https://bugs.freedesktop.org/show_bug.cgi?id=48880
--- Comment #10 from Alex Deucher agd5f@yahoo.com 2012-04-18 12:22:55 PDT --- Is it only problematic on VGA or TMDS? I.e., does one only one or the other, but not both? It seems to be something specific to your monitor. I can't reproduce this on my ontario hardware or monitors. I manually added the modeline you were using and it works fine here. Can you try slightly different timing? Some monitors are really picky about the clocks and sometimes they do not like certain pll divider combinations even though the result is the the same clock. You might try setting the RADEON_PLL_PREFER_MINM_OVER_MAXP or RADEON_PLL_USE_FRAC_FB_DIV pll flags in atombios_adjust_pll() and see if either of those helps.