Need to override HDMI resolution list

[Hadmut Danisch]

Binary package hint: xserver-xorg

Hi,

this is rather a wish than a bug report.

I connected an ubuntu machine with an HDMI cable to a small LCD TV. Unfortunately, the TV's firmware is not that good. Although the TV has a 1366x768 panel, it only lists 1920x1080 and 720x480 as possible resolutions. The X11 server refuses to use any other than those two listed resolutions, even if configured in xorg.conf. (The manufacturer says, it is not possible to use the TV with any other resolution, not even in the panel's resolution of 1366x768.) Unfortunately it is impossible to get an acceptable resolution, because 1920x1080 is scaled down and blurred, while 720x480 is scaled up and again, blurred.

But then something curios happened: Accicendtly I pulled a little bit at the HDMI cable and it slipped out of the plug just a little bit.

Then, the BIOS and Linux in console mode did not show anything on the TV anymore and instead used the analog VGA port. But once the X11 server started, it chose the resolution of 1366x768 as configured in xorg.conf, and even the TV accepted it, displaying 1360x768 as resolution (don't ask me why it detected 1360 instead of 1366), and the display was absolutely sharp and perfect, and in the correct resolution.

So I guess that the losen HDMI cable allowed to transmit the signal from the PC to the TV, but not from the TV to the PC. PC could not detect the TV and read its table.

This shows that it would be usefull to have an option to force the X11 server to ignore the resolution table reported by the screen on DVI and HDMI and to rigorosly use the given resolution as an override, in order to circumvent firmware problems as described.

regards

[Bryce Harrington (bryce)]

There are a variety of parameters available that can be set in xorg.conf to add or force various resolutions, modelines, force on/off particular options, and so on, so I'm *pretty* sure what you're asking for already exists, although it's hard to say just from the description provided. If that doesn't work for you, then we'd need to see your 'xrandr --verbose', and pairs of xorg.conf/Xorg.0.log's that you've tested and the results you found.

You also didn't specify which version of Ubuntu you tested or what video driver you're using, and a lot of this is heavily driver-dependent, so before any further analysis could be done here we'd need to know that as well. (I can suss this out of the Xorg.0.log if you don't know).

Also attach the output from 'lspci -vvnn' and 'dmidecode', in case the error is something particular to your hardware and need your product id's.

[Hadmut Danisch (hadmut)]

I could actually fix the problem about two hours ago:

- generate a modeline with cvt
- put it into xorg.conf
- xrandr --addmode HDMI-0 1360×768
- xrandr -s 1360×768

But I had to choose a width that is a multiple of 8, so I had to choose 1360 instead of the panel's physical width of 1366.

regards

[Bryce Harrington (bryce)]

I'm afraid that without the info I asked about in my previous comment, there is nothing we can do on our end to better handle this use case. We'd need to know specifics about your monitor and video card, for example, in order to generate quirks or conditional logic so the "fix" applies only to this case.

If you'd like us to pursue finding a better "just works" solution for your use case, please feel free to reopen this bug and attach all the information I requested, and we can take it up with upstream.