[Bug 105887] [GLK] no signal after switch resolution from 1080p at 23.98

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=105887

--- Comment #69 from Daniel Scheller <d.scheller.oss at gmail.com> ---
(In reply to Clinton Taylor from comment #68)

> After bashing my head against he wall for several days I have finally
> determined that the issue is caused by the HDMI level shifter on the board
> and not a driver issue. 
> 
> After changing the mode from 1080p24 to 1080p50 the scope shows a valid
> clock into the level shifter and a very low amplitude clock on the output to
> the connector. Adding a short delay after the Transcoder is disabled appears
> to solve the issue with the level shifter.
> 
> I have attached a scope shot of the waveform into the level shifter (yellow
> trace) and the output to the connector (blue trace).

Thanks for investigating this further and even finding a "proper" driver
workaround for this hardware problem. I did a test with msleep(50) at the end
of intel_ddi.c:intel_ddi_disable_transcoder_func() (see [1]) on the ASRock
board, and this helps the "No signal" issue aswell.

I went a bit further and added a modparam to switch between different GLK
tests, which by now are "no 12bpc" and "msleep(50)" ([2] is what everything
looks like right now, esp. re patches/tests) :-)

If anyone else likes to do some testing: I've put installable kernel .debs up
at  [3] for easy install (Ubuntu Bionic 4.15.0-23.25 proposed, with the patches
ontop). To enable any of the workarounds, use the parameter glkhdmi (set to 1
for the 12bit disable hack, set to 2 for the msleep test). Easiest is to add
i915.glkhdmi=X (replace X with 1 or 2) to /etc/default/grub into
GRUB_CMDLINE_LINUX_DEFAULT, then update-grub, then reboot. Note if you don't
set the option or set it to zero, you'll get stock behaviour and the "No
signal" issue back.

[1]
https://github.com/herrnst/ubuntu-bionic-kernel/commit/f0503e843b8223a2a4572c573a120cd49a8e7134
[2]
https://github.com/herrnst/ubuntu-bionic-kernel/compare/Ubuntu-4.15.0-23.25...4.15.0-23.25-glktesting
[3]
https://github.com/herrnst/ubuntu-bionic-kernel/tree/4.15.0-23.25-glktesting-deb

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