just try it... drm tries to rotate then mis-renders updates (wrong offset). gl doesnt even bother rotate the output BUT rotates input.
rage has cmdline options (-rot 0/90/180/270) to make it easy.
in addition the window does not resize to the screen correctly when rotated. to make things even more fun, enlightenment doesn't do rotation via ecore-evas etc. and thus misses out on rotation if there is no drm rotation. since we have had code to render rotated for a long time in x11 and i am sure this worked in the fb before too (fbcon), this should work here too...
don't forget to test input (e.g. a touch panel) too... :)