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

Ville Syrjälä ville.syrjala at linux.intel.com
Wed Feb 24 15:09:27 UTC 2016

On Tue, Feb 23, 2016 at 08:44:49PM +0100, Tore Anderson wrote:
> 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.

Well, just to check the details of your particular cable/dongle,
maybe you can post the dmesg with drm.debug=0xe with my branch?
Or at least the parts that refer to DP dual mode adaptors.

> 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.

Yeah, interlaced requires half the bandwidth of progressive, so it
should then fit comfortably within the 165MHz limit of the adaptor.

> > > 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. ;-)
> Tore

Ville Syrjälä
Intel OTC

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