Struct LayerScene 

pub struct LayerScene { /* private fields */ }

One CRTC’s layers and the commit machinery around them.

Layers are added once and kept: add_layer returns a LayerHandle that stays valid until the layer is removed, and a stale one resolves to nothing rather than to whatever now occupies the slot. Per frame, the caller changes what it wants — geometry through set_display, content through the source — and asks for a build_frame.

The scene is deliberately not a commit loop. It builds a plan and reads the kernel’s answer; issuing the commit, attaching a modeset, and dispatching the page-flip event belong to the caller, because a compositor has its own ideas about all three. See the crate documentation for the shape of a frame.

Four of the six pinned invariants are the scene’s: buffers released a flip late rather than an ioctl early (1), a failed commit releasing immediately because no flip will gate it (2), the FB-only fast path (4), and teardown draining an outstanding flip before letting go of what it scans out (5). None of them is something a caller opts into; they are what the type does.

Implementations

impl LayerScene

pub fn new(crtc_id: u32) -> Self

A scene driving crtc_id, with no layers.

pub const fn crtc_id(&self) -> u32

The CRTC this scene drives.

pub fn len(&self) -> usize

How many layers the scene holds.

pub fn is_empty(&self) -> bool

Whether the scene holds no layers.

pub const fn is_suspended(&self) -> bool

Whether the scene is suspended after losing DRM master.

pub const fn has_pending_flip(&self) -> bool

Whether a page-flip event is armed and undispatched.

Invariant 5. Dropping the scene while this is true tears down a framebuffer the flip still references.

pub const fn flip_landed(&mut self)

Record that the armed page-flip event was dispatched.

pub fn resume(&mut self)

Lift a suspension after the session resumes.

Whoever held the CRTC meanwhile may have left planes lit, so the next frame turns off every plane it does not use.

pub fn add_layer(&mut self, source: Box<dyn LayerBufferSource>) -> LayerHandle

Add a layer backed by source.

pub fn remove_layer(&mut self, handle: LayerHandle) -> bool

Remove a layer. A stale handle is a no-op.

If the layer’s buffers are still in flight, its source is kept alive until they come back — releasing to a dropped source would strand them. The source hears LayerBufferSource::on_retired first.

pub fn layer(&self, handle: LayerHandle) -> Option<&SceneLayer>

Borrow a layer, or None if the handle is stale.

pub fn layer_mut(&mut self, handle: LayerHandle) -> Option<&mut SceneLayer>

Borrow a layer mutably, or None if the handle is stale.

pub fn handles(&self) -> impl Iterator<Item = LayerHandle> + '_

Every live handle, in slot order.

pub fn build_frame<C: TestCommitter>( &mut self, registry: &PlaneRegistry, crtc_index: u32, kind: CommitKind, committer: &mut C, ) -> Result<FrameBuild, SceneError>

Acquire from every layer, lower it, and run the allocator.

A source with nothing to contribute is skipped and counted, never treated as a failure — see SourceError::WouldBlock.

Errors

• SceneError::Suspended while the scene is suspended.
• SceneError::Source if a source failed for a real reason.
• SceneError::NotMaster if the allocator lost DRM master.

pub fn enable_composition( &mut self, device: &Device, width: u32, height: u32, ) -> Result<(), DumbError>

Give the scene a composition canvas.

Layers the allocator cannot place are blended into this surface and it goes on a plane of its own. Without one they are reported unassigned and simply do not reach the screen.

Separate from new because the search itself is device-free and this is not: the canvas owns two dumb buffers.

The canvas is ARGB8888, which every plane that matters takes. enable_composition_on is for the ones that do not.

Errors

drmkit_dumb::DumbError if either buffer cannot be allocated.

pub fn enable_composition_on( &mut self, device: &Device, width: u32, height: u32, plane: &PlaneCapabilities, ) -> Result<(), DumbError>

Give the scene a canvas in a format plane can scan out.

On a controller with one plane and no ARGB8888 – tilcdc on a BeagleBone, i.MX LCDIF, the small SPI panels – an ARGB8888 canvas cannot be armed at all, so every layer that overflows the plane count is dropped rather than composited. This negotiates instead, and blends in ARGB8888 regardless: the conversion happens once at flush rather than quantizing at every layer.

Errors

drmkit_dumb::DumbError::InvalidConfig if the plane advertises nothing the canvas can write – a YUV-only overlay, say – and otherwise as enable_composition.

pub fn canvas_fourcc(&self) -> Option<u32>

What the composition canvas scans out as, if there is one.

pub fn finalize_frame( &mut self, build: FrameBuild, result: KernelResult, ) -> CommitReport

Reconcile scene state with the kernel’s answer, releasing buffers per invariants 1, 2 and 5.

pub fn finalize_frame_with_fence( &mut self, build: FrameBuild, result: KernelResult, release_fence: Option<&SyncFence>, ) -> CommitReport

Finalize a frame, handing over the commit’s OUT_FENCE.

release_fence signals once this commit’s buffers are on screen — which is the moment the buffers it displaced are off screen and safe to render into again. Sources that opted in through wants_release_fence get a duplicate of it alongside each released buffer, so a GPU producer waits on it GPU-side instead of blocking until a later release edge.

