<!--

	The FreeBSD Documentation Project

	$FreeBSD$

	Originally by: Jeroen Ruigrok van der Warven

	Date: newbus-draft.txt,v 1.8 2001/01/25 08:01:08

	Copyright (c) 2000 Jeroen Ruigrok van der Warven (asmodai@wxs.nl)

	Copyright (c) 2002 Hiten Mahesh Pandya (hiten@uk.FreeBSD.org)

	Future Additions:

		o Expand the information about device_t, and explain each and 

		  every field in it.

		o Add information about the bus_* functions.

		o Add information about bus specific (e.g. PCI) functions.

		o Add a reference section for additional information.

		o Add more newbus related structures and typedefs.

		o Add a 'Terminology' section.

		o Add information on resource manager functions, busspace 

		manager functions, newbus events related functions.

		o More cleanup ... !

	Provided under the FreeBSD Documentation License.

-->

<chapter id="newbus">

  <chapterinfo>

    <authorgroup>

      <author>

        <firstname>Jeroen</firstname>

	 <surname>Ruigrok van der Werven</surname>

	 <affiliation><address><email>asmodai@FreeBSD.org</email></address>

	 </affiliation>

	 <contrib>Written by </contrib>

      </author>

    </authorgroup>

    <authorgroup>

      <author>

        <firstname>Hiten</firstname>

	<surname>Pandya</surname>

	<affiliation><address><email>hiten@uk.FreeBSD.org</email></address>

	</affiliation>

      </author>

    </authorgroup>

  </chapterinfo>

  <title>Newbus</title>

  <para><emphasis>Special thanks to Mathew N. Dodd, Warner Losh, Bill Paul.

  Daug Rabson, Mike Smith, Peter Wemm and Scott Long.</emphasis></para>

  <para>This chapter explains the Newbus device framework in detail.</para>

  <sect1 id="devdrivers">

    <title>Device Drivers</title>

    <sect2>

      <title>Purpose of a Device Driver</title>

      <para>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 <emphasis>device driver interface (DDI)</emphasis> is 

      the defined interface between the kernel and the device driver component.

      </para>

    </sect2>

    <sect2>

      <title>Types of Device Drivers</title>

      <para>There used to be days in &unix;, and thus FreeBSD, in which there 

      were four types of devices defined:</para>

      <itemizedlist>

        <listitem><para>block device drivers</para></listitem>

        <listitem><para>character device drivers</para></listitem>

        <listitem><para>network device drivers</para></listitem>

        <listitem><para>pseudo-device drivers</para></listitem>

      </itemizedlist>

      <para><emphasis>Block devices</emphasis> performed in way that used 

      fixed size blocks [of data].  This type of driver depended on the 

      so called <emphasis>buffer cache</emphasis>, which had the purpose

      to cache accessed blocks of data in a dedicated part of the 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.</para>

    </sect2>

    <sect2>

      <title>Character devices</title>

      <para>However, in the versions of FreeBSD 4.0 and onward the

      distinction between block and character devices became non-existent.

      </para>

    </sect2>

  </sect1>

  <sect1 id="newbus-overview">

    <!--

      Real title:

      Newbus, Busspace and the Resource Manager, an Explanation of the Possibilities

    -->

    <title>Overview of Newbus</title>

    <para><emphasis>Newbus</emphasis> 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 <emphasis>Operating System</emphasis>.</para>

    <para>Its main features include amongst others:</para>

    <itemizedlist>

      <listitem><para>dynamic attaching</para></listitem>

      <listitem><para>easy modularization of drivers</para></listitem>

      <listitem><para>pseudo-busses</para></listitem>

    </itemizedlist>

    <para>One of the most prominent changes is the migration from the flat and 

    ad-hoc system to a device tree lay-out.</para>

    <para>At the top level resides the <emphasis><quote>root</quote></emphasis>

    device which is the parent to hang all other devices on.  For each 

    architecture, there is typically a single child of <quote>root</quote> 

    which has such things as <emphasis>host-to-PCI bridges</emphasis>, etc. 

    attached to it.  For x86, this <quote>root</quote> device is the 

    <emphasis><quote>nexus</quote></emphasis> device and for Alpha, various 

    different different models of Alpha have different top-level devices 

    corresponding to the different hardware chipsets, including 

    <emphasis>lca</emphasis>, <emphasis>apecs</emphasis>, 

    <emphasis>cia</emphasis> and <emphasis>tsunami</emphasis>.</para>

    <para>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 <emphasis><quote>bus</quote></emphasis>.  

    Examples of common busses in the system are ISA and PCI which manage lists 

    of devices attached to ISA and PCI busses respectively.</para>

    <para>Often, a connection between different kinds of bus is represented by 

    a <emphasis><quote>bridge</quote></emphasis> device which normally has one 

    child for the attached bus.  An example of this is a 

    <emphasis>PCI-to-PCI bridge</emphasis> which is represented by a device 

    <emphasis><devicename>pcibN</devicename></emphasis> on the parent PCI bus 

    and has a child <emphasis><devicename>pciN</devicename></emphasis> 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.</para>

    <para>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.</para>

    <tip><para>An ISA device might want to map its IO port at 

    <literal>0x23c</literal>, 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 <literal>0x23c</literal> IO port 

    request might become memory-mapped at <literal>0xb000023c</literal> on a 

    <acronym>MIPS</acronym> box by the PCI bridge.</para></tip>

    <para>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).

    </para>

    <para>In order to normalize access to memory and port mapped resources, 

    Newbus integrates the <literal>bus_space</literal> 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).</para>

    <para>This support is integrated into the resource allocation mechanism.  

