At the highest level, a device driver's job is to provide a uniform interface to a particular device, while hiding details specific to the device's implementation.
Two different ethernet drivers, for example, both allow a client to send packets out an interface, using the exact same C language function. Each driver is responsible for managing its own hardware in a way that makes the client interfaces identical, even though the hardware is different.
Note that the interfaces that are provided by the driver may be "intermediate" — that is, they might not necessarily represent the "final" device in the chain.
Consider a PCI-based ethernet device. First, a base PCI driver is required that understands how to talk to the PCI bus itself. This driver doesn't know anything about ethernet, but it does know how to deal with the specific PCI chipset present on the machine.
It enumerates the devices on that bus, collects information from the various registers on each device, and provides functions that allow its clients (such as the PCI-based ethernet driver) to perform PCI operations like allocating an interrupt or a DMA channel.
Thus, this base PCI driver provides services to the ethernet driver, allowing the ethernet driver to manage its associated hardware.
At the same time, other devices (such as a video card) could also use the base PCI driver in a similar manner to manage their hardware.
The Fuchsia model
In order to provide maximum flexibility, drivers in the Fuchsia world are allowed to bind to matching "parent" devices, and publish "children" of their own. This hierarchy extends as required: one driver might publish a child, only to have another driver consider that child their parent, with the second driver publishing its own children, and so on.
In order to understand how this works, let's follow the PCI-based ethernet example.
The system starts by providing a special "PCI root" parent. Effectively, it's saying "I know that there's a PCI bus on this system, when you find it, bind it here."
Drivers are evaluated by the system (a directory is searched), and drivers that match are automatically bound.
In this case, a driver that binds to a "PCI root" parent is found, and bound.
This is the base PCI driver. Its job is to configure the PCI bus, and enumerate the peripherals on the bus.
The PCI bus has specific conventions for how peripherals are identified: a combination of a Vendor ID (VID) and Device ID (DID) uniquely identifies all possible PCI devices. During enumeration, these values are read from the peripheral, and new parent nodes are published containing the detected VID and DID (and a host of other information).
Every time a new device is published, the same process as described above (for the initial PCI root device publication) repeats; that is, drivers are evaluated by the system, searching for drivers that match up with the new parents' characteristics.
Whereas with the PCI root device we were searching for a driver that matched a certain kind of functionality (called a "protocol," we'll see this shortly), in this case, however, we're searching for drivers that match a different protocol, namely one that satisfies the requirements of "is a PCI device and has a given VID and DID."
If a suitable driver is found (one that matches the required protocol, VID and DID), it's bound to the parent.
As part of binding, we initialize the driver — this involves such operations as setting up the card for operation, bringing up the interface(s), and publishing a child or children of this device. In the case of the PCI ethernet driver, it publishes the "ethernet" interface, which conforms to yet another protocol, called the "ethernet implementation" protocol. This protocol represents a common protocol that's close to the functions that clients use (but is one step removed; we'll come back to this).