Kernel: Introduce the DeviceManagement singleton

This singleton simplifies many aspects that we struggled with before:
1. There's no need to make derived classes of Device expose the
constructor as public anymore. The singleton is a friend of them, so he
can call the constructor. This solves the issue with try_create_device
helper neatly, hopefully for good.
2. Getting a reference of the NullDevice is now being done from this
singleton, which means that NullDevice no longer needs to use its own
singleton, and we can apply the try_create_device helper on it too :)
3. We can now defer registration completely after the Device constructor
which means the Device constructor is merely assigning the major and
minor numbers of the Device, and the try_create_device helper ensures it
calls the after_inserting method immediately after construction. This
creates a great opportunity to make registration more OOM-safe.

Kernel/Devices/Device.cpp

@@ -6,20 +6,13 @@
 
 #include <AK/Singleton.h>
 #include <Kernel/Devices/Device.h>
+#include <Kernel/Devices/DeviceManagement.h>
 #include <Kernel/FileSystem/InodeMetadata.h>
 #include <Kernel/FileSystem/SysFS.h>
-#include <Kernel/Locking/MutexProtected.h>
 #include <Kernel/Sections.h>
 
 namespace Kernel {
 
-static Singleton<MutexProtected<HashMap<u32, Device*>>> s_all_devices;
-
-MutexProtected<HashMap<u32, Device*>>& Device::all_devices()
-{
-    return *s_all_devices;
-}
-
 NonnullRefPtr<SysFSDeviceComponent> SysFSDeviceComponent::must_create(Device const& device)
 {
     return adopt_ref_if_nonnull(new SysFSDeviceComponent(device)).release_nonnull();
@@ -115,41 +108,15 @@ RefPtr<SysFSComponent> SysFSCharacterDevicesDirectory::lookup(StringView name)
     });
 }
 
-void Device::for_each(Function<void(Device&)> callback)
-{
-    all_devices().with_exclusive([&](auto& map) -> void {
-        for (auto& entry : map)
-            callback(*entry.value);
-    });
-}
-
-Device* Device::get_device(unsigned major, unsigned minor)
-{
-    return all_devices().with_exclusive([&](auto& map) -> Device* {
-        auto it = map.find(encoded_device(major, minor));
-        if (it == map.end())
-            return nullptr;
-        return it->value;
-    });
-}
-
 Device::Device(unsigned major, unsigned minor)
     : m_major(major)
     , m_minor(minor)
 {
-    u32 device_id = encoded_device(major, minor);
-    all_devices().with_exclusive([&](auto& map) -> void {
-        auto it = map.find(device_id);
-        if (it != map.end()) {
-            dbgln("Already registered {},{}: {}", major, minor, it->value->class_name());
-        }
-        VERIFY(!map.contains(device_id));
-        map.set(device_id, this);
-    });
 }
 
 void Device::after_inserting()
 {
+    DeviceManagement::the().after_inserting_device({}, *this);
     VERIFY(!m_sysfs_component);
     auto sys_fs_component = SysFSDeviceComponent::must_create(*this);
     m_sysfs_component = sys_fs_component;
@@ -160,21 +127,17 @@ void Device::after_inserting()
 
 void Device::before_removing()
 {
-    m_state = State::BeingRemoved;
     VERIFY(m_sysfs_component);
     SysFSComponentRegistry::the().devices_list().with_exclusive([&](auto& list) -> void {
         list.remove(*m_sysfs_component);
     });
+    DeviceManagement::the().before_device_removal({}, *this);
+    m_state = State::BeingRemoved;
 }
 
 Device::~Device()
 {
     VERIFY(m_state == State::BeingRemoved);
-    u32 device_id = encoded_device(m_major, m_minor);
-    all_devices().with_exclusive([&](auto& map) -> void {
-        VERIFY(map.contains(device_id));
-        map.remove(encoded_device(m_major, m_minor));
-    });
 }
 
