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DMA-BUF allocation feedback
DRM sessions publish renderer formats through a Linux DMA-BUF v4 global for each render device. Its default feedback identifies that renderer's render node (or its primary node when no render node is available). Per-surface feedback starts with this same render fallback; requesting feedback does not require a mapped window or a scanout-capable buffer.
After a successful display queue, or a successful render returning no display
changes, Ferese can add an output-specific scanout preference. It uses the
primary plane selected by that output's DRM surface, not a union of all planes.
Only an exact FourCC/modifier pair shared by that plane and the renderer is
preferred. The pair must have been observed on the primary result. This narrow
choice follows the current policy's full-format comparison with the swapchain;
it does not advertise broader primary formats, cursor formats or overlay formats.
Implicit modifiers are not equated with explicit linear modifiers, and alpha
formats are not equated with their opaque counterparts.
The preference's target device is the output's primary DRM node; its main device and renderer fallback remain the originating global's render device. Every preferred pair is also renderer-importable. Failed preference construction retains render feedback rather than failing an already queued display frame.
Surface selection and invalidation
Only the current mapped fullscreen toplevel can receive the preference. Its last successful frame must expose underlying buffer storage, opaque content covering the output and full opacity. Rounded shader clipping continues to hide storage. Current opacity or dimming also withdraws the preference immediately. The surface must be visible on exactly one registered output, and that output must belong to the originating import global. Animation, overview, locking, paused/inactive outputs and mirror targets receive render fallback. A failed scanout reported by Smithay withdraws the preference; a format-unsupported result can still receive a compatible allocation hint.
Feedback selection refreshes after event dispatch, before flushing protocol messages, even when no frame is drawn. Migration, fullscreen exit, unmapping and changes in eligibility restore render-only feedback. Multiple visible outputs do not race to replace one another's feedback. Subsurfaces, popups, layer surfaces and cursors currently retain render-only feedback; eligibility expansion is a separate hardware-gated phase.
Mode changes, surface replacement, rollback and session pause/reactivation invalidate output observations. A fresh successful frame is needed before a preference returns. Retiring a GPU withdraws scanout preferences and retires its global. Version 4 requires a main-device tranche, so existing bindings retain last-known renderer capabilities rather than receiving empty feedback or being retargeted to another GPU. These retained capabilities do not promise device availability: imports through that retired global are rejected. A reconnected device publishes a new global. Destroyed surface requests are pruned. Smithay owns the subscriptions, format-table transport, tranche encoding and suppression of unchanged updates. Its feedback state is shared by all subscriptions for one surface; the initial live request determines the originating global. Destroy all feedback objects before requesting the surface through a different global to change that association.
Cross-GPU migration keeps the originating renderer's fallback instead of advertising unverified formats or retargeting imports to another GPU. This does not establish cross-GPU composition compatibility: imports are still checked through the originating global's renderer, and the existing destination-renderer behavior needs physical validation. Version 3 clients have no per-surface feedback updates; their existing import checks remain authoritative.
Allocation feedback is an optimization hint, not proof that a particular scene or client buffer will scan out. The compositor still owns plane assignment, clipping, blur, capture privacy, lock protection, gamma and composition fallback. No plane eligibility flags change here.
Checks and hardware gate
cargo test --locked -p ferese backends::direct::feedback -- --nocapture
cargo test --locked -p ferese dmabuf_feedback -- --nocapture
Deterministic tests cover exact intersections, device identities, observed format changes, candidate geometry, queue failure, migration, multiple outputs, policy withdrawal, scanout failure and reconfiguration. A private Wayland v4 client checks initial render feedback, subscriptions, preference/fallback changes, unchanged update suppression, feedback renewal and retired-device preference withdrawal. It does not create a real DMA-BUF, use DRM or establish hardware acceptance.
Before expanding eligibility, record the build, GPU/driver/kernel, output modes, scale/transforms, application and its actual allocated FourCC/modifier. Decode v4 feedback device IDs and format tables and verify preferences against the live renderer and selected primary plane. Test fullscreen entry/exit, animations, opacity/clipping, popups, sharing and lock/unlock; compare captured pixels and allocation outcomes. Exercise mixed outputs, mirrors, migration, hotplug, reconfiguration/rollback, pause/resume, gamma and failed scanout. Include v3 clients and applications that ignore or follow feedback, and explicitly validate cross-GPU imports. Use the output-management hardware checks. Physical scanout rate, GPU work, power and presentation latency remain unmeasured until those hardware baselines are collected.