Open-source driver squeezes ultrawide output from a limited SM750 HDMI card

A developer has released sm750hdmidrm, an out-of-tree Linux DRM driver written specifically for the SE-DP750A-HDMI PCIe card, a cheap board built around Silicon Motion’s SM750 GPU and a Lattice HDMI transmitter. The project targets Linux 6.17 and newer, ships as a DKMS package for Ubuntu 24.04 and Linux Mint 22, and deliberately blacklists the aging in-kernel sm750fb driver so it can claim the card cleanly. It is explicitly experimental and scoped to one exact board revision — the shared PCI ID isn’t enough, since other SM750 cards use different transmitters or wiring.

The interesting engineering is in working around two hard limits. The SM750’s primary plane has an 11-bit right-edge field, capping real hardware scanout at 2048 pixels wide, and the card sits behind a slow PCIe 1.1 x1 link that can’t sustain responsive full-screen 32-bit updates. To get around the bandwidth ceiling, the driver converts changed regions to dithered 16-bit RGB565 just before upload — halving device-bound pixel traffic — using a custom coordinate-anchored ordered dither with green-channel correction, plus DMA batching and coalesced updates. To fake an ultrawide desktop, it renders a logical 2464x1080 or 2560x1080 workspace, compresses it horizontally into the 2048-wide HDMI signal, and relies on a physical 2560x1080 monitor’s built-in stretch mode to expand it back across the panel.

The writeup is framed as an ode to open source: the value here isn’t a mass-market product but one person reverse-engineering an obscure, spec-constrained card and publishing the workarounds, tradeoffs, and outright dangers (non-EDID modes can exceed clock limits and blank the display) in full. It’s a reminder that community drivers keep marginal hardware usable long after vendors and mainline kernels stop caring, and that meaningful performance can be reclaimed by trading a little color fidelity and aspect accuracy for bandwidth.