cam: event_loop: Add deferred calls support

Add a deferred cals queue to the EventLoop class to support queuing
calls from a different thread and processing them in the event loop's
thread.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

src/cam/event_loop.cpp

@@ -40,8 +40,10 @@ int EventLoop::exec()
 	exitCode_ = -1;
 	exit_.store(false, std::memory_order_release);
 
-	while (!exit_.load(std::memory_order_acquire))
+	while (!exit_.load(std::memory_order_acquire)) {
+		dispatchCalls();
 		event_base_loop(event_, EVLOOP_NO_EXIT_ON_EMPTY);
+	}
 
 	return exitCode_;
 }
@@ -57,3 +59,28 @@ void EventLoop::interrupt()
 {
 	event_base_loopbreak(event_);
 }
+
+void EventLoop::callLater(const std::function<void()> &func)
+{
+	{
+		std::unique_lock<std::mutex> locker(lock_);
+		calls_.push_back(func);
+	}
+
+	interrupt();
+}
+
+void EventLoop::dispatchCalls()
+{
+	std::unique_lock<std::mutex> locker(lock_);
+
+	for (auto iter = calls_.begin(); iter != calls_.end(); ) {
+		std::function<void()> call = std::move(*iter);
+
+		iter = calls_.erase(iter);
+
+		locker.unlock();
+		call();
+		locker.lock();
+	}
+}

src/cam/event_loop.h

@@ -8,6 +8,9 @@
 #define __CAM_EVENT_LOOP_H__
 
 #include <atomic>
+#include <functional>
+#include <list>
+#include <mutex>
 
 struct event_base;
 
@@ -22,6 +25,8 @@ public:
 	int exec();
 	void exit(int code = 0);
 
+	void callLater(const std::function<void()> &func);
+
 private:
 	static EventLoop *instance_;
 
@@ -29,7 +34,11 @@ private:
 	std::atomic<bool> exit_;
 	int exitCode_;
 
+	std::list<std::function<void()>> calls_;
+	std::mutex lock_;
+
 	void interrupt();
+	void dispatchCalls();
 };
 
 #endif /* __CAM_EVENT_LOOP_H__ */