 String Device::absolute_path() const

Kernel/Devices/Device.h

@@ -27,14 +27,6 @@
 
 namespace Kernel {
 
-template<typename DeviceType, typename... Args>
-inline KResultOr<NonnullRefPtr<DeviceType>> try_create_device(Args&&... args)
-{
-    auto device = TRY(adopt_nonnull_ref_or_enomem(new DeviceType(forward<Args>(args)...)));
-    device->after_inserting();
-    return device;
-}
-
 class Device : public File {
 protected:
     enum class State {
@@ -57,10 +49,6 @@ public:
     virtual bool is_device() const override { return true; }
     virtual void before_removing() override;
     virtual void after_inserting();
-
-    static void for_each(Function<void(Device&)>);
-    static Device* get_device(unsigned major, unsigned minor);
-
     void process_next_queued_request(Badge<AsyncDeviceRequest>, const AsyncDeviceRequest&);
 
     template<typename AsyncRequestType, typename... Args>
@@ -80,8 +68,6 @@ protected:
     void set_uid(UserID uid) { m_uid = uid; }
     void set_gid(GroupID gid) { m_gid = gid; }
 
-    static MutexProtected<HashMap<u32, Device*>>& all_devices();
-
 private:
     unsigned m_major { 0 };
     unsigned m_minor { 0 };

Kernel/Devices/DeviceManagement.cpp

@@ -0,0 +1,88 @@
+/*
+ * Copyright (c) 2021, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/HashTable.h>
+#include <AK/Singleton.h>
+#include <Kernel/Devices/DeviceManagement.h>
+#include <Kernel/FileSystem/InodeMetadata.h>
+#include <Kernel/Sections.h>
+
+namespace Kernel {
+
+static Singleton<DeviceManagement> s_the;
+
+UNMAP_AFTER_INIT DeviceManagement::DeviceManagement()
+{
+}
+UNMAP_AFTER_INIT void DeviceManagement::initialize()
+{
+    s_the.ensure_instance();
+}
+
+UNMAP_AFTER_INIT void DeviceManagement::attach_null_device(NullDevice const& device)
+{
+    m_null_device = device;
+}
+
+DeviceManagement& DeviceManagement::the()
+{
+    return *s_the;
+}
+
+Device* DeviceManagement::get_device(unsigned major, unsigned minor)
+{
+    return m_devices.with_exclusive([&](auto& map) -> Device* {
+        auto it = map.find(encoded_device(major, minor));
+        if (it == map.end())
+            return nullptr;
+        return it->value;
+    });
+}
+
+void DeviceManagement::before_device_removal(Badge<Device>, Device& device)
+{
+    u32 device_id = encoded_device(device.major(), device.minor());
+    m_devices.with_exclusive([&](auto& map) -> void {
+        VERIFY(map.contains(device_id));
+        map.remove(encoded_device(device.major(), device.minor()));
+    });
+}
+
+void DeviceManagement::after_inserting_device(Badge<Device>, Device& device)
+{
+    u32 device_id = encoded_device(device.major(), device.minor());
+    m_devices.with_exclusive([&](auto& map) -> void {
+        if (map.contains(device_id)) {
+            dbgln("Already registered {},{}: {}", device.major(), device.minor(), device.class_name());
+            VERIFY_NOT_REACHED();
+        }
+        auto result = map.set(device_id, &device);
+        if (result != AK::HashSetResult::InsertedNewEntry) {
+            dbgln("Failed to register {},{}: {}", device.major(), device.minor(), device.class_name());
+            VERIFY_NOT_REACHED();
+        }
+    });
+}
+
+void DeviceManagement::for_each(Function<void(Device&)> callback)
+{
+    m_devices.with_exclusive([&](auto& map) -> void {
+        for (auto& entry : map)
+            callback(*entry.value);
+    });
+}
+
+NullDevice& DeviceManagement::null_device()
+{
+    return *m_null_device;
+}
+
+NullDevice const& DeviceManagement::null_device() const
+{
+    return *m_null_device;
+}
+
+}

