Chapter 14. Newbus

Jeroen Ruigrok van der Werven (asmodai)

Hiten Pandya

Special thanks to Matthew N. Dodd, Warner Losh, Bill Paul, Doug Rabson, Mike Smith, Peter Wemm and Scott Long.

This chapter explains the Newbus device framework in detail.

14.1. Device Drivers

14.1.1. Purpose of a Device Driver

A device driver is a software component which provides the interface between the kernel’s generic view of a peripheral (e.g., disk, network adapter) and the actual implementation of the peripheral. The device driver interface (DDI) is the defined interface between the kernel and the device driver component.

14.1.2. Types of Device Drivers

There used to be days in UNIX®, and thus FreeBSD, in which there were four types of devices defined:

  • block device drivers

  • character device drivers

  • network device drivers

  • pseudo-device drivers

Block devices performed in a way that used fixed size blocks [of data]. This type of driver depended on the so-called buffer cache, which had cached accessed blocks of data in a dedicated part of memory. Often this buffer cache was based on write-behind, which meant that when data was modified in memory it got synced to disk whenever the system did its periodical disk flushing, thus optimizing writes.

14.1.3. Character Devices

However, in the versions of FreeBSD 4.0 and onward the distinction between block and character devices became non-existent.

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 layout.

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. For Alpha, various 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.

An ISA device might want to map its IO port at 0x230, so it asks its parent, in this case the ISA bus. The ISA bus hands it over to the PCI-to-ISA bridge which in its turn asks the PCI bus, which reaches the host-to-PCI bridge and finally the nexus. The beauty of this transition upwards is that there is room to translate the requests. For example, the 0x230 IO port request might become memory-mapped at 0xb0000230 on a MIPS box by the PCI bridge.

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 be named foo_if.m).

Example 1. Newbus Methods
      # 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.

14.3. Newbus API

As the Newbus API is huge, this section makes some effort at documenting it. More information to come in the next revision of this document.

14.3.1. Important Locations in the Source Hierarchy

src/sys/[arch]/[arch] - Kernel code for a specific machine architecture resides in this directory. For example, the i386 architecture, or the SPARC64 architecture.

src/sys/dev/[bus] - device support for a specific [bus] resides in this directory.

src/sys/dev/pci - PCI bus support code resides in this directory.

src/sys/[isa|pci] - PCI/ISA device drivers reside in this directory. The PCI/ISA bus support code used to exist in this directory in FreeBSD version 4.0.

14.3.2. Important Structures and Type Definitions

devclass_t - This is a type definition of a pointer to a struct devclass.

device_method_t - This is the same as kobj_method_t (see src/sys/kobj.h).

device_t - This is a type definition of a pointer to a struct device. device_t represents a device in the system. It is a kernel object. See src/sys/sys/bus_private.h for implementation details.

driver_t - This is a type definition which references struct driver. The driver struct is a class of the device kernel object; it also holds data private to the driver.

driver_t Implementation

	  struct driver {
		KOBJ_CLASS_FIELDS;
		void	*priv;			/* driver private data */
	  };

A device_state_t type, which is an enumeration, device_state. It contains the possible states of a Newbus device before and after the autoconfiguration process.

Device States _device_state_t

	  /*
	   * src/sys/sys/bus.h
	   */
	  typedef enum device_state {
		DS_NOTPRESENT,	/* not probed or probe failed */
		DS_ALIVE,		/* probe succeeded */
		DS_ATTACHED,	/* attach method called */
		DS_BUSY			/* device is open */
	  } device_state_t;

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