Terrible display flickering after FC5 update today

Adam Jackson ajackson at redhat.com
Fri Oct 27 17:12:51 UTC 2006


Johannes Erdfelt wrote:
> On Wed, Oct 25, 2006, Tom Horsley <tomhorsley at adelphia.net> wrote:
>> On Wed, 25 Oct 2006 14:02:48 -0700
>> Johannes Erdfelt <johannes at erdfelt.com> wrote:
>>> What monitor do you use?
>> It's kinda weird - I use a Westinghouse LVM-42w2 HD monitor, but I have both
>> FC5 and FC6 supposedly using the refresh rates it gets from the monitor
>> for the 1920x1080 display mode, so the fact that it is much more frequent
>> on FC6 (and I've never seen it happen on Windows) makes me think it is more
>> like a random bug of some kind than a refresh rate problem. (The monitor
>> will tell me if the signal is out of range, and it doesn't do that, it
>> just turns off).
>>
>> It also shows some tendency to happen more frequently when running things like
>> opengl screensavers than when running a simple 2d desktop.
> 
> It's not exactly a refresh rate problem. I'm not a graphics technology
> guru, but I'll try to explain.
> 
> The speed of the dotclock is causing either the ATI card to produce a
> slightly dirty signal or the monitor is having trouble handling it. I
> never found out which is the problem (and from reading around, it might
> be either or both depending on your setup), but I did find out that
> reducing the dotclock helps the problem.
> 
> You can reduce the speed of the dotclock by using reduced blanking
> interval timings. Since you're using an LCD, you don't need all of the
> extra blanking intervals that CRTs need. As a result, you can keep your
> high resolution without sacrificing refresh rate and hopefully solving
> your problem.
> 
> The problem with the radeon driver is that it rejects timings with a
> reduced blanking interval. You can hack the driver to fix that.

It shouldn't be doing so anymore, at least in FC6.

The other problem is that your card only has a fixed amount of memory 
bandwidth.  The DAC or TMDS controller has to compete with the rendering 
engine for access to pixels; if it loses, the frame will just go black. 
  1920x1080 at 60 Hz is 124 megapixels per second, and at 32bpp that's 
500MB/sec.  VRAM is fast but not always that fast.

- ajax




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