x11-session-research/phase0_evidence/wayland_baseline_2026-05-03/a1_summary.md

A1 baseline — verdict

CORRIGENDUM 2026-05-03 ~17:00 CEST — the "Major reframing finding" section below interpreted 0 % kwin CPU as "KWin doing nothing per frame, direct-scanout engaged, campaign hypothesis structurally weakened." That interpretation is wrong. A subsequent drm_info-during-playback probe (see drmprobe_findings.md in this directory) shows Plane 39 rotates triple-buffered ABGR8888 framebuffers throughout playback — KWin IS doing per-frame GL composite, but the work is GPU-side (Panfrost) and invisible to top / perf-on-userspace. The campaign's mechanism is intact; the matrix needs GPU-aware metrics. The original text below is preserved as the on-the-day record of what I concluded with insufficient instrumentation.

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Three in-session reps of chromium-fourier 149 / brave_drops_test.html / Plasma Wayland 6.6.4 + kwin-fourier 6.6.4-3 + qt6-base-fourier 6.11.0-3 acquired 2026-05-03 14:23–14:31 CEST.

Per the campaign-contained-data discipline, this is the only Wayland baseline this campaign uses. Predecessor numbers are referenced below as instrument-sanity context, not as comparison targets.

Results

Metric	Rep 1	Rep 2	Rep 3	Median (cluster)
frames_total (over 70 s)	1685	1685	1686	1685
effective fps	24.04	24.04	24.05	24.04
drops_total	0	0	0	0
drops_post_warmup (after t=10s)	0	0	0	0
kwin_wayland %CPU median (1 Hz × 70 samples)	0.00	0.00	0.00	0.00
kwin_wayland %CPU mean	0.07	0.04	0.04	0.04
kwin_wayland %CPU max	4.00	1.00	3.00	3.00
perf samples on kwin_wayland (99 Hz × 70 s)	39	28	(similar)	tens, not thousands

IQR / median for the spread metrics is 0 (cluster is degenerate at the lower bound). Per the protocol's exit-condition tree, this is the "tight cluster" branch: Phase 1 binding cells can use these as the anchor with a sub-1% tolerance band on kwin %CPU and a ≤ 1-frame tolerance on drops_post_warmup.

Source video plays at 24 fps for 70.09 s ≈ 1682 frames; observed 1685 frames matches within rounding (chromium counts the DROPS_TRAJECTORY playing-event frame separately).

What the perf reports say kwin was actually doing

For all three reps, the perf samples on kwin_wayland during playback are dominated by event-loop bookkeeping, not by any GL-composite or dmabuf-import path:

Rep	Top symbol(s) at non-trivial %
1	__pi_memcpy_generic (97.18 %) — single memcpy event in a 39-sample run
2	libz.so (37.57 %), call_filldir (31.91 %) — readdir + zlib
3	dbus_message_unref (38.80 %), QUnixEventDispatcherQPA::processEvents (36.72 %), libz.so (23.30 %) — DBus + Qt event loop

Zero samples anywhere in:

• glEGLImageTargetTexture2DOES (the GL EGL image bind path the predecessor's kwin_overlay_subsurface campaign Phase 2 hypothesised would dominate per-frame KWin cost on this hardware)
• panfrost_* (Mesa Panfrost driver routines)
• wp_subsurface_* / WaylandSurface_* (overlay/subsurface protocol handling)
• any Compositor::* / OpenGLBackend::* / OutputLayer::* KWin internal symbols

The total cycle count across all three reps is in the millions, not billions — kwin_wayland was scheduled out for >99 % of the 70 s capture window in every rep.

What this means

The "KWin is the bottleneck" framing the campaign was built around is structurally weakened by these data.

The campaign's load-bearing hypothesis (README.md § 1) was that "the campaign's load-bearing hypothesis is that this plane-allocation freedom translates into measurable browser-video speedup." That hypothesis was built on top of the predecessor's observation that kwin_wayland consumed ~36 % CPU during similar playback, attributable per the predecessor's Phase 2 source-read to per-frame GL composite of NV12 → RGB. Today's A1 reps show kwin_wayland at 0 % median, with no GL-composite work in the perf samples. There is no Wayland-side KWin-induced overhead for the X11 cells to be faster than.

The most likely mechanism (hypothesis, not yet verified)

KWin 6.6.4 (with kwin-fourier 6.6.4-3 patches applied) appears to have engaged its direct-scanout code path for the chrome-window-displaying-video workload. KWin's direct-scanout support has been there for years on the Wayland backend and has been progressively widened: when there's a single visible "top" surface (the chrome window) whose buffer matches a hardware plane's format/modifier capabilities, KWin can hand that buffer to DRM directly without first GL-compositing it into KWin's own framebuffer. The browser's RGB or NV12 dmabuf goes onto Plane 39 (the Primary plane on rockchip-drm RK3568) without any per-frame KWin GPU work.

This is not the wp_subsurface route the predecessor was investigating — it's a different, simpler scanout path that doesn't require the client to opt into overlay protocol. It just requires the client's surface to be the only visible non-trivial top-level plus a buffer format/modifier that DRM can scan out.

If this hypothesis is correct, two things follow:

1. The campaign's X11-vs-Wayland delta is much smaller than originally expected. Both sessions can avoid per-frame compositor work for the single-window video case. The X11 cells will not be measurably faster than Wayland for this workload.
2. The campaign's mechanism is realised under Wayland too, when conditions are right. "Plane-allocation freedom" is not X11-exclusive — it's just easier to engage on X11 because there's no compositor in the path at all. On Wayland, KWin engages an equivalent fast-path when its heuristics allow.

