Add Classic DOS Text-Mode / VGA (720x400@70, 640x400@70) and Genlocked 70Hz Scaler Timings (1080p70, 1440p70)

[haxman]

Feature Request: Add Classic DOS Text-Mode / VGA (720x400@70, 640x400@70) and Genlocked 70Hz Scaler Timings (1080p70, 1440p70)

Summary

Add native timing and format validation support in open_gc573 for classic IBM VGA / DOS text-mode and Mode 13h resolutions (720x400 @ 70.086 Hz and line-doubled 640x400 @ 70.086 Hz), as well as 70.00 Hz / 70.09 Hz frame intervals for standard HD and QHD resolutions (1920x1080@70 and 2560x1440@70).

Motivation

1. Direct Retro PC & VGA Capture: Connecting classic retro PCs, industrial systems, or VGA-to-HDMI adapters running native DOS command prompts (720x400 @ 70Hz) or line-doubled VGA games (320x200 scaled to 640x400 @ 70Hz) currently results in the kernel driver rejecting the signal because neither 400p height nor 70Hz frame intervals are present in the open_gc573 validation table.
2. Genlocked / Frame-Locked Retro Scaler Workflows: When using external high-end scalers (such as the RetroTINK 4K Pro) set to Frame Lock (Genlock) on a native 70Hz DOS source, the scaler outputs a true 70Hz HDMI container (e.g., 1920x1080 @ 70.086 Hz or 2560x1440 @ 70.086 Hz). Currently, capturing these genlocked 70Hz streams requires forcing an intermediate 60Hz/120Hz triple-buffering conversion on the scaler, introducing unnecessary frame pacing judder. Exposing 70Hz intervals in the driver allows pixel-perfect 1:1 frame-locked capture of classic PC hardware.

Proposed Implementation

• Add DOS Timings to open_gc573: Update the timing validation matrix and VIDIOC_ENUM_DV_TIMINGS discrete tables to recognize and accept 720x400 @ 70.086 Hz and 640x400 @ 70.086 Hz.
• Add 70Hz Frame Intervals: Include 70.000 / 70.086 fps intervals in the frame-interval descriptor arrays for standard resolutions (1920x1080 and 2560x1440).
• Ensure Aspect Ratio & Scaling in everlive_display: Verify that everlive_display correctly handles the non-standard pixel aspect ratios associated with 720x400 text modes so DOS characters render with proper geometry.

[Everlite]

Hi @haxman,

We have implemented native DOS VGA modes and 70Hz genlocked timing support in commit 0b32cf2:

1. Classic DOS VGA Modes (720x400@70, 640x400@70): Added to gc555_video_modes in open_gc573/gc573-video.c and wired into FPGA SD broadcast validation/routing in open_gc573/gc573-fpga.c.
2. Genlocked 70Hz Timings: Added 70Hz frame rate intervals to 1920x1080 and 2560x1440 modes in the driver mode table and widened the low-rate HD FPGA validation ceiling to 72Hz so 70.086Hz signals pass natively without frame drops or judder.
3. Aspect Ratio Correction: Added automatic 4:3 CRT geometry scaling in everlive_display for 720x400 and 640x400 inputs so DOS text characters and classic games render with their authentic 4:3 display proportions.

Please pull latest main, rebuild with ./build.sh (or reload the module via open_gc573/load.sh), test with your DOS / RetroTINK 4K 70Hz setup, and let us know how it works for you!