modesetting: adds default modes the monitor never advertised (needs an option to turn off) #4037

Select the version

25.1.X

Describe your issue

On connectors with a "scaling mode" property (amdgpu exposes it), modesetting's drmmode_output_add_gtf_modes() (drmmode_display.c) adds the full default VESA mode list on top of the modes the kernel reports, whenever the EDID doesn't say GTF is supported. The resolution list then fills with modes the monitor never advertised, and picking one can give a scaled or out-of-range picture.

The amdgpu X driver doesn't do this: it lists only the kernel's modes. There's no way to turn it off in modesetting.

I patched in an option for it locally: Option "ScaledModes" "false" returns
early from drmmode_output_add_gtf_modes() (default true, so nothing changes
unless it's set).

I'm new to this so I don't know how to properly make a PR just yet.

Steps to reproduce

1. Use modesetting on an amdgpu GPU, with a monitor whose EDID doesn't flag GTF support (often CRTs, or a custom EDID)
2. Run xrandr
3. The output lists many more modes than the monitor's EDID has; compare with the connector's modes in /sys/class/drm/cardN/modes

What did you expect?

Only the modes the kernel reports for the connector (as the amdgpu X driver does), or at least an option to turn the added default modes off.

Additional Information

A patch file: modesetting-scaledmodes-option.patch

Extra fields

• I have checked the existing issues
• I have read the Contributing Guidelines
• I'd like to work on this issue