Bounded GPU validation

The initial failure and corrected run were captured on the physical board on 2026-09-22 while the shell, seatd, and persistent Wi-Fi remained active. Their service invocation IDs match before and after the corrected run.

initial-format-failure.txt: the vendor VG_LITE_RGB565 layout reversed red/blue relative to DRM/Pixman RGB565; exact checks failed as intended.
corrected-format-pass.txt: source at 8850144, executable /nix/store/78mysi1vsrk7rzgikwggqkx87dpvjkim-k230-vglite-validation-riscv64-unknown-linux-gnu-0-1104236db4d1e47873bd68924f912747b820228c/bin/k230-vglite-validation. Vendor VG_LITE_BGR565 matches the DRM/Pixman memory layout. RGB565 sampled scale output, one premultiplied-alpha case, and a private DRM dumb-buffer export/import with a full CPU pixel check pass. The probe does not modeset.

The matched full-panel nearest-scale case measured 1.969 ms per GPU operation with a finish, 1.876 ms batched, and 1.950 ms for Pixman. These single-run wall times do not establish a useful speedup. The follow-up three-round process-CPU results are recorded below. Broad RGBA fidelity and live compositor/scanout integration remain unproven. Sway still uses Pixman.

The corrected transcript removes console command echo, prompts and ANSI formatting; the marked command output is retained. Neither a passed private buffer test nor an alpha sample proves general compositor compatibility.

Three-round completion and integration decision

cpu-timing-pass.txt records the final four-subtest run on the same physical board, source revision 576471b. The optional package was rebuilt with nix build .#k230-vglite-validation --max-jobs 1 --cores 4 --no-link --print-out-paths and resolved to:

/nix/store/sd5qy0s03hzy8mqf507bi9zx586wrnxq-k230-vglite-validation-riscv64-unknown-linux-gnu-0-1104236db4d1e47873bd68924f912747b820228c

The source-built closure was transferred over the existing Wi-Fi link, its export SHA256 checked, and imported into the running Nix store. It was not added to the system closure. The board still ran system /nix/store/0sbdslp3na1y1bkjm30wdz24ivcn6rgd-nixos-system-nixos-26.11.20260919.20b1ddd. Boot ID was 4f33a8c4-5824-4998-913c-c1307defa6c5; Linux had one online CPU. There was no video player running. No reboot, service restart, display commit, permission change, or image flash was performed.

The coordinator ran each rgb565, alpha, dmabuf, and benchmark mode with timeout --signal=TERM --kill-after=3s 25s <package>/bin/k230-vglite-validation "$mode" /dev/dri/card0. Every mode exited zero. The console records systemctl is-active shell seatd k230-wifi and each unit’s InvocationID before/after; all remain active with identical IDs. The probe source calls only allocation, PRIME export, mapping, and destruction DRM APIs: it does not request master, modeset, add a framebuffer, or submit a plane/atomic update. The shell retained display ownership.

Median of three successive rounds, milliseconds per operation:

Geometry	GPU finish wall	GPU finish process CPU	GPU batch wall	GPU batch process CPU	Pixman wall	Pixman process CPU
128x128 → 256x256	0.192292	0.031133	0.157448	0.004690	0.170406	0.163621
284x616 → 568x1232	1.968375	0.101825	1.882701	0.017740	1.781209	1.704792

Each GPU-finish round contains 20 operations; batched GPU and Pixman each contain 200. Validation and warmups precede timing. Both implementations use the same nonuniform quadrant pattern, RGB565 output, and nearest-neighbor 2x transform. Sixteen boundary/corner samples match before timings are accepted. The imported dumb-buffer test additionally checks every CPU-visible pixel.

The full-panel per-operation GPU path used about 94% less process CPU time while taking about 11% longer elapsed time than Pixman. This is repeatable CPU offload in this workload, not a frame-rate improvement. Process CPU time excludes interrupt work and other processes. Repeated operations use private warm buffers; these numbers omit application-buffer import/copy, general texture operations, scanout synchronization and compositor integration costs. Batched throughput does not describe per-frame interactive latency.

Decision: keep Pixman in the default shell and pursue a separate opt-in renderer experiment. The CPU result justifies that experiment, but switching renderers now is blocked by unproved compositor buffer import/cache/fence ownership, general RGBA fidelity, complete renderer operations, and physical scanout/regression/performance evidence. An exact premultiplied blend sample is not a full alpha/color conformance suite. A successful private DRM-buffer import is not proof of scanout or video zero-copy. The follow-up must test these boundaries and retain a working Pixman fallback before any default change. The bounded validation proposal itself is complete.