[Nouveau] unstable refresh rate

[James]

On 2019-08-06 12:32 p.m., Ilia Mirkin wrote:
> 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.
There is nothing in dmesg.
Maybe nouveau needs a module parameter for debug logging?
I'm going to try a different cable anyways.
Is there something in /proc to check if "scrambling" is on?
I do have a 4k LG tv.
> 
> 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