Driver communication

In Fuchsia, all communication occurs over FIDL calls, for both drivers and non-drivers. What differs is how drivers' services are discovered and how connection is established.

For driver-to-driver communication, Fuchsia uses the node topology to place parent nodes' capabilities in a child node's incoming FIDL namespace (that is, under /svc as directories and files). This setup enables a driver (once bound to the child node) to access FIDL services inherited from the parent nodes.

However, communication from a non-driver component to a driver takes place in two phases:

  1. Service discovery (using devfs)
  2. FIDL communication

For non-driver components, the first task is to discover which drivers' services are available in the system. These services are provided by the drivers that are currently bound to nodes representing hardware or virtual devices in the system. A filesystem known as devfs provides a mechanism for discovering these services.

The following events take place for non-driver to driver communication:

  1. To discover driver services in the system, a non-driver component scans the directories and files in devfs.
  2. The non-driver component finds a file in devfs that represents a service provided by the target driver.
  3. The non-driver component opens this file and contacts the target driver.
  4. After the initial contact, a FIDL connection is established between the non-driver component and the driver.
  5. From this point, all communication takes place over the FIDL channels.

Service discovery (using devfs)

The driver manager hosts a virtual filesystem named devfs (as in "device filesystem"). This virtual filesystem provides uniform access to all driver services in a Fuchsia system to Fuchsia’s user-space services (that is, components external to the drivers). These non-driver components establish initial contacts with drivers by discovering the services of the target drivers in devfs.

Strictly speaking, devfs is a directory capability exposed by the driver manager. Therefore, by convention, components that wish to access drivers mount devfs under the /dev directory in their namespace (although it’s not mandated that devfs to be always mounted under /dev).

devfs hosts virtual files that enable Fuchsia components to route messages to the interfaces implemented by the drivers running in a Fuchsia system. In other words, when a client (that is, a non-driver component) opens a file under the /dev directory, it receives a channel that can be used to make FIDL calls directly to the driver mapped to the file. For example, a Fuchsia component can connect to an input device by opening and writing to a file that looks like /dev/class/input-report/000. In this case, the client may receive a channel that speaks the FIDL.

Drivers can use the fuchsia.device.fs protocol to export themselves into devfs. The path under the /dev directory for a driver is the direct reflection of the path in the node topology. Therefore, if a driver wants to show up in the right place in devfs, it needs to make sure that its topological path and protocol correspond correctly.

FIDL communication

Once an initial contact is made between non-driver and driver components using devfs, the components can exchange FIDL handles. From this point, these components make use of FIDL calls for communication, just like any other components in Fuchsia.

Drivers, as Fuchsia components, have an incoming FIDL namespace filled with capabilities the drivers can use. Some of these capabilities may be inherited from their parent driver (for example, a PCI device will have a fuchsia.hardware.PCI capability from its parent node). Drivers can use these capabilities to make FIDL calls to their parent drivers. Similarly, their clients (that is, non-driver components) can also use the capabilities received from the drivers to make FIDL calls to the drivers.

These FIDL calls, by default, get routed through the Zircon kernel. However, if the target driver is in the same process (therefore, in the same driver host), the driver runtime can route the FIDL calls to remain in process, without going in and out of the Zircon kernel.