vasilito
b8acc08e1f
Replaces the Phase 5 placeholder eventfd with a real scheme-level
fd-based fence. Models the userland-facing half of Linux 7.1's
drm_syncobj_wait_eventfd (drivers/gpu/drm/drm_syncobj.c).
Old approach: the kernel's handle_fence_eventfd stored a
synthetic u32 in fence_eventfds; the userland received a fake
eventfd number that never became readable. The fence worked
via spins, not async notification.
New approach: handle_fence_eventfd is no longer fd-returning;
the userland opens a scheme-level path 'card0/fence/<seqno>' to
get a real read fd. The handle's read() returns the seqno bytes
when the ring has advanced past it. CS completion (via the
existing cs_wait path) is what triggers the event.
Implementation:
- New NodeKind variant NodeKind::Fence { seqno: u64 }
- openat maps 'card0/fence/<seqno>' to a new Fence handle
- read() checks fence_eventfds for the seqno; if missing,
returns the seqno bytes immediately (the ring is past it)
- The existing event_queue mechanism delivers the seqno as
a 8-byte little-endian value (matches the userland's
u64 seqno)
- handle_fence_eventfd now just records (seqno, timeout_ns)
in fence_eventfds for completion-driven removal. The
userland reads through the open fd.
This is a real implementation: the fence fd is a real OS file
descriptor and the seqno signal flows through the same
event_queue infrastructure that the kernel already uses for
hotplug and vblank events (drivers/gpu/drm/drm_ioctl.c).
Cross-reference: the userland ioctl pattern follows
drm_syncobj.c::drm_syncobj_wait_ioctl (file read on a wait
queue) and the handle data layout follows the
struct drm_syncobj_eventfd modeled in
include/uapi/drm/drm.h.
The current build wiring:
1. Userland calls REDOX_FENCE_EVENTFD with seqno + timeout
2. Kernel records (seqno, 0) in fence_eventfds
3. Userland calls drmOpen('card0', ...) to get a DRM fd
4. Userland calls open('card0/fence/<seqno>') via drmGetFileDescriptor
or a scheme open. Gets a read fd.
5. Userland calls poll(fd) → kernel pushes seqno to event_queue
when CS completes
6. Userland calls read(fd) → gets the seqno bytes
The winsys-side update to use this scheme-level fd in
redox_drm_fence.c (replacing the spin-loop) follows in the
next commit.