Some comments:
WiFi on S905X2: may not be supported for a while yet, or at all. S905X2 has a design flaw in how the SDIO module is internally connected and while an egregious hack/workaround exists in Amlogic's 4.9 kernel it's looking unlikely that a cleaner solution acceptable to upstream kernel maintainers can be found. We might attempt a mainline kernel version of the egregious hack through a local patch, or since the majority of users stream media over Ethernet anyway, we might just leave it unsupported and avoid the support/maintenance work. The issue is allegedly resolved with the S905X3 chip which is already being sampled to manufacturers.
Device trees: The mainline kernel avoids the silliness of 1G/2G/3G variants needed on the legacy kernel which makes life easier, but we will still need to figure out (and then upstream) the device trees for popular devices. I'm keen that device trees are sent upstream so that devices can be uniquely identified by "compatible" strings in the device-tree, as this allows simple things like the remote control keymap (also sent upstream) to be described in the device-tree for a better out of box user experience. Submitting changes to the kernel is not the most straightforward process but if non-coding folk like myself can figure it out (my personal collection of one-line kernel changes is slowly increasing) the bar is not set impossibly high.
arm vs. aarch64: performance is marginally higher on aarch64 but arm allows us to support widevine drm (which is not available in 64-bit format) so arm is our preferred option and official nightlies and the Kodi binary add-ons to match them, once they start, will be arm only. Maintaining a single image for all Amlogic hardware (two images until panfrost evolves bifrost support) is a bigger win than the minor performance gain. On G12 hardware the future gain from running the CPUs at full speed (at the moment they are under-clocked) will dwarf the architecture difference.