Kernel/Devices/DeviceManagement.h

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2021, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Badge.h>
+#include <AK/NonnullRefPtrVector.h>
+#include <AK/OwnPtr.h>
+#include <AK/RefPtr.h>
+#include <AK/Time.h>
+#include <AK/Types.h>
+#include <Kernel/API/KResult.h>
+#include <Kernel/API/TimePage.h>
+#include <Kernel/Arch/x86/RegisterState.h>
+#include <Kernel/Devices/Device.h>
+#include <Kernel/Devices/NullDevice.h>
+#include <Kernel/UnixTypes.h>
+
+namespace Kernel {
+
+class DeviceManagement {
+    AK_MAKE_ETERNAL;
+
+public:
+    DeviceManagement();
+    static void initialize();
+    static DeviceManagement& the();
+    void attach_null_device(NullDevice const&);
+
+    void after_inserting_device(Badge<Device>, Device&);
+    void before_device_removal(Badge<Device>, Device&);
+
+    void for_each(Function<void(Device&)>);
+    Device* get_device(unsigned major, unsigned minor);
+    NullDevice const& null_device() const;
+    NullDevice& null_device();
+
+    template<typename DeviceType, typename... Args>
+    static inline KResultOr<NonnullRefPtr<DeviceType>> try_create_device(Args&&... args)
+    {
+        auto device = TRY(adopt_nonnull_ref_or_enomem(new DeviceType(forward<Args>(args)...)));
+        device->after_inserting();
+        return device;
+    }
+
+private:
+    RefPtr<NullDevice> m_null_device;
+    MutexProtected<HashMap<u32, Device*>> m_devices;
+};
+
+}

Kernel/Devices/FullDevice.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <AK/Memory.h>
+#include <Kernel/Devices/DeviceManagement.h>
 #include <Kernel/Devices/FullDevice.h>
 #include <Kernel/Sections.h>
 #include <LibC/errno_numbers.h>
@@ -13,7 +14,7 @@ namespace Kernel {
 
 UNMAP_AFTER_INIT NonnullRefPtr<FullDevice> FullDevice::must_create()
 {
-    auto full_device_or_error = try_create_device<FullDevice>();
+    auto full_device_or_error = DeviceManagement::try_create_device<FullDevice>();
     // FIXME: Find a way to propagate errors
     VERIFY(!full_device_or_error.is_error());
     return full_device_or_error.release_value();

Kernel/Devices/FullDevice.h

@@ -12,14 +12,15 @@ namespace Kernel {
 
 class FullDevice final : public CharacterDevice {
     AK_MAKE_ETERNAL
+    friend class DeviceManagement;
+
 public:
     static NonnullRefPtr<FullDevice> must_create();
     virtual ~FullDevice() override;
 
-    // FIXME: We expose this constructor to make try_create_device helper to work
+private:
     FullDevice();
 
-private:
     // ^CharacterDevice
     virtual KResultOr<size_t> read(OpenFileDescription&, u64, UserOrKernelBuffer&, size_t) override;
     virtual KResultOr<size_t> write(OpenFileDescription&, u64, const UserOrKernelBuffer&, size_t) override;

Kernel/Devices/HID/PS2KeyboardDevice.cpp

@@ -9,6 +9,7 @@
 #include <AK/Singleton.h>
 #include <AK/Types.h>
 #include <Kernel/Debug.h>
+#include <Kernel/Devices/DeviceManagement.h>
 #include <Kernel/Devices/HID/HIDManagement.h>
 #include <Kernel/Devices/HID/PS2KeyboardDevice.h>
 #include <Kernel/IO.h>
@@ -83,7 +84,7 @@ bool PS2KeyboardDevice::handle_irq(const RegisterState&)
 
