Introduction to Haiku's Device Driver Architecture

This document tries to give you a short introduction into the new device manager, and how to write drivers for it. Haiku still supports the legacy device driver architecture introduced with BeOS.

The new device driver architecture of Haiku is still a moving target, although most of its details are already specificed.

1. The Basics

The device manager functionality builds upon device_node objects. Every driver in the system publishes one or more of such nodes, building a tree of device nodes. This tree is in theory a dynamic representation of the current hardware devices in the system, but in practice will also contain implementation specific details; since every node comes with an API specific to that node, you'll find device nodes that only come with a number of support functions for a certain class of drivers.

Structurally, a device_node is a set of a module, attributes, and resources, as well as a parent and children. At a minimum, a node must have a module, all other components are optional.

When the system starts, there is only a root node registered. Only primary hardware busses register with the root node, such as PCI, and ISA on x86. Since the PCI bus is an intelligent bus, it knows what hardware is installed, and registers a child node for each device on the bus.

Every driver can also publish a device in /dev for communication with userland applications. All drivers and devices are kernel modules.

2. Exploring the Device Tree

So how does it all work? When building the initial device tree, the system only explores a minimum of device drivers only, resulting in a tree that basically only shows the hardware found in the computer.

Now, if the system requires disk access, it will scan the device file system for a driver that provides such functionality, in this case, it will look for drivers under "/dev/disk/". The device manager has a set of built-in rules for how to translate a device path into a device node, and vice versa: every node representing a device of an intelligent bus (such as PCI) will also contain device type information following the PCI definitions. In this case, the "disk" sub-path will translate into the PCI_mass_storage type, and hence, the device manager will then completely explore all device nodes of that type.

It will also use that path information to only ask drivers that actually are in a matching module directory. In the above example of a disk driver, this would be either in "busses/scsi", "busses/ide", "drivers/disk", ...

For untyped or generic busses, it will use the context information gained from the devfs query directly, and will search for drivers in that sub directory only. The only exception to this rule are the devfs directories "disk", "ports", and "bus", which will also allow to search matching drivers in "busses". While this is relatively limited, it is a good way to cut down the number of drivers to be loaded.

3. Writing a Driver

The device manager assumes the following API from a driver module:

• supports_device()
  Determines wether or not the driver supports a given parent device node, that is the hardware device it represents (if any), and the API the node exports.
• register_device()
  The driver should register its device node here. The parent driver is always initialized at this point. When registering the node, the driver can also attach certain I/O resources (like I/O ports, or memory ranges) to the node -- the device manager will make sure that only one node can claim these resources.
• init_driver()
  Any initialization necessary to get the driver going. For most drivers, this will be reduced to the creation of a private data structure that is going to be used for all of the following functions.
• uninit_driver()
  Uninitializes resources acquired by init_driver().
• register_child_devices()
  If the driver wants to register any child device nodes or to publish any devices, it should do so here. This function is called only during the initial registration process of the device node.
• rescan_child_devices()
  Is called whenever a manual rescan is triggered.
• device_removed()
  Is called when the device node is about to be unregistered when its device is gone, for example when a USB device is unplugged.
• suspend()
  Enters different sleep modes.
• resume()
  Resumes a device from a previous sleep mode.

To ensure that a module exports this API, it must end its module name with "driver_v1" to denote the version of the API it supports. Note that suspend() and resume() are currently never called, as Haiku has no power management implemented yet.

If your driver can give the device it is attached to a nice name that can be presented to the user, it should add the B_DEVICE_PRETTY_NAME attribute to the device node.

The B_DEVICE_UNIQUE_ID should be used in case the device has a unique ID that can be used to identify it, and also differentiate it from other devices of the same model and vendor. This information will be added to the file system attributes of all devices published by your driver, so that user applications can identify, say, a USB printer no matter what USB slot it is attached to, and assign it additional data, like paper configuration, or recognize it as the default printer.

If your driver implements an API that is used by a support or bus module, you will usually use the B_DEVICE_FIXED_CHILD attribute to specify exactly which child device node you will be talking to. If you support several child nodes, you may want to have a closer look at the section explaining how to write a bus driver.

In addition to the child nodes a driver registers itself, a driver can either have dynamic children or fixed children, never both. Also, fixed children are registered before register_child_devices() is called, while dynamic children are registered afterwards.

