The state of mainline hardware decoding

[sigmaris]

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This seems to indicate at least mpeg2 and vp8 are exposed. Not sure where H.264 is, maybe it needs something in rkvdec?
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This looks like the Hantro decoder device then. The rkvdec device, which supports H.264, would normally appear as another V4L2 device e.g. /dev/video1, but I just checked mainline Linux's device tree for the RK3328, and it doesn't actually include the rkvdec device even though the SoC has one, so your OS probably doesn't know the rkvdec exists. LibreELEC includes a patch that adds the rkvdec device node. I sort of mentioned this earlier, but if you want H.264 acceleration I'd recommend compiling your own kernel with the patches from LibreELEC (and their kernel config to enable e.g. CONFIG_VIDEO_ROCKCHIP_VDEC).

As a side note, the Hantro VPU on the SoC does support H.264 decoding as well, but the register layout is slightly different from other Hantro implementations and so support for it isn't implemented (there is not much benefit as it's limited to 1920x1080 while the rkvdec can decode 4K resolution H.264).