14.2 Overview of Newbus

Newbus is the implementation of a new bus architecture based on abstraction layers which saw its introduction in FreeBSD 3.0 when the Alpha port was imported into the source tree. It was not until 4.0 before it became the default system to use for device drivers. Its goals are to provide a more object oriented means of interconnecting the various busses and devices which a host system provides to the Operating System.

Its main features include amongst others:

• dynamic attaching

• easy modularization of drivers

• pseudo-busses

One of the most prominent changes is the migration from the flat and ad-hoc system to a device tree lay-out.

At the top level resides the “root” device which is the parent to hang all other devices on. For each architecture, there is typically a single child of “root” which has such things as host-to-PCI bridges, etc. attached to it. For x86, this “root” device is the “nexus” device and for Alpha, various different different models of Alpha have different top-level devices corresponding to the different hardware chipsets, including lca, apecs, cia and tsunami.

A device in the Newbus context represents a single hardware entity in the system. For instance each PCI device is represented by a Newbus device. Any device in the system can have children; a device which has children is often called a “bus”. Examples of common busses in the system are ISA and PCI which manage lists of devices attached to ISA and PCI busses respectively.

Often, a connection between different kinds of bus is represented by a “bridge” device which normally has one child for the attached bus. An example of this is a PCI-to-PCI bridge which is represented by a device pcibN on the parent PCI bus and has a child pciN for the attached bus. This layout simplifies the implementation of the PCI bus tree, allowing common code to be used for both top-level and bridged busses.

Each device in the Newbus architecture asks its parent to map its resources. The parent then asks its own parent until the nexus is reached. So, basically the nexus is the only part of the Newbus system which knows about all resources.

Resource allocation can be controlled at any place in the device tree. For instance on many Alpha platforms, ISA interrupts are managed separately from PCI interrupts and resource allocations for ISA interrupts are managed by the Alpha's ISA bus device. On IA-32, ISA and PCI interrupts are both managed by the top-level nexus device. For both ports, memory and port address space is managed by a single entity - nexus for IA-32 and the relevant chipset driver on Alpha (e.g. CIA or tsunami).

In order to normalize access to memory and port mapped resources, Newbus integrates the bus_space APIs from NetBSD. These provide a single API to replace inb/outb and direct memory reads/writes. The advantage of this is that a single driver can easily use either memory-mapped registers or port-mapped registers (some hardware supports both).

This support is integrated into the resource allocation mechanism. When a resource is allocated, a driver can retrieve the associated bus_space_tag_t and bus_space_handle_t from the resource.

Newbus also allows for definitions of interface methods in files dedicated to this purpose. These are the .m files that are found under the src/sys hierarchy.

The core of the Newbus system is an extensible “object-based programming” model. Each device in the system has a table of methods which it supports. The system and other devices uses those methods to control the device and request services. The different methods supported by a device are defined by a number of “interfaces”. An “interface” is simply a group of related methods which can be implemented by a device.

In the Newbus system, the methods for a device are provided by the various device drivers in the system. When a device is attached to a driver during auto-configuration, it uses the method table declared by the driver. A device can later detach from its driver and re-attach to a new driver with a new method table. This allows dynamic replacement of drivers which can be useful for driver development.

The interfaces are described by an interface definition language similar to the language used to define vnode operations for file systems. The interface would be stored in a methods file (which would normally named foo_if.m).

      # Foo subsystem/driver (a comment...)
      
      INTERFACE foo

        METHOD int doit {
            device_t dev;
        };
        
        # DEFAULT is the method that will be used, if a method was not
        # provided via: DEVMETHOD()
        
        METHOD void doit_to_child {
            device_t dev;
            driver_t child;
        } DEFAULT doit_generic_to_child;
     

When this interface is compiled, it generates a header file “foo_if.h” which contains function declarations:

      int FOO_DOIT(device_t dev);
      int FOO_DOIT_TO_CHILD(device_t dev, device_t child);
   

A source file, “foo_if.c” is also created to accompany the automatically generated header file; it contains implementations of those functions which look up the location of the relevant functions in the object's method table and call that function.

The system defines two main interfaces. The first fundamental interface is called “device” and includes methods which are relevant to all devices. Methods in the “device” interface include “probe”, “attach” and “detach” to control detection of hardware and “shutdown”, “suspend” and “resume” for critical event notification.

The second, more complex interface is “bus”. This interface contains methods suitable for devices which have children, including methods to access bus specific per-device information [1], event notification (child_detached, driver_added) and resource management (alloc_resource, activate_resource, deactivate_resource, release_resource).

Many methods in the “bus” interface are performing services for some child of the bus device. These methods would normally use the first two arguments to specify the bus providing the service and the child device which is requesting the service. To simplify driver code, many of these methods have accessor functions which lookup the parent and call a method on the parent. For instance the method BUS_TEARDOWN_INTR(device_t dev, device_t child, ...) can be called using the function bus_teardown_intr(device_t child, ...).

Some bus types in the system define additional interfaces to provide access to bus-specific functionality. For instance, the PCI bus driver defines the “pci” interface which has two methods read_config and write_config for accessing the configuration registers of a PCI device.