 UNMAP_AFTER_INIT RefPtr<PS2KeyboardDevice> PS2KeyboardDevice::try_to_initialize(const I8042Controller& ps2_controller)
 {
-    auto keyboard_device_or_error = try_create_device<PS2KeyboardDevice>(ps2_controller);
+    auto keyboard_device_or_error = DeviceManagement::try_create_device<PS2KeyboardDevice>(ps2_controller);
     // FIXME: Find a way to propagate errors
     VERIFY(!keyboard_device_or_error.is_error());
     if (keyboard_device_or_error.value()->initialize())

Kernel/Devices/HID/PS2KeyboardDevice.h

@@ -21,6 +21,8 @@ namespace Kernel {
 class PS2KeyboardDevice final : public IRQHandler
     , public KeyboardDevice
     , public I8042Device {
+    friend class DeviceManagement;
+
 public:
     static RefPtr<PS2KeyboardDevice> try_to_initialize(const I8042Controller&);
     virtual ~PS2KeyboardDevice() override;
@@ -35,10 +37,9 @@ public:
         enable_irq();
     }
 
-    // FIXME: We expose this constructor to make try_create_device helper to work
+private:
     explicit PS2KeyboardDevice(const I8042Controller&);
 
-private:
     // ^IRQHandler
     virtual bool handle_irq(const RegisterState&) override;
 

Kernel/Devices/HID/PS2MouseDevice.cpp

@@ -6,6 +6,7 @@
 
 #include <AK/Memory.h>
 #include <Kernel/Debug.h>
+#include <Kernel/Devices/DeviceManagement.h>
 #include <Kernel/Devices/HID/PS2MouseDevice.h>
 #include <Kernel/Devices/VMWareBackdoor.h>
 #include <Kernel/IO.h>
@@ -174,7 +175,7 @@ void PS2MouseDevice::set_sample_rate(u8 rate)
 
 UNMAP_AFTER_INIT RefPtr<PS2MouseDevice> PS2MouseDevice::try_to_initialize(const I8042Controller& ps2_controller)
 {
-    auto mouse_device_or_error = try_create_device<PS2MouseDevice>(ps2_controller);
+    auto mouse_device_or_error = DeviceManagement::try_create_device<PS2MouseDevice>(ps2_controller);
     // FIXME: Find a way to propagate errors
     VERIFY(!mouse_device_or_error.is_error());
     if (mouse_device_or_error.value()->initialize())

Kernel/Devices/HID/PS2MouseDevice.h

@@ -17,6 +17,8 @@ namespace Kernel {
 class PS2MouseDevice : public IRQHandler
     , public MouseDevice
     , public I8042Device {
+    friend class DeviceManagement;
+
 public:
     static RefPtr<PS2MouseDevice> try_to_initialize(const I8042Controller&);
     bool initialize();
@@ -32,10 +34,9 @@ public:
         enable_irq();
     }
 
-    // FIXME: We expose this constructor to make try_create_device helper to work
+protected:
     explicit PS2MouseDevice(const I8042Controller&);
 
-protected:
     // ^IRQHandler
     virtual bool handle_irq(const RegisterState&) override;
 

Kernel/Devices/HID/VMWareMouseDevice.cpp

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <Kernel/Devices/DeviceManagement.h>
 #include <Kernel/Devices/HID/VMWareMouseDevice.h>
 #include <Kernel/Devices/VMWareBackdoor.h>
 #include <Kernel/Sections.h>
@@ -16,7 +17,7 @@ UNMAP_AFTER_INIT RefPtr<VMWareMouseDevice> VMWareMouseDevice::try_to_initialize(
         return {};
     if (!VMWareBackdoor::the()->vmmouse_is_absolute())
         return {};
-    auto mouse_device_or_error = try_create_device<VMWareMouseDevice>(ps2_controller);
+    auto mouse_device_or_error = DeviceManagement::try_create_device<VMWareMouseDevice>(ps2_controller);
     // FIXME: Find a way to propagate errors
     VERIFY(!mouse_device_or_error.is_error());
     if (mouse_device_or_error.value()->initialize())

