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Why does v4l thinks there is 1 plane in a multiplanar format?

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I am using the vivid kernel module to create a virtual video device.

When I set the format to NV12, it thinks there is only one plane:

$ v4l2-ctl -d /dev/video2 --verbose --set-fmt-video=pixelformat=NV12
VIDIOC_QUERYCAP: ok
VIDIOC_G_FMT: ok
VIDIOC_S_FMT: ok
Format Video Capture Multiplanar:
    Width/Height      : 1280/720
    Pixel Format      : 'NV12' (Y/UV 4:2:0)
    Field             : None
    Number of planes  : 1
    Flags             : 
    Colorspace        : sRGB
    Transfer Function : Default
    YCbCr/HSV Encoding: Default
    Quantization      : Default
    Plane 0           :
      Bytes per Line : 1280
      Size Image     : 1382400

The NV12 format has two planes: one for Y and one for UV combined.

But v4l2-ctl seems to think (or vivid seems to think) that it has only one plane.

I sit in a Tesla and translated this thread with Ai:

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