[kernel] Let endpoints get interrupt notifications

- Add a tag field to all endpoint messages, which doubles as a
  notification field
- Add a endpoint_bind_irq syscall to enable an endpoint to listen for
  interrupt notifications. This mechanism needs to change.
- Add a temporary copy of the serial port code to nulldrv, and let it
  take responsibility for COM2

src/kernel/objects/endpoint.cpp

@@ -1,10 +1,11 @@
+#include "device_manager.h"
 #include "objects/endpoint.h"
 #include "objects/process.h"
 #include "objects/thread.h"
 #include "vm_space.h"
 
 endpoint::endpoint() :
-	kobject(kobject::type::endpoint)
+	kobject {kobject::type::endpoint}
 {}
 
 endpoint::~endpoint()
@@ -17,15 +18,20 @@ void
 endpoint::close()
 {
 	assert_signal(j6_signal_endpoint_closed);
-	for (auto &data : m_blocked)
-		data.th->wake_on_result(this, j6_status_closed);
+	for (auto &data : m_blocked) {
+		if (data.th)
+			data.th->wake_on_result(this, j6_status_closed);
+	}
+
+	device_manager::get().unbind_irqs(this);
 }
 
 j6_status_t
-endpoint::send(size_t len, void *data)
+endpoint::send(j6_tag_t tag, size_t len, void *data)
 {
 	thread_data sender = { &thread::current(), data };
 	sender.len = len;
+	sender.tag = tag;
 
 	if (!check_signal(j6_signal_endpoint_can_send)) {
 		assert_signal(j6_signal_endpoint_can_recv);
@@ -48,9 +54,10 @@ endpoint::send(size_t len, void *data)
 }
 
 j6_status_t
-endpoint::receive(size_t *len, void *data)
+endpoint::receive(j6_tag_t *tag, size_t *len, void *data)
 {
 	thread_data receiver = { &thread::current(), data };
+	receiver.tag_p = tag;
 	receiver.len_p = len;
 
 	if (!check_signal(j6_signal_endpoint_can_recv)) {
@@ -69,20 +76,54 @@ endpoint::receive(size_t *len, void *data)
 
 	// TODO: don't pop sender on some errors
 	j6_status_t status = do_message_copy(sender, receiver);
-	sender.th->wake_on_result(this, status);
+
+	if (sender.th)
+		sender.th->wake_on_result(this, status);
+
 	return status;
 }
 
+void
+endpoint::signal_irq(unsigned irq)
+{
+	j6_tag_t tag = j6_tag_from_irq(irq);
+
+	if (!check_signal(j6_signal_endpoint_can_send)) {
+		assert_signal(j6_signal_endpoint_can_recv);
+
+		for (auto &blocked : m_blocked)
+			if (blocked.tag == tag)
+				return;
+
+		thread_data sender = { nullptr, nullptr };
+		sender.tag = tag;
+		m_blocked.append(sender);
+		return;
+	}
+
+	thread_data receiver = m_blocked.pop_front();
+	if (m_blocked.count() == 0)
+		deassert_signal(j6_signal_endpoint_can_send);
+
+	*receiver.len_p = 0;
+	*receiver.tag_p = tag;
+	receiver.th->wake_on_result(this, j6_status_ok);
+}
+
 j6_status_t
 endpoint::do_message_copy(const endpoint::thread_data &sender, endpoint::thread_data &receiver)
 {
 	if (sender.len > *receiver.len_p)
 		return j6_err_insufficient;
 
-	vm_space &source = sender.th->parent().space();
-	vm_space &dest = receiver.th->parent().space();
-	vm_space::copy(source, dest, sender.data, receiver.data, sender.len);
+	if (sender.len) {
+		vm_space &source = sender.th->parent().space();
+		vm_space &dest = receiver.th->parent().space();
+		vm_space::copy(source, dest, sender.data, receiver.data, sender.len);
+	}
+
 	*receiver.len_p = sender.len;
+	*receiver.tag_p = sender.tag;
 
 	// TODO: this will not work if non-contiguous pages are mapped!!