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chromium-fourier: - patch 3/3 nv12-external-oes-on-modifier-external-only.patch — adds NativePixmapEGLBinding::ModifierRequiresExternalOES helper, extends OzoneImageGLTexturesHolder::GetBinding to honor EGL external_only flag for NV12 dmabufs on panfrost / panthor. Validated on ohm (RK3566 hantro mainline 6.19.10): bbb_1080p30_h264.mp4 plays at 34.7 % combined CPU vs ~131 % pre-patch baseline (~3.8x). - PKGBUILD pkgrel 1->2, source array + sha256sums + prepare() hook for patch 4, patch numbering 1/2,2/2 -> 1/3,2/3,3/3. - NEXT.md appended with 2026-04-28 section: patch 4 design, validation log, KWin GL_ALPHA bug pinpoint (preexisting since 2026-03-06, affects every wayland video client; unrelated to chromium-fourier), device-renumbering note (/dev/video1 = encoder post-reboot). - KWIN_PIVOT.md: 4-phase plan to identify and patch KWin's glTexImage2D(internalFormat=GL_ALPHA) site, ohm-only test plan, scope discipline. - patches/ now tracked (compiler-rt-adjust-paths, enable-v4l2, wayland-allow-direct-egl-gles2, nv12-external-oes); the dead-end chromeos-pipeline-bypass.patch removed. firefox-fourier: - 4 patches (gfxinfo v4l2 stateless fourccs, libwrapper hwdevice ctx, ffmpegvideo v4l2-request route, prefs v4l2-request default). - PKGBUILD bumped to firefox 150.0.1, Arch toolchain glue patches layered in, mozconfig with --without-wasm-sandboxed-libraries for ALARM, package() launcher fix (rm -f symlink before cat > to avoid ENOENT through the dangling /usr/local symlink mach install drops). - 150.0.1-1-aarch64.pkg.tar.zst built on boltzmann (95 MB), pending fresnel power-on for V4L2 stateless validation on RK3399.
182 lines
8.6 KiB
Markdown
182 lines
8.6 KiB
Markdown
# KWin pivot — fix the `glTexImage2D(GL_ALPHA)` stall
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## What we know
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KWin 6.6.4-1 on Arch Linux ARM (Plasma 6.6.4-1, mesa 26.0.5-1, libdrm
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2.4.131-1) on ohm (PineTab2 / RK3566 / panfrost) silently corrupts its
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GL command queue mid-frame whenever a wayland client posts a video
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buffer. The journal carries a rolling stream of:
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```
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kwin_wayland: 0x4: GL_INVALID_VALUE in glTexImage2D(internalFormat=GL_ALPHA)
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kwin_wayland: 0x4: GL_INVALID_OPERATION in glTexSubImage2D(invalid texture level 0) × N
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```
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`GL_ALPHA` is not a valid `internalFormat` for `glTexImage2D` under
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**OpenGL ES 3.x** (it was the GLES1.x single-channel alpha format;
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GLES3 deprecates it for sized formats — `GL_R8`, `GL_LUMINANCE8_ALPHA8`,
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etc.). Once the texture allocation fails, the `glTexSubImage2D` calls
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that should populate it all error at level 0. KWin keeps retrying the
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same broken upload every frame, never recovers, and the present-callback
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path that depends on that texture stops acking client frames. Every
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wayland video client deadlocks on the missing ack.
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First occurrence in this box's journal: **2026-03-06** — the bug
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predates any chromium-fourier work by roughly seven weeks.
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## Triangulation already in hand
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| Client | Outcome |
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|---|---|
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| chromium-fourier 149-r2 (with patch 3/3) | plays ~3 s @ 34.7 % CPU then renderer/GPU park in `futex_do_wait` |
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| chromium-fourier 149-r2 (without patch 3/3) | plays ~10 s (slower path delays surfacing) then identical deadlock |
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| VLC | `cannot convert decoder/filter output to any format supported by the output` → `could not initialize video chain` |
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| mpv `--vo=null --hwdec=v4l2request` | `Could not create device.` (mpv-side bug, separate, unrelated) |
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| ffmpeg `-hwaccel v4l2request -i bbb -f null -` | plays through clean at 36 fps; hardware path is healthy |
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Decode path is healthy on this hardware. The wall is exclusively the
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compositor's GL backend.
