Userspace initially didn't have any sort of mechanism to handle
device hotplug (either removing or inserting a device).
This meant that after a short term of scanning all known devices, by
fetching device events (DeviceEvent packets) from /dev/devctl, we
basically never try to read it again after SystemServer initialization
code.
To accommodate hotplug needs, we change SystemServer by ensuring it will
generate a known set of device nodes at their location during the its
main initialization code. This includes devices like /dev/mem, /dev/zero
and /dev/full, etc.
The actual responsible userspace program to handle hotplug events is a
new userspace program called DeviceMapper, with following key points:
- Its current task is to to constantly read the /dev/devctl device node.
Because we already created generic devices, we only handle devices
that are dynamically-generated in nature, like storage devices, audio
channels, etc.
- Since dynamically-generated device nodes could have an infinite minor
numbers, but major numbers are decoded to a device type, we create an
internal registry based on two structures - DeviceNodeFamily, and
RegisteredDeviceNode. DeviceNodeFamily objects are attached in the
main logic code, when handling a DeviceEvent device insertion packet.
A DeviceNodeFamily object has an internal HashTable to hold objects of
RegisteredDeviceNode class.
- Because some device nodes could still share the same major number (TTY
and serial TTY devices), we have two modes of allocation - limited
allocation (so a range is defined for a major number), or infinite
range. Therefore, two (or more) separate DeviceNodeFamily objects can
can exist albeit sharing the same major number, but they are required
to allocate from a different minor numbers' range to ensure there are
no collisions.
- As for KCOV, we handle this device differently. In case the user
compiled the kernel with such support - this happens to be a singular
device node that we usually don't need, so it's dynamically-generated
too, and because it has only one instance, we don't register it in our
internal registry to not make it complicated needlessly.
The Kernel code is modified to allow proper blocking in case of no
events in the DeviceControlDevice class, because otherwise we will need
to poll periodically the device to check if a new event is available,
which would waste CPU time for no good reason.
We have 3 new components:
1. The AudioManagement singleton. This class like in other subsystems,
is responsible to find hardware audio controllers and keep a reference
to them.
2. AudioController class - this class is the parent class for hardware
controllers like the Sound Blaster 16 or Intel 82801AA (AC97). For now,
this class has simple interface for getting and controlling sample rate
of audio channels, as well a write interface for specific audio channel
but not reading from it. One AudioController object might have multiple
AudioChannel "child" objects to hold with reference counting.
3. AudioChannel class - this is based on the CharacterDevice class, and
represents hardware PCM audio channel. It facilitates an ioctl interface
which should be consistent across all supported hardware currently.
It has a weak reference to a parent AudioController, and when trying to
write to a channel, it redirects the data to the parent AudioController.
Each audio channel device should be added into a new directory under the
/dev filesystem called "audio".
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.
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.
