denise-fbdev

The legacy Linux fbdev backend for Denise, a direct-rendering UI toolkit in Rust for embedded Linux and systems without a desktop environment.

Read this before reaching for it

This is a fallback, and on any current kernel it is a fallback to DRM through a longer route. /dev/fb0 on a modern system is almost always CONFIG_DRM_FBDEV_EMULATION — DRM pretending to be fbdev. An Alpine VM reports its framebuffer's name as virtio_gpudrmfb; a Raspberry Pi running Bookworm reports vc4drmfb. Going through it means giving up page flips, vsync and buffer age, to reach the same hardware denise-drm already drives properly.

So prefer DRM. Use this when:

• the kernel is old enough to predate a usable DRM driver for the panel,
• the panel has an fbdev driver and no DRM driver at all, which still happens with small SPI displays,
• or DRM master cannot be obtained and a degraded picture beats none.

# #[cfg(target_os = "linux")]
# fn demo() -> Result<(), Box<dyn std::error::Error>> {
use denise::Surface;
use denise_fbdev::FbdevSurface;

let mut surface = FbdevSurface::open_first()?;
println!("{:?}", surface.size());
# Ok(())
# }

What it costs

No page flip and no vsync, so a frame can tear. Drawing goes through a shadow buffer and only damaged rows are copied out, which keeps the tear as small as the change that caused it — the only mitigation available here, and another reason damage tracking belongs in the core rather than in a backend.

Permissions

Writing to /dev/fb* needs the video group, or root.

Platform

Linux only; elsewhere the crate compiles to almost nothing. FbInfo and the pixel layout parsing are platform-independent and unit tested everywhere, because misreading a varinfo bitfield is exactly the bug that shows as wrong colours on one board and nowhere else. unsafe is permitted here and every block carries a // SAFETY: comment.

Where this sits

Implements denise::Surface. Pair it with denise-evdev for input.

Status

M2 complete. Part of Denise — see the repository README for the whole picture.

MIT licensed.