Alternatives to v4l2src for webcam on the Nano

On the Nano, is there a way to use nvdec to output nvmm from a
webcam source?

I use
v4l2src ! nvvidconv
but prefer something that will use nvdec.

You can use nvv4l2decoder to get a hardware acceleration.

Now that omx plugins were deprecated in L4T R32.1, I guess it is up to nvv4l2decoder if nvdec or GPU is used. But according to tegrastats, nvdec is fully utilized.

Doing this:

v4l2src ! nvv4l2decoder ! …

freezes the output after the first frame, with many warnings like this

0:00:06.755537532 26550 0x7f74003a30 WARN v4l2videodec gstv4l2videodec.c:1026:gst_v4l2_video_dec_loop: Decoder is producing too many buffers
0:00:06.759199843 26550 0x55b1e1b0a0 INFO v4l2src gstv4l2src.c:957:gst_v4l2src_create: sync to 0:00:06.299999937 out ts 0:00:06.561280765

What is the pipeline I should use?

These are probably more important warnings:

0:00:00.176722907 29198 0x558b714120 WARN v4l2 gstv4l2object.c:4408:gst_v4l2_object_probe_caps:nvv4l2decoder0:src Failed to probe pixel aspect ratio with VIDIOC_CROPCAP: Unknown error -1
0:00:00.176800409 29198 0x558b714120 WARN v4l2 gstv4l2object.c:2370:gst_v4l2_object_add_interlace_mode:0x558b703380 Failed to determine interlace mode

Please check if the following post helps:
You should not need hardware decoder if your source is in YUV422.