x-raw,format=BGRx ! videoconvert ! video/x-raw,format=BGR ! appsink