arm: more uart async & debug stuff

arm/async.h

@@ -30,6 +30,64 @@ struct task_final_suspend {
     std::coroutine_handle<> parent;
 };
 
+template <typename T>
+struct gimme {
+    // child interface
+    bool await_ready() { return false; }
+    void await_suspend(std::coroutine_handle<> h) {
+        ha = h;
+        waiting = true;
+        TRACE(tracing::TraceEvent::kAsyncGimmeWaiting);
+
+        if (parent) {
+            TRACE(tracing::TraceEvent::kAsyncCallParent);
+            parent.resume();
+            TRACE(tracing::TraceEvent::kAsyncCallParentDone);
+        }
+    }
+    T await_resume() {
+        waiting = false;
+        TRACE(tracing::TraceEvent::kAsyncGimmeResume);
+        return std::move(stuff);
+    }
+
+    // parent interface
+    auto feed(T&& s) {
+        struct awaitable {
+            bool await_ready() {
+                if (g.waiting) {
+                    return true;
+                }
+                // TODO: handle g.ha.done()
+                return false;
+            }
+            void await_suspend(std::coroutine_handle<> h) {
+                g.parent = h;
+            }
+            void await_resume() {}
+
+            gimme<T>& g;
+        };
+
+        if (!waiting) {
+           __builtin_trap();
+        }
+        if (!ha) {
+           __builtin_trap();
+        }
+        stuff = s;
+        parent = {};
+        ha.resume();
+
+        return awaitable{.g = *this};
+    }
+
+    bool waiting = false;
+    std::coroutine_handle<> ha;
+    T stuff;
+    std::coroutine_handle<> parent;
+};
+
 template <typename T = void>
 struct task;