Radeon graphics problems after updating from Linux-libre 4.5.4 to 4.6.1

[Dan T.]

Hello,

just want to give you this heads-up

I'm using Parabola GNU+Linux-libre on my HP Pavilion laptop.

Ever since updating Linux-libre from 4.5.4 to 4.6.1 in June (2016),
video has
started freezing at boot. I can log in and operate the computer just
fine blindly, but
video becomes completely frozen just after systemd initialization and
file system check, before the virtual terminal appears (so I just see
the fsck results forever).

The laptop has two graphics processors: ATI Mobility Radeon HD 4250 and
AMD Radeon HD 6650M. Graphics used to work fine before. I suppose the
former was in use.

I noticed that the problem can be bypassed by disabling kernel mode
setting in the
GRUB bootloader (with radeon.modeset=0). However, after disabling the
kernel mode setting, Redshift (software that is supposed to change
screen colour according to the sun) doesn't work at all and performance
is extremely poor, I experience a lot of crashes when watching videos
etc. etc., very laggy! Again, it used to work fine before that update.

this persists with Linux-libre 4.8

Thanks

Dan