https://bugzilla.kernel.org/show_bug.cgi?id=85421
--- Comment #39 from EmanueL Czirai emanueLczirai@cryptoLab.net --- Created attachment 172311 --> https://bugzilla.kernel.org/attachment.cgi?id=172311&action=edit same error as OP (dmesg)
Sorry, I thought it was the same thing(bug).
I actually did get the same error as OP once (*ERROR* radeon: failed testing IB on GFX ring (-35)) with radeon.lockup_timeout=4 (but was intended to be 4000)
My apologies though.