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Protocols in drivers

What is a protocol?

A protocol is a strict interface definition.

The ethernet driver published an interface that conforms to ZX_PROTOCOL_ETHERNET_IMPL. This means that it must provide a set of functions defined in a data structure (in this case, ethernet_impl_protocol_ops_t).

These functions are common to all devices implementing the protocol — for example, all ethernet devices must provide a function that queries the MAC address of the interface.

Other protocols will of course have different requirements for the functions they must provide. For example a block device will publish an interface that conforms to the "block implementation protocol" (ZX_PROTOCOL_BLOCK_IMPL) and provide functions defined by block_protocol_ops_t. This protocol includes a function that returns the size of the device in blocks, for example.

In many cases a Protocol is used to allow drivers to be simpler by taking advantage of a common implementation of an Interface. For example, the "block" driver implements the common block interface, and binds to devices implementing the Block Core Protocol, and the "ethernet" driver does the same thing for the Ethernet Interface and Ethermac Protocol. Some protocols, such as the two cited here, make use of shared memory, and non-rpc signaling for more efficient, lower latency, and higher throughput than could be achieved otherwise.

Classes represent a promise that a device implements an Interface or Protocol. Devices exist in the Device Filesystem under a topological path, like /sys/pci/00:02:00/e1000. If they are a specific class, they also appear as an alias under /dev/class/CLASSNAME/.... The e1000 driver implements the Ethermac interface, so it also shows up at /dev/class/ethermac/000. The names within class directories are unique but not meaningful, and are assigned on demand.

Example protocols:

  • the PCI root protocol (ZX_PROTOCOL_PCIROOT),
  • the PCI device protocol (ZX_PROTOCOL_PCI), and
  • the ethernet implementation protocol (ZX_PROTOCOL_ETHERNET_IMPL).

The names in brackets are the C language constants corresponding to the protocols, for reference.

Platform dependent vs platform independent

Above, we mentioned that ZX_PROTOCOL_ETHERNET_IMPL was "close to" the functions used by the client, but one step removed. That's because there's one more protocol, ZX_PROTOCOL_ETHERNET, that sits between the client and the driver. This additional protocol is in place to handle functionality common to all ethernet drivers (in order to avoid code duplication). Such functionality includes buffer management, status reporting, and administrative functions.

This is effectively a "platform dependent" vs "platform independent" decoupling; common code exists in the platform independent part (once), and driver-specific code is implemented in the platform dependent part.

This architecture is repeated in multiple places. With block devices, for example, the hardware driver binds to the bus (e.g., PCI) and provides a ZX_PROTOCOL_BLOCK_IMPL protocol. The platform independent driver binds to ZX_PROTOCOL_BLOCK_IMPL, and publishes the client-facing protocol, ZX_PROTOCOL_BLOCK.

You'll also see this with the display controllers, I2C bus, and serial drivers.

Process / protocol mapping

In order to keep the discussions above simple, we didn't talk about process separation as it relates to the drivers. To understand the issues, let's see how other operating systems deal with them, and compare that to the Fuchsia approach.

In a monolithic kernel, such as Linux, many drivers are implemented within the kernel. This means that they share the same address space, and effectively live in the same "process."

The major problem with this approach is fault isolation / exploitation. A bad driver can take out the entire kernel, because it lives in the same address space and thus has privileged access to all kernel memory and resources. A compromised driver can present a security threat for the same reason.

The other extreme, that is, putting each and every driver service into its own process, is used by some microkernel operating systems. Its major drawback is that if one driver relies on the services of another driver, the kernel must effect at least a context switch operation (if not a data transfer as well) between the two driver processes. While microkernel operating systems are usually designed to be fast at these kinds of operations, performing them at high frequency is undesirable.

The approach taken by Fuchsia is based on the concept of a driver host. A driver host is a process that contains a protocol stack — that is, one or more protocols that work together. The driver host loads drivers from ELF shared libraries (called Dynamic Shared Objects, or DSOs). In the simple drivers section, we'll see the meta information that's contained in the DSO to facilitate the discovery process.

The protocol stack effectively allows the creation of a complete "driver" for a device, consisting of platform dependent and platform independent components, in a self-contained process container.

For the advanced reader, take a look at the dm dump command available from the Fuchsia command line. It displays a tree of devices, and shows you the process ID, DSO name, and other useful information.

Here's a highly-edited version showing just the PCI ethernet driver parts:

1. [root]
2.    [sys]
3.       <sys> pid=1416 /boot/driver/bus-acpi.so
4.          [acpi] pid=1416 /boot/driver/bus-acpi.so
5.          [pci] pid=1416 /boot/driver/bus-acpi.so
            ...
6.             [00:02:00] pid=1416 /boot/driver/bus-pci.so
7.                <00:02:00> pid=2052 /boot/driver/bus-pci.proxy.so
8.                   [e1000] pid=2052 /boot/driver/e1000.so
9.                      [ethernet] pid=2052 /boot/driver/ethernet.so

From the above, you can see that process ID 1416 (lines 3 through 6) is the Advanced Configuration and Power Interface (ACPI) driver, implemented by the DSO bus-acpi.so.

During primary enumeration, the ACPI DSO detected a PCI bus. This caused the publication of a parent with ZX_PROTOCOL_PCI_ROOT (line 5, causing the appearance of the [pci] entry), which then caused the driver host to load the bus-pci.so DSO and bind to it. That DSO is the "base PCI driver" to which we've been referring throughout the discussions above.

During its binding, the base PCI driver enumerated the PCI bus, and found an ethernet card (line 6 detects bus 0, device 2, function 0, shown as [00:02:00]). (Of course, many other devices were found as well, but we've removed them from the above listing for simplicity).

The detection of this device then caused the base PCI driver to publish a new parent with ZX_PROTOCOL_PCI and the device's VID and DID. Additionally, a new driver host (process ID 2052) was created and loaded with the bus-pci.proxy.so DSO (line 7). This proxy serves as the interface from the new driver host (pid 2052) to the base PCI driver (pid 1416).

This is where the decision was made to "sever" the device driver into its own process — the new driver host and the base PCI driver now live in two different processes.

The new driver host 2052 then finds a matching child (the e1000.so DSO on line 8; it's considered a match because it has ZX_PROTOCOL_PCI and the correct VID and DID). That DSO publishes a ZX_PROTOCOL_ETHERNET_IMPL, which binds to a matching child (the ethernet.so DSO on line 9; it's considered a match because it has a ZX_PROTOCOL_ETHERNET_IMPL protocol).

What's not shown by this chain is that the final DSO (ethernet.so) publishes a ZX_PROTOCOL_ETHERNET — that's the piece that clients can use, so of course there's no further "device" binding involved.