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Phase 2 of the GPU-rendering plan deliberately targeted a single head
(GpuContext::output: Option<(crtc::Handle, GpuOutput)>) as a narrow
proof that GBM+EGL+DrmCompositor rendering works at all on this
hardware. DrmOutputManager already supports driving several crtcs at
once - initialize_output is a per-crtc call on one shared manager,
the same way anvil drives multiple outputs - so this was purely an
unexercised restriction, not an architectural limit.
GpuContext::output is now GpuContext::outputs: Vec<(crtc::Handle,
GpuOutput)>, and udev/platform.rs calls initialize_output for every
connected head in its own bring-up loop instead of only the first
after that loop finishes. A head this fails for individually (already
logged, not fatal) still just has no entry and falls back to the
existing legacy Pixman path, unchanged from before.
session.rs's VBlank handler and its VT-switch resume path (which
excludes GPU-driven crtcs from the legacy set_crtc reassert loop, a
different device fd that must never issue mode-set commands against a
crtc DrmOutputManager already owns) both now look a crtc up in the
Vec instead of comparing against a single stored one.
render.rs's own render-loop lookup uses direct field access
(gpu.outputs.iter_mut().find(...)) rather than an equivalent
&mut self method: the borrow checker treats a method call as
borrowing all of GpuContext, including gpu.renderer needed a few
lines later for the same head, where direct field access lets it see
the two borrows are disjoint.
Still gated behind SRDWM_GPU=1 (unset by default) and untested on
real multi-monitor hardware with the flag on - this machine has one
display, so the actual multi-head path itself only gets exercised
whenever it's set on hardware that has more than one.
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Extends the SRDWM_GPU=1 opt-in path (Phase 1, fa0c7f1) from a
capability probe into an actual, working GPU render pipeline for
exactly one head, per the plan this was built from
the plan file.
gpu.rs: probe now goes all the way through EGLContext, GlesRenderer,
DrmDevice (real DRM device, separate duped fd from the existing legacy
Card), and DrmOutputManager construction, returning both a GpuContext
and its DrmDeviceNotifier on success. GpuContext::initialize_output
drives one crtc/mode/connector through DrmOutputManager, storing the
resulting GpuOutput for the render loop to find.
Confirmed while reading smithay's own source directly (not assumed):
DrmDevice::new's disable_connectors parameter is not an atomic-vs-
legacy switch - DrmDevice::create_internal tries atomic capability
first and falls back to a Legacy internal variant automatically,
exposed via DrmDevice::is_atomic(), logged here rather than assumed.
platform.rs: probes at startup, calls initialize_output for the first
head only (Phase 2's deliberate scope - see the plan), registers the
DrmDeviceNotifier as its own calloop event source alongside (not
replacing) the existing legacy DRM-fd registration.
render.rs: render_udev_frame's per-head loop checks, before any of the
existing Pixman-specific element-building logic runs, whether this
head's crtc matches the GPU context's initialized output; if so,
renders a plain clear color through render_frame/queue_frame and
continues to the next head, completely bypassing the Pixman path for
that head. Every other head, and this same head whenever the GPU
context or its output is absent, is entirely unaffected.
session.rs: register_gpu_drm_notifier handles DrmEvent::VBlank by
calling frame_submitted() on the matching GpuOutput (required per
queue_frame's own doc comment, or the swapchain runs out of buffers)
and logs DrmEvent::Error without treating it as fatal.
Deliberately out of scope for this phase (documented in the plan):
window content/decorations/cursor on the GPU head (clear color only),
multi-monitor GPU rendering (one head only), and VT-switch pause/
resume for the GPU head specifically (DrmOutputManager's own pause()/
activate() calls are a different API surface from the
existing manual set_crtc+DPMS reassertion, and porting that pairing
correctly needs its own isolated verification pass).
SRDWM_GPU unset (the default) is unaffected: every step above only
runs when it's set to "1", and every failure at any step falls back
to the existing, untouched Pixman path with a logged reason, same
fallback contract Phase 1 already established.
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The udev backend is, by explicit design, 100% software: PixmanRenderer
compositing into legacy KMS dumb buffers. That was a deliberate choice
for portability (dumb buffers work on essentially any DRM driver,
including a VM with no GBM/3D support), not an oversight - but this
machine's real hardware (Intel UHD 620, i915) should support real
GBM+EGL rendering, and the user has asked for a genuine GPU-accelerated
path, GPU preferred with CPU fallback, built as a separate track that
doesn't risk the working software path.
Adds `udev/gpu.rs::probe`: gated behind `SRDWM_GPU=1` (unset by
default - a no-op, zero behavior change for every session that
doesn't set it), attempts GBM device creation on a duped DRM fd, EGL
display/device creation, and a software-rasterizer check, logging
exactly which step failed if any and falling back silently. Wired in
at udev backend startup, right after the DRM fd is opened.
Deliberately does not yet create an EGLContext, a GlesRenderer, or
touch scanout at all. Reading smithay's own reference compositor
(anvil/src/udev.rs) confirmed it wires GBM+EGL rendering together with
atomic-KMS scanout as one unit via DrmCompositor, not as a renderer
swapped into the existing legacy set_crtc/page_flip flip loop this
backend uses today. Adopting DrmCompositor is separate, larger-scoped
work than "swap the renderer" - it replaces the same UdevHead
mode-set/flip machinery the VT-switch fixes
(register_session_notifier's ActivateSession arm, copy_and_flip's
retry backoff) live in, and needs its own plan. This probe answers the
first question - does the hardware even support it at all - safely,
before that larger integration is scoped and attempted.
Cargo.toml: added backend_egl/backend_gbm smithay features, additive
to the existing renderer_pixman path (unchanged, still the default).
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