What the data does NOT establish

• That direct-scanout is the cause. The data is consistent with direct-scanout but a perf-only diagnosis can't pin it down. Phase 1 should add drm_info snapshots during playback (which plane is programmed with the chrome window's buffer FOURCC + modifier?) and KWin debug logging (KWIN_DRM=1 dumps direct-scanout decisions) to confirm.
• That this behavior holds for multi-window scenarios. A single visible non-trivial top-level window is the simplest case for direct-scanout. If the operator works multi-windowed (panel + chrome + terminal + Konsole), the fast-path may decline and kwin %CPU may rebound. The matrix's relevance to daily-driver scenarios depends on this.
• That this behavior holds across browsers. chromium-fourier 149 specifically has the patches that enable smooth NV12 dmabuf production. Brave 147 stock and Firefox 150 may produce buffers in different shapes (RGB-pre-composited, different modifier) that don't satisfy the direct-scanout predicate. Phase 1 reps for those browsers will tell.

Cross-check: predecessor's same-condition reps

For instrument sanity (NOT as comparison target — these are the predecessor's kwin_timing_nodebug_rep[1-3] numbers from 2026-05-02/03):

	Predecessor median	This campaign median
frames_total	1688	1685
drops_total	44	0
drops_post_warmup	28	0
kwin %CPU median	35.9	0.00

The frames_total match indicates the test page + chromium-fourier emit at the same rate in both campaigns. The drops + kwin %CPU divergence is too large to be measurement noise — something about the runtime conditions changed between 2026-05-02 (predecessor's reps) and 2026-05-03 14:23 (today's reps), even though the package versions are identical and the test page is the same file.

Possible non-package causes worth listing here so a follow-up can investigate (out of A1 scope):

• Boot generation: predecessor's reps were on a session that had been running ~9 hours; today's reps were on a session that had been running ~50 minutes after autologin (revert.log entry 6).
• Cumulative session state: predecessor's session likely had multiple browser instances and other windows open during the campaign's preceding work; today's session was freshly autologin'd from greeter, only the test chrome window visible.
• Thermal: predecessor's temp_pre.txt for rep 1 isn't in our scope to check (would be predecessor data import); but today's reps had cpu-thermal at 36 °C pre-rep, well below thermal-throttle thresholds.
• kwin / qt patches state: packages identical per pacman -Q, but the runtime state of KWin's heuristics (window-rule cache, scanout-decision history) might differ between sessions. This is a normal property of compositors and explains some run-to-run variance even on the same binary.

The discipline rule already required the in-session re-measurement this campaign just did. The predecessor's number is no longer the reference; this campaign's measured median (0 drops, 0 % kwin) is the reference for any X11 cells the campaign will later compare against.

Phase 1 implications

The matrix design needs revisiting before Phase 1 cells lock:

1. The mpv --vo=xv cell remains the most informative single point for the campaign's original mechanism (does the X server route NV12 to Plane 39 directly?), per the browser overlay inventory's verdict.
2. The browser X11 cells become a measurement of "do browsers under X11 get the equivalent direct-scanout benefit they get under Wayland?" rather than the original "does X11 win over Wayland?" framing. Three plausible outcomes:
   • X11 cells match Wayland baseline (both engage direct scanout) → "compositor-or-not is irrelevant for the single-window case on this hardware"
   • X11 cells slightly faster than Wayland (X11 path has less per-frame X protocol overhead) → small but real win for X11 daily-driver
   • X11 cells slower than Wayland (X11 path has issues KWin's direct-scanout doesn't) → unexpected; would need re-investigation
3. A multi-window variant of the with-KWin baseline should be added before Phase 1 binding-cell lock — otherwise the matrix only measures the easiest scenario. Suggested add: A1' rep with chrome + Konsole + Plasma panel all visible, see if kwin %CPU rebounds. If it does, the matrix's daily-driver-relevance picture is more nuanced.

The campaign continues with the matrix as defined, but with the understanding that the original framing is partially invalidated. Phase 1 will lock around the reframed sub-questions in phase0_evidence/browser_overlay_inventory_2026-05-03.md § "Implications for the matrix" + the multi-window add above.

Files in this evidence dir

a1_rep1/  a1_rep2/  a1_rep3/   — three rep evidence dirs
01_live_session.txt              — Wayland session state at A1 capture time
02_predecessor_assets.txt        — verification that predecessor scripts/assets reusable
a1_protocol.md                   — protocol spec, run beforehand
a1_summary.md                    — this file

Each rep directory contains:

• start.txt / end.txt / capture_start.txt — wall-clock
• temp_pre.txt / temp_post.txt — cpu-thermal temp
• top_kwin.txt — kwin_wayland top samples (70 × 1 Hz)
• top_full.txt — system top samples (70 × 1 Hz)
• stderr.log — chromium stderr (full)
• drops_trajectory.txt — DROPS_TRAJECTORY lines (73 each)
• drops_summary.txt — frames_total / drops_total / drops_post_warmup
• kwin_cpu_summary.txt — kwin %CPU stats
• perf_record_stderr.txt — perf recorder's own stderr
• perf_report_self.txt / perf_report_top50.txt — perf flamegraph (text)

perf.data files (~400 KB each) are root-owned on ohm (created by sudo perf record) and were not synced to noether. They remain at /home/mfritsche/phase3_prime_runs/x11research_a1_rep[1-3]/perf.data on ohm if re-analysis is needed.