Going to put this in for tracking. e should really support all rotations regardless of drm device/output. evas can render rotated (gl surely can) and it works in almost all engines. it should work in ecore_evas drm and be used by e for rotation.
IMHO the ecore_evas + evas engine should, IF the drm device can more optimally rotate itself, then use that feature transparently if possible. the raw interface should be at ecore_evas ... or in future at the evas output level.
don't forget to ALSO rotate input. especially touch input (possibly/probably also handle specifically mapping specific touch devices to specific outputs WITH specific calibrations (rotations) as the touch device may or may not report in the same orientation as "native rendering without rotation".
this is necessary for pretty much every device like a phone, tablet, smart fridge, and watch as well as pc monitors and laptops/convertible displays etc.
don't forget that any "optimizations" like hw plane fiddling, direct buffer swapping of clients to bypass the compositor if they are fullscreen etc. should account for rotation etc.
but as it stands now... it's pretty broken/useless on many devices i am trying (touch laptops). i know many people who also rotate their desktop monitors... i do not but if i did i'd be in trouble too.