Comment 5 for bug 1066294

I hope this isn't just a me-too, but I want to add that I'm amazed that I somehow lived through both 12.04 (and the 12.10 upgrade process) completely oblivious to the fact that PAE had become 'temporarily requisite' for 12.04 at launch and now definitely requisite for 12.10 absent 'unsupported' hacks and workarounds.*

The upgrade process on a Dell Latitude D600 laptop (first-generation Pentium M, no PAE) did warn that my graphics were at risk for being unsupported (as prior versions have), but there's no equivalent check and warning for the 'CPU surprise.' Since Ubuntu does leave old kernels lying around in /boot, the upgraded system was still bootable with 3.2.0, but between the kernel issue and the fact that Unity still isn't satisfied with what GL features the radeon driver can offer with an ""ATI Radeon Mobility 9000 (M9) Lf (AGP)" (ChipID = 0x4c66)" - yet no longer falls back to unity-2d, despite it still being installed** - I decided to back out rather than fight.

For users bit by this scenario who want to 'downgrade' back to a known state, the following abuse of apt (as posted on the Ubuntu Wiki) works:

Replace /etc/apt/sources.list with the helpfully-saved sources.list.distUpgrade in the same directory;

Create the file /etc/apt/preferences containing the following:

Package: *
Pin: release a=precise
Pin-Priority: 1001

Then: apt-get update; apt-get dist-upgrade; wait, wait some more, and you'll be back to 12.04. (That this managed to result in circular dependencies for CUPS on my system, so printing may be dead until you resolve that by hand, but elsewise things are back to normal.)

*The 'official' workarounds aren't terribly bad if you're starting from a fresh install: Rely on xubuntu (XFCE) or lubuntu (LXDE), then install the "ubuntu-desktop" package if you want Unity alongside the lighter-weight desktops - but that sort of 'crossgrade' is a little more annoying on an installed system you'd just intended to update rather than modify. If you're determined, some minor adjustments to the above recipe should work.

However, it's unclear how long the 'lightweight' distributions will continue to offer non-PAE kernels if the main Ubuntu kernel team is tired of maintaining some of the security patches; if there's a 'position statement' from those distros re: how long support can be expected, please link!

**(The absence of unity-2d fallback is surely surmountable with editing of the GDM config; I just wasn't in the mood. I'm still somehow surprised that Unity demands more GL features than compiz.)