[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 #39 from Martin Andersen <martin.x.andersen at gmail.com> ---
This bug is still very much relevant. I just tested this again under 4.16.11,
and it still behaves in the *exact same manner* as previously reported. This
was on the IVB system, not the HSW system I tested on previously. However, the
results (or lack thereof) were identical.

I had some (albeit minor) hopes that the 12bpc validation patches which went in
a while ago would solve this (*if* the error is indeed related to this, as
previously suggested). Since there apparently is no option to disable 12bpc or
force 8bpc, it is impossible to tell with any certainly whether that is the
case.

[btw.-I did look at the proposed patches to force a certain bits per channel,
'force_bpc', but I cannot see that these made it into the i915 driver as it was
not a valid parameter to the module]

The behaviour is as follows (same as reported earlier)

- 50 & 60Hz modes show no output whatsoever on the second display. Nothing
during boot (KMS/console), nothing after X-server startup. Output on the
primary LCD (HDMI1) is working without any issues.

- The 23.976Hz and 24.000Hz modes are garbled, both showing rectangular patters
and intermittent static.

The source source & sink have been hooked up directly and it behaves the same
way, eliminating cabling issues.

Reverting to the 3.18 branch (3.18.109) fixes the issue.

However, based on the previous "weakness in source devices" comment, I guess
Intel's "solution" to this issue is to replace a fully-working system which
works flawlessly under the 3.x kernel branch with something else? Or am I
misunderstanding something?

Stay tuned for more logs with debugging information enabled. (If anyone is
still interested in solving this, that is.)

--Martin

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