arch linux kernel 4.7 ati radeon issues
Downgrading the kernel
If you are unable to boot after a kernel update, then you can downgrade the kernel via a live CD. Use a fairly recent Arch Linux installation medium. Once it has booted, mount the partition where your system is installed to /mnt, and if you have /boot or/var on separate partitions, mount them there as well (e.g. mount /dev/sdc3 /mnt/boot). Then chroot into the system:
# arch-chroot /mnt /bin/bash
# pacman -U linux-3.5.6-1-x86_64.pkg.tar.xz linux-headers-3.5.6-1-x86_64.pkg.tar.xz virtualbox-host-modules-4.2.0-5-x86_64.pkg.tar.xz
Exit the chroot (with exit), reboot and you should be done.
Enable early KMS
Tip: If you have problems with the resolution, Kernel mode setting#Forcing modes and EDID may help.
Kernel mode setting (KMS) is supported by the radeon driver and is mandatory and enabled by default.
KMS is typically initialized after the initramfs stage. It is possible, however, to enable KMS during the initramfs stage. To do this, add the radeon module to the MODULES line in /etc/mkinitcpio.conf:
MODULES="... radeon ..."
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