Kernel/Devices: Introduce the Device Control Device

This device will assist userspace to manage hotplug events.
A userspace application reads a DeviceEvent entry until the return value
is zero which indicates no events that are queued and waiting for
processing.
Trying to read with a buffer smaller than sizeof(DeviceEvent) results in
EOVERFLOW.
For now, there's no ioctl mechanism for this device but in the future an
acknowledgement mechanism can be implemented via ioctl(2) interface.

Kernel/Devices/DeviceControlDevice.cpp

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2021, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/Devices/DeviceControlDevice.h>
+#include <Kernel/Devices/DeviceManagement.h>
+
+namespace Kernel {
+
+UNMAP_AFTER_INIT NonnullRefPtr<DeviceControlDevice> DeviceControlDevice::must_create()
+{
+    auto device_control_device_or_error = DeviceManagement::try_create_device<DeviceControlDevice>();
+    // FIXME: Find a way to propagate errors
+    VERIFY(!device_control_device_or_error.is_error());
+    return device_control_device_or_error.release_value();
+}
+
+bool DeviceControlDevice::can_read(const OpenFileDescription&, size_t) const
+{
+    return true;
+}
+
+UNMAP_AFTER_INIT DeviceControlDevice::DeviceControlDevice()
+    : CharacterDevice(2, 10)
+{
+}
+
+UNMAP_AFTER_INIT DeviceControlDevice::~DeviceControlDevice()
+{
+}
+
+ErrorOr<size_t> DeviceControlDevice::read(OpenFileDescription&, u64, UserOrKernelBuffer& buffer, size_t size)
+{
+    auto device_event = DeviceManagement::the().dequeue_top_device_event({});
+    if (!device_event.has_value())
+        return 0;
+
+    if (size < sizeof(DeviceEvent))
+        return Error::from_errno(EOVERFLOW);
+    size_t nread = 0;
+    TRY(buffer.write(&device_event.value(), nread, sizeof(DeviceEvent)));
+    nread += sizeof(DeviceEvent);
+    return nread;
+}
+
+ErrorOr<void> DeviceControlDevice::ioctl(OpenFileDescription&, unsigned, Userspace<void*>)
+{
+    return Error::from_errno(ENOTSUP);
+}
+
+}