Breaks the one process = one server = one session = one context equation
that the whole stack's correctness used to rest on. Design and per-slice
shape: docs/Disaggregated/design/11-state-ownership.md.
S1 wire: bind_context ring record (opcode 84) announces the current
context at the make-current edge; CreateContext/DestroyContext join the
SurfaceOp control frames; control revision 5 -> 6; the server keeps a
per-session token table and refuses unknown tokens by name.
S2 server: SessionRuntime per session with thread-scoped resolution
(singletons become fallbacks), per-thread gPipeInputs, per-session latch
domains, per-thread segment resolvers; the in-process unix supervisor
serves up to MOBILEGL_IPC_INPROC_MAX_SESSIONS (16) concurrent sessions;
surface mode is declared per session and the display lease refuses a
second on-screen session by name.
S3 frontend: ShareGroupState owns the shared object stores and name
generators (buffer/texture/renderbuffer/sampler/program), GLContext keeps
per-context bindings/errors/container objects; pGLContext becomes
thread-local with a process default context preserving the single-context
world verbatim; eglCreateContext builds a real GLContext and joins the
sharing context's group.
S4 backend: native EGL tuples keyed by (session, context token);
InitDisplayAndContext's unconditional teardown is gone - creating a
second context or surface no longer destroys the first; eglCreateContext
sharing is wired to the native driver; bind_context is the native switch
point.
S5 applier: MGPipeApplierState splits into MGPipeObjectRecords (per
share group) and MGPipeWorkingState (per context), resolved through a
{session, token} registry with a thread-local cached fast path.
Tests: MultiSessionTest (incl. two sessions x two contexts interleaved
with real clear/readback witnesses), ApplierOwnershipTest (with a
null-resolver negative control), ContextStateOwnershipTest,
ServerLoopEglTest native-context liveness; 99/99 of the acceptance set
green on the host build.
Known debts (documented in the design doc): twin registries still keyed
by handle alone, no per-record ContextSlot, process-global backend
generations, MakeCurrent dedup per current binding.
Wayland Present() failure now returns EGL_BAD_SURFACE instead of being
dropped. In spawn mode a GLX X drawable is backed by a remote pbuffer so
the XID is never mistaken for an Android ANativeWindow; X11 pixel present
and remote resize are still unimplemented.
The whole stack assumes one process = one server = one session = one
context = one share group. The anland unified server needs that equation
broken in the library instead of fork-per-app workers. This document fixes
the four ownership layers (session, context, share group, thread current),
the wire changes (per-record context slot, CreateContext/DestroyContext
SurfaceOps, context-tagged reverse events), and the staged plan P14a-d
with acceptance tests.
A Wayland client's window is its own wl_surface, and what the compositor shows is the wl_buffer
attached to it - but the pixels of a MobileGL frame are drawn where the backend is, on a split
build in another process on another OS. So far a wl_egl_window was taken for an X11 window and
its client-side address sent to the Android server (EGL_BAD_NATIVE_WINDOW at best), and under
glvnd a Wayland display never reached this vendor at all.
A window on a Wayland display - asked for as EGL_PLATFORM_WAYLAND, or a wl_display recognised by
its first word being wl_display_interface - is now a window surface in the EGL state at the
wl_egl_window's size with a pbuffer of that size behind it in the backend, whatever
MOBILEGL_IPC_SURFACE says. At every swap the frame is read back out of the default framebuffer
through this library's own glReadPixels (the application's read framebuffer, pack buffer and pack
state put back as they were), its rows flipped and swizzled into a wl_shm buffer, and attached,
damaged and committed on the application's wl_surface. Three buffers rotate on their release
events, dispatched on an event queue of the surface's own. libwayland-client is loaded at run
time, so neither the build nor the library depends on it. EGL_KHR_platform_wayland and
EGL_EXT_platform_wayland are announced on Linux.
Measured: glmark2-wayland runs its build scene at 13 FPS (400x300) and the window shows the frame
under kwin. A window resize is not followed yet; the surface keeps its creation size.
Two defects, both found by asking the client directly rather than by reading its log.
The backend refused to initialize any EGLDisplay but the first - "already bound to a different
EGLDisplay" - and the field it compared against has no reader anywhere else in the tree: the domain
rule was the whole of the rule. What it cost was a compositor, which asks for the display it will
draw into AFTER probing another one. The probe succeeded, the real display was refused, and the
caller reported the refusal one layer up as "no usable DRM render device" - a message that names
where it stopped, not why.
The second: glvnd asks a vendor for entry points from two places. getProcAddress is how it builds
its own static EGL table, and getDispatchAddress is how it answers eglGetProcAddress for every name
it does not know itself - which is every extension entry point, the device queries among them. This
side answered the second from a GL-only table, so an application asking for eglQueryDeviceStringEXT
got a null for a function the vendor implements.
On a system with libglvnd the libEGL.so.1 an application links is a dispatcher, not an
implementation, and the supported way to be the implementation it talks to is to be a vendor
library it loads and routes to. On this machine it is also the ONLY way that reaches a caller which
does not link EGL directly: kwin goes through libepoxy, which dlopen()s libEGL.so.1 and dlsyms its
entry points at startup, so a library injected with LD_PRELOAD is invisible to it however loudly it
is named - measured, with the client in kwin's maps and not one call into it.
So this is the vendor half of that ABI: __egl_Main answers glvnd with the entry points by name, the
platforms this implementation serves, and the device it is associated with - the render node of the
machine, because that is what lets a compositor match its own node to the display it is about to
draw into, and because glvnd drops a vendor that does not answer at all.
Two details are the difference between a vendor that is loaded and a vendor that is used. glvnd
requires setDispatchIndex to exist even when a vendor has nothing to record, which this one has -
its entry points are the implementation, they never re-enter glvnd. And a vendor must hand over ITS
OWN entry points: with default visibility a reference inside this library resolves against the whole
process, where another libEGL that loaded first wins. That is what -Bsymbolic-functions is for;
declaring the entry points protected says the same thing but does not compile against the platform
headers, which declare them default.
A vendor string and an eglQueryString answer have to agree, and they are read by two different
parties: the system's libglvnd reads the first to decide which vendor a platform request should be
routed to, and an application reads the second before it decides to make that request at all. Kept
apart they drift, and the drift has a signature - a library that is loaded, initialized, and never
called, while the caller reports that it found no provider for a function it never reached.
The list gains the device family for the same reason: a compositor asks which devices exist and
reads the DRM node off one before it asks for a display on it, so an implementation that answers
"no devices" is one it cannot use no matter how well the implementation draws.
Two MIT headers from libglvnd, verbatim, because MobileGL is about to implement the vendor half of
glvnd's EGL ABI and the distribution ships the runtime without them.
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