Radeon HD 4250: screen going blank every few seconds

[Tijl Coosemans]

On Sun, 30 Dec 2018 22:46:07 +0100 Tijl Coosemans <tijl at FreeBSD.org> wrote:
> 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".

This seems to be a loader tunable so you'll have to add
compat.linuxkpi.hw_i2c=0 to /boot/loader.conf and reboot.