https://bugs.freedesktop.org/show_bug.cgi?id=76130
--- Comment #16 from Evgeny Grechnikov diamondaz+fdb@yandex.ru --- It seems that the timeout is too small. A dirty fix replacing rdev->usec_timeout with 1000000 in rv770_send_msg_to_smc (drivers/gpu/drm/radeon/rv770_smc.c) works for me (both *ERROR*s in kernel log disappear, performance returns to normal).