[Bug 91434] [IVB/HSW] 23.976Hz & 24Hz modes broken on dual-display with recent (4.0.x) kernels

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=91434

--- Comment #36 from Martin Andersen <martin.x.andersen at gmail.com> ---
(In reply to Ville Syrjala from comment #35)
> (In reply to Martin Andersen from comment #34)
> > There is no DP-HDMI dongle involved. The output is fed directly from a
> > MacBook Retina system via full-size/regular HDMI cable. (Yes, I've also
> > noticed it says DP.)
> 
> That just means the DP++ chip is likely soldered onto the motherboard. It's
> definitely somewhere because it answers when we talk to it.

As mentioned earlier, the exact same issue occurs with an Ivy Bridge system
-with no DP output or chip - alongside the other two systems tested on.

The sole reason a MacBook is being used is purely for practical reasons. 

The MacBook also behaves correctly using older kernels, which should eliminate
any perceived Apple-weirdness causing this.

The only strange issue observed with the MacBook is an apparent bug in the
Intel driver which, when setting drm.debug=0xe causes the backlight setting to
be inverted(!) (regardless of being hooked up to an external output or not).
That obviously belongs in a separate bug report.

Nevertheless, I recently tested 4.13.2 (201709132057) on the (main) IVB system
and observed the same issue with no signal from both the 50 and 60Hz modes.
Nothing during kernel boot, nothing from X.

However, I did not produce (yet another) debug output as it was pretty much
identical to the ones provided before, and since this bug appeared to be
abandoned (which I am glad is not apparently the case)

After this I tried 3.18.71 (#201709132337) – which worked without issues. So I
assume none of the drm stuff is backported to this branch. (Thankfully)

> > However; the exact same setup works 100% correctly with older kernels, as
> > outlined earlier in the ticket. By 100% i mean: never any picture dropouts,
> > never any dots (which only were seen with this most recent kernel) or other
> > graphical glitches.
> 
> That's probably because older kernels only did 8bpc, whereas newer ones do
> 12bpc whenever possible.

Maybe, though I doubt it. Why then do the other modes work? I have sent Deep
Color signals to the PJ before but frankly see no reason for this.

But surely there is a flag to disable this in the driver, so it can be ruled
out on a definitive basis?

> > I have three other sources hooked up to this projector, and none have
> > issues.
> 
> Can you tell whether any of them do 12bpc HDMI output, or just 8bpc?

The sources are 2x PS3s and one Blu-Ray player, all of which are capable of
12bpc.

> > The same behaviour is seen when hooked up directly to either of the
> > projector's two HDMI ports - with another cable - as well. So I believe
> > 'cabling issues' can be ruled out.
> 
> The cables aren't unusually long are they?
>
> It's definitely possible that the signal degradation happens either in the
> source of sink side. I could almost excuse the source side for not being
> tested to handle 12bpc iff Apple's own driver always uses 8bpc and they
> didn't expect anyone to install another OS on the thing. If the problem is
> in the projector then I think it's less excusable, and they should have
> tested to make sure the device works with the maximum TMDS clock it's
> advertising to the source.

I feel the focus on what is causing this is entirely misguided. As mentioned
before, testing has been performed when the source has been hooked up directly
to the PJ (using a 6ft cable), with the same issues.

But I'd like to emphasize: the same cabling has been used since 2012 without
any issues what-so-ever relating to signal dropouts or interference on either
60/50Hz or 23.976/24Hz material - both in 2D and 3D. This is from a total of
six sources: two PS3s, one Blu-Ray player, one analog-to-digital
converter+upscaler (to 1080p50), and two DVB set-top boxes. (Hope this
information does not not lead to a 'your cables are too old', 'your system too
complex' comment) ;)

This is also the fourth projector which is hooked up to this IVB system - if
there was something even sligthly strange relating to cabling it surely would
have been detected at a significantly earlier stage.

But to re-iterate, all of the above cabling has been bypassed and the system
hooked up directly. It is therefore with high probability *not* cabling
related.

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