[SOLVED] Modesetting with external monitor broken (intel)

Question

After recent updates, modesetting during boot no longer works if an external (vga) monitor is plugged in.
Once it gets to modesetting, both screens go black and I can't tell what happens after that (Might hang, might boot fine I just can't see it…) The external vga screen is black (with very occasional horizontal line of noise), but remains on (It doesn't go to sleep, which it usually does if not receiving a signal).
If the monitor is unplugged during boot then plugged in once the login prompt is reached, modesetting works and running X multihead with xrandr seems to work normally. (workaround until the issue is solved)
Changing from Early KMS start to Late KMS start means the black screen appears later, but it still appears.
Booting with nomodeset works with the monitor plugged in, but means X fails to start with an error message about not finding any screens.
The laptop is an old macbook, with integrated intel graphics card:
Intel Corporation Mobile 945GM/GMS, 943/940GML Express Integrated Graphics Controller (rev 03)
Last edited by Thrall (2013-03-19 11:29:04)

Answer 1

Ok, I'm trying to collect information for a bug report, so if people could confirm or correct what I have so far and add any additional relevant information, that'd be very helpful.
The fact that running the linux-lts kernel fixes the issue suggests that the problem almost certainly lies with the kernel.
Comparing the date of my first post with /var/log/pacman.log, I believe the upgrade that first caused the issue to appear was this:
upgraded linux (3.7.3-1 -> 3.7.4-1)
(There is a very slim possibility that I managed to upgrade to 3.7.4 without actually rebooting into the previous kernel, in which case the issue could have been present in an earlier version, but I wouldn't have encountered it.)
So far, the only graphics card affected appears to integrated intel chips:
Intel Corporation Mobile 945GM/GMS(/GME), 943/940GML Express Integrated Graphics Controller (rev 03)
Has anyone observed this issue with a different graphics card? (check the output from entering "lspci" in a terminal if you're not sure how to find out)
Finally, Wittfella found reinstalling from scratch fixed the issue. If someone can come up with a reasonable explanation as to *why* this fixed it (and ideally narrow it down to changes that can be made without reinstalling from scratch if at all possible), then it's possible that it's not an actual bug, so much as a poorly documented change that causes problems upgrading but is otherwise ok afterwards.