Kernel/Devices/HID/VMWareMouseDevice.h

@@ -17,13 +17,14 @@ namespace Kernel {
 
 class VMWareMouseDevice final : public PS2MouseDevice {
 public:
+    friend class DeviceManagement;
     static RefPtr<VMWareMouseDevice> try_to_initialize(const I8042Controller&);
     virtual ~VMWareMouseDevice() override;
 
     // ^I8042Device
     virtual void irq_handle_byte_read(u8 byte) override;
 
-    // FIXME: We expose this constructor to make try_create_device helper to work
+private:
     explicit VMWareMouseDevice(const I8042Controller&);
 };
 

Kernel/Devices/KCOVDevice.cpp

@@ -6,6 +6,7 @@
 
 #include <AK/Assertions.h>
 #include <AK/NonnullOwnPtr.h>
+#include <Kernel/Devices/DeviceManagement.h>
 #include <Kernel/Devices/KCOVDevice.h>
 #include <Kernel/Devices/KCOVInstance.h>
 #include <Kernel/FileSystem/OpenFileDescription.h>
@@ -20,7 +21,7 @@ HashMap<ThreadID, KCOVInstance*>* KCOVDevice::thread_instance;
 
 UNMAP_AFTER_INIT NonnullRefPtr<KCOVDevice> KCOVDevice::must_create()
 {
-    auto kcov_device_or_error = try_create_device<KCOVDevice>();
+    auto kcov_device_or_error = DeviceManagement::try_create_device<KCOVDevice>();
     // FIXME: Find a way to propagate errors
     VERIFY(!kcov_device_or_error.is_error());
     return kcov_device_or_error.release_value();

Kernel/Devices/KCOVDevice.h

@@ -12,6 +12,7 @@
 namespace Kernel {
 class KCOVDevice final : public BlockDevice {
     AK_MAKE_ETERNAL
+    friend class DeviceManagement;
 
 public:
     static HashMap<ProcessID, KCOVInstance*>* proc_instance;
@@ -25,10 +26,9 @@ public:
     KResultOr<Memory::Region*> mmap(Process&, OpenFileDescription&, Memory::VirtualRange const&, u64 offset, int prot, bool shared) override;
     KResultOr<NonnullRefPtr<OpenFileDescription>> open(int options) override;
 
-    // FIXME: We expose this constructor to make try_create_device helper to work
+protected:
     KCOVDevice();
 
-protected:
     virtual StringView class_name() const override { return "KCOVDevice"; }
 
     virtual bool can_read(const OpenFileDescription&, size_t) const override final { return true; }

Kernel/Devices/MemoryDevice.cpp

@@ -6,6 +6,7 @@
 
 #include <AK/Memory.h>
 #include <AK/StdLibExtras.h>
+#include <Kernel/Devices/DeviceManagement.h>
 #include <Kernel/Devices/MemoryDevice.h>
 #include <Kernel/Firmware/BIOS.h>
 #include <Kernel/Memory/AnonymousVMObject.h>
@@ -15,7 +16,7 @@ namespace Kernel {
 
 UNMAP_AFTER_INIT NonnullRefPtr<MemoryDevice> MemoryDevice::must_create()
 {
-    auto memory_device_or_error = try_create_device<MemoryDevice>();
+    auto memory_device_or_error = DeviceManagement::try_create_device<MemoryDevice>();
     // FIXME: Find a way to propagate errors
     VERIFY(!memory_device_or_error.is_error());
     return memory_device_or_error.release_value();

