CNA-GAP-029: EasyGL GPU timers read 32-bit nanosecond results and cannot detect GL disjoint events

CNA snapshot 009d40f5  ·  Known Issues › Functional gaps  ·  source links pinned to 009d40f5

EasyGLGpuTimerRenderer reads GL_TIME_ELAPSED as a 32-bit value because meta-gl exposes only glGetQueryObjectuiv, not a 64-bit query-object read, and it never queries GL_GPU_DISJOINT_EXT, so results saturate near 4.29 s and only an all-ones result is rejected as undefined.

Identifier

CNA-GAP-029

Category

Functional gap

Subsystem

Graphics & renderers

Status

Open

Verified against

CNA 009d40f5 (009d40f5dd085c4e674d3479675fac84b12b3e0a)

Evidence basis

Source-verified: read at TARGET, not executed

Tests touching this area

Yes: see Current tests

Affected contract

CNA::Graphics::GpuTimer and RenderPipeline pass timings (CNAEXT) on the EasyGL GL profiles

Expected behaviour

GL_EXT_disjoint_timer_query results are read as 64-bit values and believed only after GL_GPU_DISJOINT_EXT reports no disjoint event.

Actual behaviour at TARGET

EasyGLGpuTimerRenderer::IsResultAvailable and ElapsedNanoseconds in EasyGLRenderer.cpp read the result with glGetQueryObjectuiv into a 32-bit value and reject only the all-ones saturation value. A reading taken across a disjoint event that is not exactly 0xFFFFFFFF is reported as a duration, and a real pass longer than about 4.29 s saturates. The in-code comments say meta-gl exposes only a 32-bit result read and no general glGetIntegerv, so the disjoint state cannot be asked. The first is true (meta-gl declares only glGetQueryObjectuiv). The second is not: meta-gl declares glGetIntegerv(GetParameter, GLint*) as a plain pass-through, and this same file already casts a raw value into metagl::QueryTarget for GL_TIME_ELAPSED, so GL_GPU_DISJOINT_EXT could be read the same way. That query exists only on the OpenGL ES path (GL_EXT_disjoint_timer_query); desktop GL 3.3 timers come from ARB_timer_query and WebGL reports no timer support.

Source locations

• modules/renderers/easygl/src/EasyGLRenderer.cpp — EasyGLGpuTimerRenderer::IsResultAvailable and ElapsedNanoseconds
• modules/graphics-ext/src/GpuTimer.cpp — the CNAEXT GpuTimer consumer

Evidence

Checked by reading at 009d40f5 and meta-gl develop @ 20c8b2dc in the local checkout (no ui64v read and no disjoint token in include/); not executed. The limitation is imposed by the sibling API; the pass durations the engine layer measures are far below the saturation point.

Independent re-verification: Checked by reading at 009d40f5 and meta-gl develop @ 20c8b2dc in the local checkout (no glGetQueryObjectui64v, no disjoint token, but a typed glGetIntegerv that forwards any value); meta-gl is an unpinned sibling, so its state at the moment of any given build is not fixed by the CNA commit. Not executed. The pass durations the engine layer measures are far below the saturation point.

Focused reproduction

No focused reproduction is known. Nothing has been invented here; the evidence above is what exists.

Current tests

GpuTimerTests.cpp and PassTimingTests.cpp cover the timer contract; nothing injects a disjoint event.

Regression test

Once meta-gl exposes glGetQueryObjectui64v and GL_GPU_DISJOINT_EXT: a test that a disjoint flag invalidates the sample.

Blast radius

GPU pass timings on the EasyGL family only; the first sample after a context change is the known disjoint case. Rendering is unaffected.

Workaround

Treat outlying samples, especially the first after a context change, as invalid.