Re: [PATCH v6 0/4] Add SCDC information to connector debugfs

[Maxime Ripard]

On Mon, Jun 15, 2026 at 10:07:02AM +0200, Nicolas Frattaroli wrote:
> On Saturday, 13 June 2026 08:57:29 Central European Summer Time Hans Verkuil wrote:
> > Hi Nicolas,
> > 
> > On 11/06/2026 14:57, Nicolas Frattaroli wrote:
> > > HDMI uses the DDC I2C bus for communicating various bits of link status
> > > out of band with the actual HDMI video signal. This information can be
> > > useful for debugging issues like questionable cables sabotaged by feline
> > > teeth, Enthusiast Grade cables made of cow fencing wire, and other such
> > > problems that ruin one's media viewing plans.
> > > 
> > > Consequently, this series exposes various bits of pertinent information
> > > from the SCDC protocol in an HDMI connector's debugfs. To continually
> > > poll the link status, userspace can poll the debugfs file.
> > 
> > Something is not quite right: I've been testing this series with my i915
> > based laptop with HDMI connector, and I never see the scdc_status file.
> > And that's because CONFIG_DRM_BRIDGE_CONNECTOR is not set for my configuration.
> 
> That's to be expected. i915 does not use bridge connectors, and neither
> does amdgpu iirc. I am working on embedded boards that do use bridge
> connectors, along with all the rest of the HDMI state helpers.
> 
> Implementing something new in DRM usually involves having to triplicate
> certain parts of the work in order to have i915 and amdgpu behave the
> same as everyone else.
> 
> > So I think you are creating the debugfs entry in the wrong place.
> 
> I can't create it for all drm_connectors as Maxime suggested due to the
> cyclical dependency that would create. If someone has any idea on how
> to break that dependency cycle, I'm open to suggestions.

Back when we introduce the audio support, we floated the idea to
de-midlayer this and turn it into a debugfs_init helper. We don't have a
lot of users yet so it might be the best time to do so, and would solve
your issue.

Maxime

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