IMPORTANT

This was my first message in this bug report: 

I have the same problem with my HD 7950; using hangouts, playing Left for Dead
2, or watching a flash video my screen goes crazy with vertical lines or grey
fog. Started when i upgraded to testing repo (Archlinux) and downloaded the
newest linux-firmware package, who includes TAHITI_mc2.bin. I suffered this bug
on kernels 3.14 and 3.15. 

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In Archlinux i was stable with kernel 3.14, and the problem started when i was
using the new firmware. I thought that the new firmware was the cause of this
bug, but NO, because i had the same bug using the old firmare, so this bug it
was caused by one of this radeon commits backported to kernel 3.14.6 (the first
kernel using newest firmware). I am 100% sure.

https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/log/?id=refs/tags/v3.14.21&ofs=1300