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## Constraint: ohm is the only test box on hand
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ampere (RK3588 / panthor) is in the boxes-from-Shenzhen pile, currently
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DOWN. fresnel (RK3399 / Pinebook Pro) is offline. boltzmann (Rock 5
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ITX+ build host) doesn't run KWin. We do every step on ohm; we accept
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the wifi flakiness and the occasional reboot.
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## Phase 1 — Reproduce outside chrome and bound the trigger (1 evening)
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Goal: a deterministic, headless-or-near-headless reproduction that
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doesn't require launching a 800-MB browser.
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1. **Smallest-possible client.** Build a 50-line C wayland client that
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creates a `wp_linux_dmabuf_v1` buffer, pumps frames at 30 fps, and
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exits when KWin first errors. Use `weston-simple-dmabuf-egl` from
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the `weston` package as a starting template — already does exactly
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this but without our specific format/modifier matrix.
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2. **Vary the format/modifier matrix.** Run the smallest-possible
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client with each of: NV12 + LINEAR, NV12 + AFBC, NV12 + AFRC,
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AR24 + LINEAR, XR24 + LINEAR. We already know NV12 paths trigger;
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confirming AR24/XR24 do *not* trigger localizes the bug to KWin's
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YUV import path (vs a generic dmabuf import bug).
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3. **Vary the buffer dimensions.** Some KWin texture-cache paths
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allocate fixed-size internal scratch textures; non-power-of-two,
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non-multiple-of-16, or specifically odd-aspect cases sometimes
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trigger paths that healthy aspect ratios skip. Test 1920×1080,
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1280×720, 854×480, 640×360 and a deliberately weird 1366×768.
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4. **Vary KWin scene type.** Switch
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`kwin_wayland --scene-type=opengl` vs `--scene-type=opengl-es`
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(current default on this hardware). If the bug only fires under
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GLES, that's a strong signal — the offending site is in a
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GLES-only fallback.
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By the end of Phase 1 we should have a one-line `weston-simple-dmabuf-egl
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-format=NV12 -modifier=…` that triggers the GL_ALPHA error within
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seconds, plus a yes/no answer to "does AR24 also trigger".
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## Phase 2 — Identify the call site (1–2 evenings)
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The crime scene is somewhere in `kwin/src/scene/*` or
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`kwin/src/effects/*`. Suspects, ranked:
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- **`SurfaceItemWayland::createPixmapTexture` → `GLTexture::create`
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with `GL_ALPHA`.** This is the most likely path: KWin allocates a
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fallback per-plane texture when the dmabuf import path can't take
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the buffer whole. NV12 has a Y plane (single-channel) and a CbCr
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plane (two-channel); historically the Y plane has been allocated as
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`GL_ALPHA` in software fallbacks. If the EGL dmabuf import returned
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`EGL_BAD_ATTRIBUTE` for `external_only` modifiers and KWin fell
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through to per-plane, this is exactly where it would land.
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- **`BlurEffect::initBlurTexture` / `BackgroundContrastEffect::*`.**
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Single-channel noise textures for blur dither. Less likely (these
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fire on every frame regardless of video clients) but listed for
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completeness.
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- **Window-decoration text glyph cache.** Qt's QGLTexture historically
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requested `GL_ALPHA` for monochrome glyph atlases. Plasma 6 should
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have moved to `GL_RED` long ago, but a stale code path in a
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third-party theme or systray icon could still hit it.
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- **Cursor texture upload via `wl_shm_pool` + ARGB8888.** KWin's
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cursor scene sometimes uploads via glTexImage2D — but the format
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there is `GL_RGBA`, not `GL_ALPHA`. Probably not the suspect.
