Radeon HD 4250: screen going blank every few seconds

[Tijl Coosemans]

On Sun, 30 Dec 2018 20:29:49 +0100 Andrea Venturoli <ml at netfence.it> wrote:
> I've got a desktop which used a Radeon HD 4200 with no troubles: I was 
> using a 1920x1200 monitor on the DVI port and occasionally a second one 
> on the VGA port; this system had no HDMI port.
> 
> 
> 
> Now I changed mainboard with an Asus M5A88-V Evo (latest BIOS 
> installed), which features a Radeon HD 4250 and I'm getting some troubles.
> 
> Focusing only on one monitor, it works fine if connected to the VGA 
> port, but I'd like to avoid this setup because:
> a) begin analogic, it shows some background noise;
> b) it won't allow me to use the second monitor.
> 
> So I'd like to use either the HDMI or the DVI port, but in this case:
> _ the system start fine and is stable as long as BIOS resolution is used;
> _ as soon as I "service kld start" and the resolution switchs to 
> 1920x1200 the screen starts flickering: every 1-2 seconds the image goes 
> black (the monitor displaying "DVI Input" or "HDMI Input" as if a new 
> signal was detected) and gets back to normal in another 1-2 seconds;
> _ once XOrg is up and running the problem does NOT disappear.
> 
> As I suspected an hardware problem, I tried booting Knoppix: initially I 
> get the same behaviour (works fine, then starts flickering once the 
> resolution is increased); however, after XOrg is running the problem stops.
> So, either it isn't an hardware problem or there is a workaround.
> 
> Also worth mentioning:
> _ if I decrease the resolution to 1920x1080 everything is ok, of course 
> apart seeing two black areas on the top and bottom of my screen :(
> _ while keeping full resolution, attaching the second monitor on the VGA 
> port *and enabling it* will reduce the flickering from every 1-2 seconds 
> to every 5-10 seconds.
> 
> 
> 
> 
> So I'm asking for some help, since either using the VGA port or 
> decreasing the resolution is not a workaround I'm eager to adopt; is 
> there anything that can be done? Some sysctl? Some BIOS settings? Some 
> xorg.conf tweak? Some update to test?
> 
> I tried and searched if this was a known (or at least previously seen) 
> problem, but came up empty.
> Even some pointer to documentation/knowledge base would help...

Try running "sysctl compat.linuxkpi.hw_i2c=0".