Ask wants_release_fence whether to arm OUT_FENCE_PTR at all: with no source listening, the fence costs a descriptor per commit and nothing reads it.

Passing None is finalize_frame, and is correct whenever the CRTC has no OUT_FENCE_PTR or the commit did not ask for one.

pub const fn set_force_full_property_writes(&mut self, force: bool)

Re-emit every property on every commit, for a driver that mishandles a partial write.

Off by default. A quirk escape hatch, not a tuning knob: it defeats invariant 4’s minimal-write path and multiplies per-frame property traffic, and the only reason to reach for it is a driver that gets the minimal set wrong. Exposed here because the allocator it configures is the scene’s, and a caller stuck on such a driver otherwise has no way in.

pub const fn force_full_property_writes(&self) -> bool

Whether full property writes are being forced.

pub fn content_changed(&self) -> bool

Whether this frame is worth committing at all.

The whole-commit skip. When every live source says it has nothing new and no layer has moved, the frame can be dropped without issuing an atomic commit — the display goes on scanning out what is already there. That is a power win on any device and the thing that lets a self-refresh panel stay in self-refresh, which it cannot do if a commit arrives every vblank saying nothing changed.

Answers true for anything it cannot rule out. A source that cannot tell whether its buffer changed reports “changed” by default, so a CPU producer painting into a dumb buffer never gets skipped; the layer set changing counts, geometry changing counts, and the first frame always counts, because nothing is on screen yet for “unchanged” to describe.

pub fn find_by_identity_tag(&self, tag: u64) -> Option<LayerHandle>

The first layer carrying tag, if any.

The lookup that makes SceneLayer::set_identity_tag worth setting: a caller that survived a rebind, a session resume, or its own restart can find its layers again from its own identifiers rather than from handles it may no longer hold.

pub const fn pending_detach(&self) -> Option<&PendingDetach>

What a rebind left lit on the CRTC the scene moved away from, until it is committed.

pub fn detach_committed(&mut self)

Record that the pending detach reached the kernel.

commit_detach calls this after a successful commit; a caller committing the detach some other way must too.

pub fn rebind(&mut self, crtc_id: u32, width: u32, height: u32) -> RebindReport

Move this scene to another output, keeping its layers.

For an output that changed underneath the caller: a mode set, a different CRTC after a session resume, a display swapped for another. Layer handles survive — that is the point, since a caller that had to rebuild its layers would have to rebuild everything it knows about them too.

Everything cached about the previous output is dropped – the assignment, the committed baseline, the failure cache – and the next commit is a full emit, with which the caller must set the new mode.

Moving to another CRTC leaves the old one’s planes lit, and the scene cannot turn them off itself: it does not commit. They are queued as a PendingDetach, and until the caller commits it through commit_detach, build_frame refuses with SceneError::DetachPending. Two reasons it cannot be skipped: the kernel will not move a plane from one CRTC to another in a single commit, so a plane both pipes can use is rejected outright on the new one; and a plane the new pipe cannot use would go on showing this scene’s last frame on the old output indefinitely. A rebind to the same CRTC queues nothing. Upstream fixed the same defect in a335bef (drm-cxx#340), from inside its rebind, which owns a device.

Returns what will not fit. A layer whose destination lies outside the new mode is not dropped — the caller may be about to move it, and deciding on its behalf would be worse than saying so.

pub fn wants_release_fence(&self) -> bool

Whether any live source wants this commit’s OUT_FENCE.

The commit path asks before arming OUT_FENCE_PTR: the fence costs a descriptor per commit that has to be closed, and with nothing listening it is a descriptor leak dressed as a feature.

pub fn release_displaced(&mut self, commit_seq: u64)

Hand back now the buffers that the page flip of real commit commit_seq (its CommitReport::commit_seq) took off screen, instead of at the next real commit.

A buffer leaves the screen when the flip of the commit after the one that showed it completes, and the scene hands it back at the commit after that (invariant 1) – right for a caller that commits ahead of its flips. A caller that waits for each flip before committing again, feeding producers that wait for a free buffer before rendering, may never make that next commit: a producer with two buffers then waits for good, and one with three gets each buffer a frame late. Call this once the flip completes – after dispatching its event, or at the start of the next frame – to hand those buffers back then.

A no-op unless commit_seq names the most recent real commit, so a late or repeated call cannot release a buffer still on screen. Port of upstream’s release_displaced (ee90026).

pub fn drain(&mut self)

Hand every held buffer back, without waiting on the kernel.

Invariant 5. For teardown, session pause, and rebind. Landing an armed flip first is the caller’s job — see has_pending_flip.

Trait Implementations

impl Debug for LayerScene

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Drop for LayerScene

fn drop(&mut self)

Executes the destructor for this type.

fn pin_drop(self: Pin<&mut Self>)

🔬This is a nightly-only experimental API. (pin_ergonomics)

Execute the destructor for this type, but different to Drop::drop, it requires self to be pinned.