[Bug 89008] [BYT] No HDMI hotplugging

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=89008

--- Comment #12 from Imre Deak <imre.deak at intel.com> ---
(In reply to Ville Syrjala from comment #10)
> (In reply to Egbert Eich from comment #9)
> > (In reply to Jani Nikula from comment #8)
> > > Ville on #intel-gfx:
> > > ... 
> The GPIO approach would involve changing the pin muxing depending on the
> state of the power well (or maybe leave them as GPIO pins all the time?). I
> can't recall if Imre actually tried this approach, but I do recall we
> discussed it a few time when he was adding the BYT power well stuff. The bad
> thing is that it's a very platform specific solution. I believe we should be
> able to do it on BYT/CHV, but probably not on other platforms.

Antti investigated the GPIO approach. The result of that was that using GPIOs
in this way is an unofficial configuration (not explicitly described by the
spec) and that the i915 driver would need to interface with the kernel's pin
control/GPIO subsystem which makes it more difficult to implement. There wasn't
either a clear conclusion if it's physically doable on CHV/BYT at all; based on
what I understood from the GPIO/MUX specs it is, but I haven't actually tried.
There is no information on whether we can use GPIOs in this way on non-BYT/CHV
platforms. As a sidenote besides D3 that Ville mentioned for non-BYT/CHV
platforms HPD also goes away on those if we turn off their display power well
(at least on HSW/BDW).

Another issue is S0ix. It's possible that in that state even the GPIO subsystem
is powered down. Granted, it would be somewhat strange.

All that said, I think it's still worth investigating this further for someone
with the time.

And yes, brand new platforms (at least Gen9LP based) have explicit HW support
to keep HPD working even during low power states.

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