4. Publishing a Device

	publish_device(device_node *node, const char *path,
		const char *deviceModuleName);

A device module must export the following API:

• init_device()
  This is called when the open() is called on this device for the first time. You may want to create a private data structure that is passed on to all subsequent calls of the open() function that your device exports.
• uninit_device()
  Is called when the last file descriptor to the device had been closed.
• device_removed()
  When the device node your device belongs to is going to be removed, you're notified about this in this function.
• open()
  Called whenever your device is opened.
• close()
  
• free()
  Free the private data structure you allocated in open().
• read()
  
• write()
  
• io()
  This is a replacement for the read(), and write() calls, and allows, among other things, for asynchronous I/O. This functionality has not yet been implemented, though (see below).
• control()
  
• select()
  
• deselect()
  

5. Writing a Bus Driver

A bus driver is a driver that represents a bus where one or more arbitrary devices can be attached to.

There are two basic types of busses: intelligent busses like PCI or USB that know a lot about the devices attached to it, like a generic device type, as well as device and vendor ID information, and simple untyped/generic busses that either have not all the information (like device type) or don't even know what and if any devices are attached. The device manager has been written in such a way that device exploration makes use of additional information the bus can provide in order to find a responsible device driver faster, and with less overhead.

5.1. Writing an Intelligent Bus Driver

If your bus knows what type of device is attached to, and also has vendor and device ID information about that device, it is considered to be an intelligent bus. The bus driver is supposed to have one parent node representing the bus, and to create a child node for each device attached to the bus.

The additional information you have about the devices are attached to the device node in the following attributes:

• B_DEVICE_VENDOR_ID
  The vendor ID - this ID has only to be valid in the namespace of your bus.
• B_DEVICE_ID
  The device ID.
• B_DEVICE_TYPE
  The device type as defined by the PCI class base information.
• B_DEVICE_SUB_TYPE
  The device sub type as defined by the PCI sub class information.
• B_DEVICE_INTERFACE
  The device interface type as defined by the PCI class API information.

You can use the B_DEVICE_FLAGS attribute to define how the device manager finds the children of the devices you exported. For this kind of bus drivers, you will usually only want to specify B_FIND_CHILD_ON_DEMAND here, which causes the driver only to be searched when the system asks for it.

5.2. Writing a Simple Bus Driver

A bus can be simple in a number of ways:

1. It may not know how many or if any devices are attached to it
2. It cannot retrieve any type information about the devices it has, but knows all devices that are attached to it

An example of the latter would be the Zorro bus of the Amiga - it only has information about the vendor and device ID, but no type information. It should be implemented like an intelligent bus, though, with the type information simply omitted.

Therefore, this section is about the former case, that is, a simple bus like the ISA bus. Since it doesn't know anything about its children, it does not publish any child nodes, instead, it will just specify the B_FIND_MULTIPLE_CHILDREN and B_FIND_CHILD_ON_DEMAND flags for its device node. Since there is no additional information about this bus, the device manager will assume a simple bus, and will try to find drivers on demand only.

The generic bus

6. Open Issues

6.1. generic/simple busses

6.2. Unpublishing

6.4. Versioning

The way the device manager works, it makes versioning of modules (which are supposed to be one of the strong points of the module system) much harder or even impossible. While the device manager could introduce a new API and could translate between a "driver_v1", and a "driver_v2" API on the fly, it's not yet possible for a PCI sub module to do the same thing.

Proposed Solution: Add attribute B_DEVICE_ALTERNATE_VERSION that specifies alternate versions of the module API this device node supports. We would then need a request_version() or set_version() function (to be called from supports_device()) that allows to specify the version of the parent node this device node wants to talk to.

6.5. Unregistering Nodes

6.6. Support for generic drivers is missing

This should probably be done by simply adding a simple bus driver named "generic" that generic drivers need to ask for.

6.7. Mappings, And Other Optimizations

Due to the way the device tree is built, the device manager could remember which driver served a given device node. That way, it wouldn't need to search for a driver anymore, but could just pick it up. Practically, the device manager should cache the type (and/or vendor/device) information of a node, and assign one or more drivers (via module name) to this information. It should also remember negative outcome, that is if there is no driver supporting the hardware.

This way, only the first boot would require an actual search for drivers, as subsequent boots would reuse the type-driver assignments. If a new driver is installed, the cached assignments would need to be updated immediately. If a driver has been installed outside of the running system, the device manager might want to create a hash per module directory to see if anything changed to flush the cache. Alternatively or additionally, the boot loader could have a menu causing the cache to be ignored.

It would be nice to find a way for generic and simple busses to reduce the amount of searching necessary for them. One way would be to remember which driver supports which bus - but this information is currently only accessible derived from what the driver does, and is therefore not reliable or complete. A separately exported information would be necessary for this.

Also, when looking for a generic or simple bus driver, actual directories could be omitted; currently, driver search is always recursive, as that's how the module mechanism is working. Eventually, we might want to extend the open_module_list_etc() call a bit more to accomplish that.