[Intel-gfx] A wrong DDI encoder override from HDMI to DP at hotplug

[Takashi Iwai]

On Wed, 18 Nov 2015 22:30:32 +0100,
Dave Airlie wrote:
> 
> On 19 November 2015 at 02:00, Takashi Iwai <tiwai at suse.de> wrote:
> > On Wed, 18 Nov 2015 16:38:03 +0100,
> > Ville Syrjälä wrote:
> >>
> >> On Wed, Nov 18, 2015 at 04:23:06PM +0100, Takashi Iwai wrote:
> >> > Hi,
> >> >
> >> > currently a DDI port may register both DP and HDMI and it shares the
> >> > same encoder.  The bug we've got a report is about this encoder type:
> >> > namely, a machine using DDI port D for HDMI is screwed up because the
> >> > encoder is switched to DP suddently.  The details are found in:
> >> >   http://bugzilla.opensuse.org/show_bug.cgi?id=955190
> >> >
> >> > The problem happens in intel_dp_hpd_pulse().  Since the machine
> >> > declares both DP and HDMI, the driver registers this callback.  And at
> >> > a hotplug event, the function changes the encoder type like:
> >> >
> >> >     if (intel_dig_port->base.type != INTEL_OUTPUT_EDP)
> >> >             intel_dig_port->base.type = INTEL_OUTPUT_DISPLAYPORT;
> >> >
> >> > After this point, the encoder is handled as DP although the same HDMI
> >> > monitor is connected.
> >> >
> >> > Changing this to exclude INTEL_OUTPUT_HDMI makes the thing working in
> >> > the bug report above, but I'm not sure what's the right fix.
> >> >
> >> > Any suggestions?
> >>
> >> This has been causing all sorts of troubles for years. I've suggested
> >> several times that we should just split the encoder into two, like we
> >> have for older platforms, but no one has taken the bait thus far.
> >>
> >> This particular piece of code to change the encoder type in the
> >> hpd_pulse hook came about with the mst code drop from Dave IIRC. At
> >> least I never figured out what it's trying to do since -ENOCOMMENT.
> >
> > Yeah, this was introduced in the commit
> >
> >   0e32b39ceed665bfa4a77a4bc307b6652b991632
> >     drm/i915: add DP 1.2 MST support (v0.7)
> >
> > so the problem exists for quite some time, since 3.17 kernel.
> >
> > I guess the patch just overlooked the DDI use case with HDMI.
> > Maybe the code was meant like below?
> >
> >         if (intel_dig_port->base.type == INTEL_OUTPUT_UNKNOWN ||
> >             intel_dig_port->base.type == INTEL_OUTPUT_DP_MST)
> >                 intel_dig_port->base.type = INTEL_OUTPUT_DISPLAYPORT;
> >
> 
> That code was copied from intel_dp_detect, which does that check.
> 
> But if the HDMI hdp is going via that path then yes it should check
> more than EDP.

OK, so we'd just need to add the check of INTEL_OUTPUT_HDMI in each
place, as a quick fix?  Since this bug has been present for a long
time (since 3.17), I'd love to have a quick fix that is applicable to
stable kernels, at first.


thanks,

Takashi