[Nouveau] unstable refresh rate

[Ilia Mirkin]

Hi James,

I semi-recently added support for HDMI 2.0 (in 4.20+, so you're good),
which is why you got 60Hz in the first place. In order for the high
rates to work, something called "scrambling" must be enabled. This is
done by 2-party agreement between the sink and the source. The sink
will inform the source that it's about to use scrambling (by writing
to the SCDC register on the display over i2c).

In my personal experiments, on a LG C6 series TV, I found that while
EDID works while the TV is off, the SCDC write does not. So you can
actually "turn on" the display on the computer while the TV itself is
off, and the scrambling setting will not have been propagated. Then
when you turn the TV on, it doesn't work. In order to remedy this, you
need to disable the display linux-side, and re-enable it.

The indicator for this sort of issue would be something about SCDC
reads/writes failing in dmesg -- do you see anything of the sort?
(Something like "Failure to write SCDC_TMDS_CONFIG")

If not, it's probably something else. But this seems like a likely candidate.

On Tue, Aug 6, 2019 at 12:14 PM James <bjlockie at lockie.ca> wrote:
>
> I have a Gigabyte GeForce 1050 connected by DVI to a monitor (1920x1200
> resolution @ 59.9502 Hz) and a TV via HDMI (3840x2160 @30 Hz).
> The problem is the TV used  to work at 59.9685 Hz but then it started
> showing "No signal" on the TV.
> I was changing settings trying to get it to work again and I happened to
> change it to 30Hz and it worked.
> The specs are here:
> https://www.gigabyte.com/Graphics-Card/GV-N1050OC-2GD/sp#sp
>  >Digital max resolution: 7680x4320
> I should be ok resolution wise.
> Could it be a poor cable?
> kernel: 5.0.0-23-generic
>
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