Just a quick update although not much has happened:
The letux-4.7 kernel with my patches has been semi-successfully tested on pyra hardware. Strangely, although it initially works correctly, Michael reported the display turns off after 10 seconds. Dumping properties after the display turned off shows DPMS = Off and no modes listed on the connector, all crtcs and layers inactive. Having any process open /dev/dri/card0 (implicitly grabbing DRM master) prevents this from happening, but then the display turns off the moment the file descriptor is closed.
This didn't happen with the "non-rotated" kernel. I've personally never seen such behaviour on the uevm + HDMI and also have absolutely no idea what could be causing it -- the only relevant difference should be the dsi panel driver and it shouldn't be aware at all of what my rotation patches are doing.
I've rebased my patches to letux-4.8-rc1 but this resulted in kernel tracebacks for me, escalated to panics if booting with HDMI connected. Booting without HDMI connected usually worked, as did modprobe -r omapdrm; connect HDMI; modprobe omapdrm. I merged 4.7-rc2 to see if it would help but it didn't. I've not yet had the time to investigate.
Until this is sorted out I suggest rebasing my patches to a known-working kernel if you want to try them. Rebasing to 4.7 results in a minor conflict in one of the patches, but it is easily resolved by hand.
Matthijs