video/x-raw(memory:NVMM) element of gstreamer don't work on jetson nano

Hi guys,
I want to use H264 harwdare decoding of jetson nano for RTSP stream, and I want to aviod memory copies and keep decoded data in BUFFER DMA for GPU memory,
when I use vide/x-raw in the gstream elemets like below, I get correct stream, but when I use video/x-raw(memory:NVMM),It doesn’t work and I don’t get any decoded data.
why? How to use video/x-raw(memory:NVMM) in correct way?

 gst_str = ('rtspsrc location=<rtsp address> latency=300 ! '
                   'rtph264depay ! h264parse ! omxh264dec ! '
                   'nvvidconv ! '
                   'video/x-raw(memory:NVMM), '
                   'format=(string)BGRx ! videoconvert ! '
                   'appsink ')
cv2.VideoCapture(gst_str, cv2.CAP_GSTREAMER)

Opencv videoCapture only read frames in CPU memory.
For your case you would try nvivafilter. (Here is an example for fisheye undistort, you would adapt for your case)