Low resolution modelines such as 480i or 576i for uses with HDMI-VGA adapter to output to 15kHz RGB SDTV cannot be added directly because HDMI requires minimum pixel clock of 25 MHz. Raspberry Pi for example seems to be able to automatically double the pixel clock on these modes so that they work transparently. This doesn’t seem to be the case with RK3588, at least with BSP kernels, trying to set such modelines results in error.
I can double the values of pixel clock and horizontal timings on modeline manually and make it work, but then the pixel aspect ratio is off by 2. This can be corrected on X by using --scale parameter on xrandr, but this seems to be done at GPU level, probably causing some performance loss. Also while this is easy to do within X, I’m somewhat at loss how to do it with Kodi running in GBM or Wayland, as hardware decoding doesn’t seem to be available on Kodi running on X with current ffmpeg-rockchip fork.
Is the video output hardware itself able to do this pixel doubling (?) on lower level natively, without GPU involvement, and could a support for it be added to allow using such modelines (or EDID with such modes) without the extra effort?