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Tooling to identify *which*:
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1. **`apitrace trace --api egl kwin_wayland …`** then
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`apitrace dump trace.trace | grep -B5 GL_ALPHA`. Apitrace gives
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us the C++ call stack at the offending site if KWin was built with
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debug symbols.
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2. **`MESA_GL_DEBUG=context KWIN_GL_DEBUG=1 kwin_wayland --replace`**
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plus `glDebugMessageCallback` already installed in KWin's
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`OpenGLBackend` will print the source/type/severity for each
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`GL_INVALID_VALUE`. Whether the file/line in the message includes
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the user-space caller depends on Mesa's debug-extension support;
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on panfrost it usually does include the GL function name and an
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ID, but not the C++ source — that is what apitrace adds.
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3. **Build kwin from source** (`extra/kwin` PKGBUILD on Arch ARM,
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patch in `-DDEBUG=ON`, `-DCMAKE_BUILD_TYPE=Debug`) so the call
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stacks resolve to file:line.
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## Phase 3 — Write the patch (½ evening once Phase 2 is done)
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If the offender is a `GL_ALPHA` allocation in a GLES3 context, the
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fix is mechanical:
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```diff
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- glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA, width, height, 0,
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- GL_ALPHA, GL_UNSIGNED_BYTE, data);
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+ glTexImage2D(GL_TEXTURE_2D, 0, GL_R8, width, height, 0,
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+ GL_RED, GL_UNSIGNED_BYTE, data);
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```
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…and adjust the consuming shader's swizzle:
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```diff
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- gl_FragColor = vec4(texture2D(s, uv).a, …);
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+ gl_FragColor = vec4(texture2D(s, uv).r, …);
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```
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If the offender is a per-plane fallback in the dmabuf import path
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(suspect #1 above), the patch is larger because the right fix is to
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*not fall through to the broken path* — handle the `external_only`
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case by binding `GL_TEXTURE_EXTERNAL_OES` instead. That mirrors the
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chromium-fourier patch 3/3 done at the chromium layer; symmetry says
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KWin should do the same in its `glTexImage` consumer.
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## Phase 4 — Ship and upstream (1 evening)
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1. **Local Arch package** as `kwin-fourier` under
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`marfrit-packages/arch/kwin-fourier/`, sibling to chromium-fourier
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and firefox-fourier. PKGBUILD inherits from `extra/kwin`, drops
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in our patch, bumps `pkgrel`. Same `provides=kwin conflicts=kwin`
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pattern.
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2. **Validate on ohm** by running the chromium-fourier 149-r2 build +
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the bbb sample for a minute uninterrupted. Success = no GL_ALPHA
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in the journal, no stall, smooth playback at the 34.7 % CPU
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number from the chromium validation.
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3. **Upstream** via:
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- File a `kwin` bug on bugs.kde.org with: apitrace fragment, our
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hardware (Mali-G52 panfrost on RK3566 mainline), exact mesa
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version, repro steps via `weston-simple-dmabuf-egl` if Phase 1
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produced one.
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- Push an MR to invent.kde.org/plasma/kwin against `master`.
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4. **Document** the fix in `chromium-fourier/docs/dmabuf-zero-copy.md`
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so the next person who lands on the same wall finds the breadcrumb
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trail.
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## What success looks like
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`chromium-fourier-149-r2` on ohm under KWin Wayland plays
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`bbb_1080p30_h264.mp4` end-to-end at the 34.7 % CPU figure already
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recorded by the architectural validation, with zero `GL_INVALID_VALUE`
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in the journal during playback. That number is the goal of the entire
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chromium-fourier campaign for RK3566 — it is currently blocked on a
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bug that has nothing to do with chromium.
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## Scope discipline
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We do not turn this into "audit the entire KWin GLES backend." If
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Phase 2 surfaces additional latent GL_INVALID_* errors that don't
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matter for video playback, we note them in the bug report and move
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on. The pivot is explicitly "remove this single wall so the
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chromium-fourier patch series can ship a working stack on RK3566."
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