I've been running with 4 monitors on my 7850 for a long time but after a Debian jessie upgrade the two monitors on the DP outputs are no longer enabled. Please see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=760881
Can you bisect?
I can but I don't think it's caused by the kernel driver alone since even older kernels like 3.14 do this now and it used to work in the past with 3.14 through 3.16. Maybe somebody can give me a hint as to what to look for and have a look at the logs. I also want to run a jessie fresh install and see if a fresh start will make a difference.
Looking for older kernels I found 3.13-0.bpo.1-amd64 and voila, it fixed the behavior. Then I booted 3.14-2 and 3.16-1 and both worked again! I compared the initramfs for 3.16-1 before and after and the only changes were in etc/modprobe.d/radeon-kms.conf and etc/boottime.kmap.gz but copying those changes by hand into the now-working initramfs did not duplicate the failure.
Resolution to the mystery: It seems that I had changed /etc/default/grub before today's testing but not run update-grub. When I installed the 3.13-0.bpo.1 kernel the kernel command lines were fixed. What caused the original failure were the explicit mode settings I had on the kernel command line, namely "video=DP-1:2560x1440-24@60e video=DP-2:2560x1440-24@60e video=DVI-0:2560x1440-24@60e". I had to add those settings in the past (see https://bugs.freedesktop.org/show_bug.cgi?id=72167.) Without them settings it works. I'll attach some kern.logs - there all sorts of interesting things going on that are apparently needed to get the DP outputs up and running with my DP->DVI converters. Setting the resolutions explicitly keeps those interesting things from happening.
Created attachment 106309 [details] kern.log with explicit settings; monitors don't work
Created attachment 106310 [details] kern.log without explicit mode settings on kernel line; monitors work
I can reproduce this at will now; let me know if I should run more tests.
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