Platform Bus

The Platform Bus is a userspace service designed to be a core part of most Poplar systems. It manages an abstract "bus" of devices that can be added by userspace bus drivers and consumed by userspace device drivers. Drivers talk to the Platform Bus via channels obtained by subscribing to the Platform Bus's services, platform_bus.bus_driver and platform_bus.device_driver.

Platform Bus is entirely a userspace concept, and so systems can be built around the Poplar kernel without it. However, many drivers and applications will expect the Platform Bus service to be present, and systems not using it will have to handle many low-level systems, such as PCI device enumeration, themselves, and so it is expected that the vast majority of systems would use Platform Bus as a fundamental building block of their userspace.

Device representation on the Platform Bus

Devices on the Platform Bus can be quite abstract, or represent literal devices that are part of the platform, or plugged in as peripherals. Examples include a framebuffer "device" provided by a driver for a graphics-capable device, a power-management chip built into the platform, and USB devices, respectively.

Devices are described by a series of properties, which are typed pieces of data associated with a label. Device properties are used to identify devices, and are given to every device driver that claims it may be able to drive a device. Handoff properties are only transfered to a driver once it has been selected to drive a device, and can contain handles to kernel objects needed to drive the device. These handles are transferred to the task implementing the device driver, which is why they cannot be send arbitrarily to drivers to query support.

Device registration


Device hand-off to device driver


Standard devices

The Platform Bus library defines expected properties and behaviour for a number of standard device classes, in an attempt to increase compatability across drivers and device users. Additional properties may be added as necessary for an individual device.

PCI devices

Platform Bus will use information provided by the kernel to create devices for each enumerated PCI device. Standard properties:

pci.vendor_idIntegerVendor ID of the PCI device
pci.device_idIntegerDevice ID of the PCI device
pci.classIntegerClass of the PCI device
pci.sub_classIntegerSub-class of the PCI device
pci.interfaceIntegerInterface of the PCI device
pci.interruptEventIf configured, an Event that is triggered when the PCI device gets an IRQ
pci.barN.sizeIntegerN is a number from 0-6. The size of the given BAR, if present.
pci.barN.handleMemoryObjectN is a number from 0-6. A memory object mapped to the given BAR, if present.

Generally, specific devices (e.g. a specific GPU) can be detected with a combination of the vendor_id and device_id properties, while a type of device can be identified via the class, sub_class, and interface properties. Drivers should filter against the appropriate properties depending on the devices they can drive.

USB devices

USB devices may be added to the Platform Bus by a USB Host Controller driver, and can be consumed by a wide array of drivers. Standard properties:

usb.vendor_idIntegerClass of the USB device
usb.product_idIntegerClass of the USB device
usb.classIntegerClass of the USB device
usb.sub_classIntegerSub-class of the USB device
usb.protocolIntegerProtocol of the USB device
usb.config0BytesByte-stream of the first configuration descriptor of the device
usb.channelChannelControl channel to configure and control the device via the bus driver

HID devices