Split embassy crate into embassy-executor, embassy-util.

1 files changed, 53 insertions, 0 deletions

diff --git a/embassy-executor/src/executor/raw/waker.rs b/embassy-executor/src/executor/raw/waker.rs
new file mode 100644
index 000000000..f6ae332fa
--- /dev/null
+++ b/embassy-executor/src/executor/raw/waker.rs
@@ -0,0 +1,53 @@
+use core::mem;
+use core::ptr::NonNull;
+use core::task::{RawWaker, RawWakerVTable, Waker};
+
+use super::TaskHeader;
+
+const VTABLE: RawWakerVTable = RawWakerVTable::new(clone, wake, wake, drop);
+
+unsafe fn clone(p: *const ()) -> RawWaker {
+    RawWaker::new(p, &VTABLE)
+}
+
+unsafe fn wake(p: *const ()) {
+    (*(p as *mut TaskHeader)).enqueue()
+}
+
+unsafe fn drop(_: *const ()) {
+    // nop
+}
+
+pub(crate) unsafe fn from_task(p: NonNull<TaskHeader>) -> Waker {
+    Waker::from_raw(RawWaker::new(p.as_ptr() as _, &VTABLE))
+}
+
+/// Get a task pointer from a waker.
+///
+/// This can be used as an optimization in wait queues to store task pointers
+/// (1 word) instead of full Wakers (2 words). This saves a bit of RAM and helps
+/// avoid dynamic dispatch.
+///
+/// You can use the returned task pointer to wake the task with [`wake_task`](super::wake_task).
+///
+/// # Panics
+///
+/// Panics if the waker is not created by the Embassy executor.
+pub fn task_from_waker(waker: &Waker) -> NonNull<TaskHeader> {
+    // safety: OK because WakerHack has the same layout as Waker.
+    // This is not really guaranteed because the structs are `repr(Rust)`, it is
+    // indeed the case in the current implementation.
+    // TODO use waker_getters when stable. https://github.com/rust-lang/rust/issues/96992
+    let hack: &WakerHack = unsafe { mem::transmute(waker) };
+    if hack.vtable != &VTABLE {
+        panic!("Found waker not created by the Embassy executor. `embassy_executor::time::Timer` only works with the Embassy executor.")
+    }
+
+    // safety: we never create a waker with a null data pointer.
+    unsafe { NonNull::new_unchecked(hack.data as *mut TaskHeader) }
+}
+
+struct WakerHack {
+    data: *const (),
+    vtable: &'static RawWakerVTable,
+}