https://bugs.freedesktop.org/show_bug.cgi?id=29787
--- Comment #10 from j.bormolini@gmail.com 2010-10-04 20:38:36 PDT --- I see essentially the same thing as Arno. I have an AMD 780G motherboard (radeon hd3200 / rs780). The only time I noticed the problem was when running mplayer - it pauses for a split second every minute or so. I'm using HDMI for video and audio.
The option to disable polling is (now?) part of drm_kms_helper, so adding "drm_kms_helper.poll=0" to the kernel cmdline disables polling and completely fixes the problem for me. I'm using kernel 2.6.36-rc6. Hopefully this helps anyone else who stumbles on this problem.
I just remembered (too late, doh!), that a while ago I set X and hd-audio[01] to RT priority when running mplayer as an experiment to test performance. I'm pretty sure I saw the problem before that, but if I ever get around to rebooting I'll test with polling enabled and the priorities unchanged. Presumably Arno does not change his priorities and still sees the problem.