Unable to fix vertical underscanning on monitor

[Devin Pohl]

Hey there, hope this is the right place for this.

I have an old iMac G3, CRT display. I'm running the display through a
different mainboard running Linux. The display signal is generated from an
HDMI port on the board, and goes through a few adapters (HDMI->VGA->DB15).
The video signal is generated with standard intel video drivers.

The issue I'm having is that the CRT isn't drawing over the entire monitor.
At first, the screen was underscanning on both X and Y axes, making the
screen have margins on all sides. With some modeline tweaking I've been
able to correct the underscanning on the X axis so that the screen takes up
the entire horizontal width of the monitor.

However, I'm stuck on the vertical component. As it stands, the CRT is
underscanning in the Y axis and my display is letterboxed. Changing the
final 4 values of the modeline seem to only be able to move the screen
around within the letterboxed area.

The reference board is able to draw to the whole screen without issues.
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