Kernel/Devices/MemoryDevice.h

@@ -15,16 +15,17 @@ namespace Kernel {
 
 class MemoryDevice final : public CharacterDevice {
     AK_MAKE_ETERNAL
+    friend class DeviceManagement;
+
 public:
     static NonnullRefPtr<MemoryDevice> must_create();
     ~MemoryDevice();
 
     virtual KResultOr<Memory::Region*> mmap(Process&, OpenFileDescription&, Memory::VirtualRange const&, u64 offset, int prot, bool shared) override;
 
-    // FIXME: We expose this constructor to make try_create_device helper to work
+private:
     MemoryDevice();
 
-private:
     virtual StringView class_name() const override { return "MemoryDevice"; }
     virtual bool can_read(const OpenFileDescription&, size_t) const override { return true; }
     virtual bool can_write(const OpenFileDescription&, size_t) const override { return false; }

Kernel/Devices/NullDevice.cpp

@@ -5,22 +5,18 @@
  */
 
 #include <AK/Singleton.h>
+#include <Kernel/Devices/DeviceManagement.h>
 #include <Kernel/Devices/NullDevice.h>
 #include <Kernel/Sections.h>
 
 namespace Kernel {
 
-static Singleton<NullDevice> s_the;
-
-UNMAP_AFTER_INIT void NullDevice::initialize()
+UNMAP_AFTER_INIT NonnullRefPtr<NullDevice> NullDevice::must_initialize()
 {
-    s_the.ensure_instance();
-    s_the->after_inserting();
-}
-
-NullDevice& NullDevice::the()
-{
-    return *s_the;
+    auto null_device_or_error = DeviceManagement::try_create_device<NullDevice>();
+    // FIXME: Find a way to propagate errors
+    VERIFY(!null_device_or_error.is_error());
+    return null_device_or_error.release_value();
 }
 
 UNMAP_AFTER_INIT NullDevice::NullDevice()

Kernel/Devices/NullDevice.h

@@ -12,14 +12,15 @@ namespace Kernel {
 
 class NullDevice final : public CharacterDevice {
     AK_MAKE_ETERNAL
+    friend class DeviceManagement;
+
 public:
-    NullDevice();
     virtual ~NullDevice() override;
 
-    static void initialize();
-    static NullDevice& the();
+    static NonnullRefPtr<NullDevice> must_initialize();
 
 private:
+    NullDevice();
     // ^CharacterDevice
     virtual KResultOr<size_t> read(OpenFileDescription&, u64, UserOrKernelBuffer&, size_t) override;
     virtual KResultOr<size_t> write(OpenFileDescription&, u64, const UserOrKernelBuffer&, size_t) override;

Kernel/Devices/RandomDevice.cpp

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <Kernel/Devices/DeviceManagement.h>
 #include <Kernel/Devices/RandomDevice.h>
 #include <Kernel/Random.h>
 #include <Kernel/Sections.h>
@@ -12,7 +13,7 @@ namespace Kernel {
 
 UNMAP_AFTER_INIT NonnullRefPtr<RandomDevice> RandomDevice::must_create()
 {
-    auto random_device_or_error = try_create_device<RandomDevice>();
+    auto random_device_or_error = DeviceManagement::try_create_device<RandomDevice>();
     // FIXME: Find a way to propagate errors
     VERIFY(!random_device_or_error.is_error());
     return random_device_or_error.release_value();

Kernel/Devices/RandomDevice.h

@@ -12,14 +12,15 @@ namespace Kernel {
 
 class RandomDevice final : public CharacterDevice {
     AK_MAKE_ETERNAL
+    friend class DeviceManagement;
+
 public:
     static NonnullRefPtr<RandomDevice> must_create();
     virtual ~RandomDevice() override;
 
-    // FIXME: We expose this constructor to make try_create_device helper to work
+private:
     RandomDevice();
 
-private:
     // ^CharacterDevice
     virtual KResultOr<size_t> read(OpenFileDescription&, u64, UserOrKernelBuffer&, size_t) override;
     virtual KResultOr<size_t> write(OpenFileDescription&, u64, const UserOrKernelBuffer&, size_t) override;

