Crate denise_drm 

Linux DRM/KMS backend for Denise.

Opens the display directly, sets a mode, and page-flips CPU-rendered dumb buffers straight to the scanout engine. No compositor, no window server, no GPU driver stack, no X.

What this is not doing, and why

No gbm. GBM exists to allocate buffers a GPU renders into. Denise renders with the CPU, so DRM dumb buffers are exactly the right allocation: scanout-capable, CPU-mappable, and free of any C library. Adding GBM would drag in libgbm and Mesa, which would end both the single-static-binary goal and easy cross-compilation.

No atomic modesetting, yet. Atomic buys three things: FB_DAMAGE_CLIPS, plane composition, and tear-free guarantees. The first is worth little here — a page flip swaps whole buffers, so damage saves rasterisation, not bandwidth, and most drivers ignore the property anyway. The second has a legacy equivalent for the one plane that matters, the hardware cursor. So the legacy path gets this milestone working on real hardware at a third of the code, behind a seam that atomic can take over when planes actually earn their keep.

Becoming DRM master

Setting a mode requires being DRM master, and only one process can be. If a compositor or another Denise process holds it, Card::become_master fails with EBUSY or EACCES — that is the single most common reason a first run on a Pi does nothing. Three ways to have the right:

• Run on a bare VT with no display server. The usual kiosk deployment.
• Be handed a file descriptor by libseat or a systemd unit, via Card::from_fd. Preferred for anything that has to coexist.
• Run as root. Works, and is a poor way to ship a product.

Testing

mode and swapchain are platform-independent on purpose and are unit tested everywhere, including on machines with no DRM device. They hold the decisions that are hard to debug in the field and easy to check on a laptop. Everything else is a thin wrapper over ioctls and can only be proven on real hardware.

Re-exports

pub use mode::ConnectorInfo;

pub use mode::ConnectorKind;

pub use mode::ModeInfo;

pub use mode::ModePreference;

pub use mode::OutputPreference;

pub use mode::Selection;

pub use mode::SelectionError;

pub use swapchain::Swapchain;

Modules

mode

Choosing which output to drive and at what mode.

swapchain

Buffer rotation and buffer-age bookkeeping.

Structs

Card

An open DRM device.

DrmSurface

A display brought up under our control, scanning out CPU-rendered buffers.

SurfaceConfig

How to bring the display up.

Enums

DrmError

Something went wrong talking to the display.

PresentMode

When a queued flip actually reaches the panel.