Ayufan figured out how to add new mode lines to the current table of HDMI modes a while back by working around one of the binary blobs Allwinner left in the video driver. It is still a good bit of work to add more modes to the driver, but basically just someone needs to do it and test them now:
https://github.com/ayufan-pine64/linux-p...965950c9db Initial support for custom resolutions
https://github.com/ayufan-pine64/linux-p...24c8dd5edb Automatically calculate ptbl for hdmi bsp driver
https://github.com/ayufan-pine64/linux-p...965950c9db Initial support for custom resolutions
https://github.com/ayufan-pine64/linux-p...24c8dd5edb Automatically calculate ptbl for hdmi bsp driver
Come have a chat in the Pine IRC channel >>