Kernel/Devices/SerialDevice.cpp

@@ -5,6 +5,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <Kernel/Devices/DeviceManagement.h>
 #include <Kernel/Devices/SerialDevice.h>
 #include <Kernel/IO.h>
 #include <Kernel/Sections.h>
@@ -23,19 +24,19 @@ UNMAP_AFTER_INIT NonnullRefPtr<SerialDevice> SerialDevice::must_create(size_t co
     RefPtr<SerialDevice> serial_device;
     switch (com_number) {
     case 0: {
-        serial_device = try_create_device<SerialDevice>(IOAddress(SERIAL_COM1_ADDR), 64).release_value();
+        serial_device = DeviceManagement::try_create_device<SerialDevice>(IOAddress(SERIAL_COM1_ADDR), 64).release_value();
         break;
     }
     case 1: {
-        serial_device = try_create_device<SerialDevice>(IOAddress(SERIAL_COM2_ADDR), 65).release_value();
+        serial_device = DeviceManagement::try_create_device<SerialDevice>(IOAddress(SERIAL_COM2_ADDR), 65).release_value();
         break;
     }
     case 2: {
-        serial_device = try_create_device<SerialDevice>(IOAddress(SERIAL_COM3_ADDR), 66).release_value();
+        serial_device = DeviceManagement::try_create_device<SerialDevice>(IOAddress(SERIAL_COM3_ADDR), 66).release_value();
         break;
     }
     case 3: {
-        serial_device = try_create_device<SerialDevice>(IOAddress(SERIAL_COM4_ADDR), 67).release_value();
+        serial_device = DeviceManagement::try_create_device<SerialDevice>(IOAddress(SERIAL_COM4_ADDR), 67).release_value();
         break;
     }
     default:

Kernel/Devices/SerialDevice.h

@@ -13,6 +13,8 @@ namespace Kernel {
 
 class SerialDevice final : public CharacterDevice {
     AK_MAKE_ETERNAL
+    friend class DeviceManagement;
+
 public:
     static NonnullRefPtr<SerialDevice> must_create(size_t com_number);
 
@@ -102,10 +104,9 @@ public:
         DataReady = 0x01 << 0
     };
 
-    // FIXME: We expose this constructor to make try_create_device helper to work
+private:
     SerialDevice(IOAddress base_addr, unsigned minor);
 
-private:
     friend class PCISerialDevice;
 
     // ^CharacterDevice

Kernel/Devices/ZeroDevice.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <AK/Memory.h>
+#include <Kernel/Devices/DeviceManagement.h>
 #include <Kernel/Devices/ZeroDevice.h>
 #include <Kernel/Sections.h>
 
@@ -12,7 +13,7 @@ namespace Kernel {
 
 UNMAP_AFTER_INIT NonnullRefPtr<ZeroDevice> ZeroDevice::must_create()
 {
-    auto zero_device_or_error = try_create_device<ZeroDevice>();
+    auto zero_device_or_error = DeviceManagement::try_create_device<ZeroDevice>();
     // FIXME: Find a way to propagate errors
     VERIFY(!zero_device_or_error.is_error());
     return zero_device_or_error.release_value();

Kernel/Devices/ZeroDevice.h

@@ -12,14 +12,15 @@ namespace Kernel {
 
 class ZeroDevice final : public CharacterDevice {
     AK_MAKE_ETERNAL
+    friend class DeviceManagement;
+
 public:
     static NonnullRefPtr<ZeroDevice> must_create();
     virtual ~ZeroDevice() override;
 
-    // FIXME: We expose this constructor to make try_create_device helper to work
+private:
     ZeroDevice();
 
-private:
     // ^CharacterDevice
     virtual KResultOr<size_t> read(OpenFileDescription&, u64, UserOrKernelBuffer&, size_t) override;
     virtual KResultOr<size_t> write(OpenFileDescription&, u64, const UserOrKernelBuffer&, size_t) override;