~ $ ffmpeg -h encoder=h264_nvenc | grep formats
ffmpeg version 3.3.6 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 4.9.3 (Gentoo 4.9.3 p1.5, pie-0.6.4)
  configuration: --prefix=/usr --libdir=/usr/lib64 --shlibdir=/usr/lib64 --docdir=/usr/share/doc/ffmpeg-3.3.6/html --mandir=/usr/share/man --enable-shared --cc=x86_64-pc-linux-gnu-gcc --cxx=x86_64-pc-linux-gnu-g++ --ar=x86_64-pc-linux-gnu-ar --optflags='-O2 -pipe' --disable-static --enable-avfilter --enable-avresample --disable-stripping --enable-version3 --disable-indev=oss --disable-indev=jack --disable-outdev=oss --enable-bzlib --disable-runtime-cpudetect --disable-debug --disable-gcrypt --disable-gnutls --disable-gmp --enable-gpl --enable-hardcoded-tables --enable-iconv --disable-lzma --enable-network --disable-openssl --enable-postproc --enable-libsmbclient --enable-ffplay --enable-sdl2 --disable-vaapi --disable-vdpau --enable-xlib --enable-libxcb --enable-libxcb-shm --enable-libxcb-xfixes --enable-zlib --disable-libcdio --disable-libiec61883 --disable-libdc1394 --disable-libcaca --disable-openal --enable-opengl --disable-libv4l2 --disable-libpulse --disable-libopencore-amrwb --disable-libopencore-amrnb --disable-libfdk-aac --disable-libopenjpeg --disable-libbluray --disable-libcelt --disable-libgme --disable-libgsm --disable-mmal --disable-libmodplug --disable-libopus --disable-libilbc --disable-librtmp --disable-libssh --disable-libschroedinger --disable-libspeex --enable-libvorbis --disable-libvpx --disable-libzvbi --disable-libbs2b --disable-chromaprint --disable-libflite --disable-frei0r --disable-libfribidi --disable-fontconfig --disable-ladspa --disable-libass --enable-libfreetype --disable-librubberband --disable-netcdf --disable-libzmq --disable-libzimg --disable-libsoxr --enable-pthreads --disable-libvo-amrwbenc --enable-libmp3lame --disable-libkvazaar --enable-nvenc --disable-libopenh264 --disable-libsnappy --enable-libtheora --disable-libtwolame --disable-libwavpack --disable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --disable-amd3dnow --disable-amd3dnowext --disable-fma4 --disable-xop --disable-doc --disable-htmlpages --enable-manpages
  libavutil      55. 58.100 / 55. 58.100
  libavcodec     57. 89.100 / 57. 89.100
  libavformat    57. 71.100 / 57. 71.100
  libavdevice    57.  6.100 / 57.  6.100
  libavfilter     6. 82.100 /  6. 82.100
  libavresample   3.  5.  0 /  3.  5.  0
  libswscale      4.  6.100 /  4.  6.100
  libswresample   2.  7.100 /  2.  7.100
  libpostproc    54.  5.100 / 54.  5.100
    Supported pixel formats: yuv420p nv12 p010le yuv444p yuv444p16le bgr0 rgb0 cuda

~ $ v4l2-ctl --list-formats
ioctl: VIDIOC_ENUM_FMT
        Index       : 0
        Type        : Video Capture
        Pixel Format: 'YUYV'
        Name        : YUYV 4:2:2

        Index       : 1
        Type        : Video Capture
        Pixel Format: 'MJPG' (compressed)
        Name        : Motion-JPEG

~ $ ffmpeg -hwaccel cuvid -pixel_format yuv444p -f v4l2 -framerate 20 -video_size 1920x1080 -c:v h264_cuvid -i /dev/video0 -c:v h264_nvenc -f alsa -i plughw:CARD=Mic test.avi
...
[video4linux2,v4l2 @ 0x145b0c0] Cannot find a proper format for codec 'h264' (id 28), pixel format 'yuv444p' (id 5)
Assertion *codec_id != AV_CODEC_ID_NONE failed at src/libavdevice/v4l2.c:808