[Intel-gfx] [BUG] HDMI 12bpc causing occasional flickering and blanking

Tore Anderson tore at fud.no
Tue Feb 23 19:44:49 UTC 2016

Hi Ville,

> "The monitor is connected with a DP+-to-HDMI cable"
> This and some reading of the DP dual mode spec gave me another idea;
> The DP->HDMI adaptor may simply be degrading the signal quality too
> much. According to the DP dual mode spec we're supposed to limit the
> TMDS clock based on the type of adapter used, but currently we have
> no code to do that. I've cooked up a few patches that should do what
> we want:
> git://github.com/vsyrjala/linux.git dp_dual_mode
> I've quickly tested it locally, and it seemed to do the right thing
> with a few different types of adaptors.

I've run 32fa589 for a few hours now and it have not seen a single
blank or flicker. So it seems you've nailed it - thanks a lot!

Let me know if you want me to test more patches, post debug logs, or
anything else.

BTW, also discovered right before you sent that e-mail that downgrading
to a 1920x1080i mode (rather than the monitor's native 1920x1080) would
also stop the flickering. I'd assume that also fits well with your
diagnosis (less bandwidth needed => better tolerance for degraded signal
quality), but I thought I'd let you know in case not.

> > By the way: Is it possible to disable HDMI 12bpc in a way that
> > doesn't require me to patch and rebuild the kernel drivers, such as
> > a kernel module parameter or sysfs setting? (I prefer to simply use
> > the upstream Fedora kernel RPMs, but this issue is currently making
> > that impossible.)  
> We don't have any knob to control this.

I don't need it anymore, so no worries. ;-)


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