    When a resource is allocated, a driver can retrieve the associated 

    <structfield>bus_space_tag_t</structfield> and 

    <structfield>bus_space_handle_t</structfield> from the resource.</para>

    <para>Newbus also allows for definitions of interface methods in files 

    dedicated to this purpose.  These are the <filename>.m</filename> files 

    that are found under the <filename>src/sys</filename> hierarchy.</para>

    <para>The core of the Newbus system is an extensible 

    <quote>object-based programming</quote> 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 

    <quote>interfaces</quote>.  An <quote>interface</quote> is simply a group 

    of related methods which can be implemented by a device.</para>

    <para>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 <emphasis>auto-configuration</emphasis>, it uses the method 

    table declared by the driver.  A device can later 

    <emphasis>detach</emphasis> from its driver and 

    <emphasis>re-attach</emphasis> to a new driver with a new method table.  

    This allows dynamic replacement of drivers which can be useful for driver 

    development.</para>

    <para>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

    <filename>foo_if.m</filename>).</para>

    <example>

      <title>Newbus Methods</title>

      <programlisting>

      # 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;

      </programlisting>

    </example>

    <para>When this interface is compiled, it generates a header file 

    <quote><filename>foo_if.h</filename></quote> which contains function 

    declarations:</para>

    <programlisting>

      int FOO_DOIT(device_t dev);

      int FOO_DOIT_TO_CHILD(device_t dev, device_t child);

    </programlisting>

    <para>A source file, <quote><filename>foo_if.c</filename></quote> 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.</para>

    <para>The system defines two main interfaces.  The first fundamental 

    interface is called <emphasis><quote>device</quote></emphasis> and 

    includes methods which are relevant to all devices.  Methods in the 

    <emphasis><quote>device</quote></emphasis> interface include 

    <emphasis><quote>probe</quote></emphasis>, 

    <emphasis><quote>attach</quote></emphasis> and 

    <emphasis><quote>detach</quote></emphasis> to control detection of 

    hardware and <emphasis><quote>shutdown</quote></emphasis>, 

    <emphasis><quote>suspend</quote></emphasis> and 

    <emphasis><quote>resume</quote></emphasis> for critical event 

    notification.</para>

    <para>The second, more complex interface is 

    <emphasis><quote>bus</quote></emphasis>.  This interface contains 

    methods suitable for devices which have children, including methods to

    access bus specific per-device information 

    <footnote><para>&man.bus.generic.read.ivar.9; and 

    &man.bus.generic.write.ivar.9;</para></footnote>, event notification 

    (<emphasis><literal>child_detached</literal></emphasis>, 

    <emphasis><literal>driver_added</literal></emphasis>) and resource 

    management (<emphasis><literal>alloc_resource</literal></emphasis>, 

    <emphasis><literal>activate_resource</literal></emphasis>, 

    <emphasis><literal>deactivate_resource</literal></emphasis>, 

    <emphasis><literal>release_resource</literal></emphasis>).

    <para>Many methods in the <quote>bus</quote> 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 

    <literal>BUS_TEARDOWN_INTR(device_t dev, device_t child, ...)</literal>

    can be called using the function 

    <literal>bus_teardown_intr(device_t child, ...)</literal>.</para>

    <para>Some bus types in the system define additional interfaces to 

    provide access to bus-specific functionality.  For instance, the PCI

    bus driver defines the <quote>pci</quote> interface which has two 

    methods <emphasis><literal>read_config</literal></emphasis> and 

    <emphasis><literal>write_config</literal></emphasis> for accessing the 

    configuration registers of a PCI device.</para>

  </sect1>

  <sect1 id="newbus-api">

    <title>Newbus API</title>

    <para>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.</para>

    <sect2>

      <title>Important locations in the source hierarchy</title>

      <para><filename>src/sys/[arch]/[arch]</filename> - Kernel code for a 

      specific machine architecture resides in this directory. for example,

      the <literal>i386</literal> architecture, or the 

      <literal>SPARC64</literal> architecture.</para>

      <para><filename>src/sys/dev/[bus]</filename> - device support for a 

      specific <literal>[bus]</literal> resides in this directory.</para>

      <para><filename>src/sys/dev/pci</filename> - PCI bus support code

      resides in this directory.</para>

      <para><filename>src/sys/[isa|pci]</filename> - PCI/ISA device drivers

      reside in this directory.  The PCI/ISA bus support code used to exist

      in this directory in FreeBSD version <literal>4.0</literal>.</para>

    </sect2>

    <sect2>

      <title>Important structures and type definitions</title>

      <para><literal>devclass_t</literal> - This is a type definition of a

      pointer to a <literal>struct devclass</literal>.</para>

      <para><literal>device_method_t</literal> - This is same as 

      <literal>kobj_method_t</literal> (see 

      <filename>src/sys/kobj.h</filename>).</para>

      <para><literal>device_t</literal> - This is a type definition of a 

      pointer to a <literal>struct device</literal>.

      <literal>device_t</literal> represents a device in the system.  It is

      a kernel object.  See <filename>src/sys/sys/bus_private.h</filename> 

      for implementation details.</para>

      <para><literal>driver_t</literal> - This is a type definition which,

      references <literal>struct driver</literal>.  The 

      <literal>driver</literal> struct is a class of the 

      <literal>device</literal> kernel object; it also holds data private 

      to for the driver.</para>

      <figure>

        <title><emphasis>driver_t</emphasis> implementation</title>

	<programlisting>

	  struct driver {

	     	KOBJ_CLASS_FIELDS;

	      	void	*priv;			/* driver private data */

	  };

	</programlisting>

      </figure>

      <para>A <literal>device_state_t</literal> type, which is

      an enumeration, <literal>device_state</literal>.  It contains

      the possible states of a Newbus device before and after the

      autoconfiguration process.</para>

      <figure>

        <title>Device states<emphasis>device_state_t</emphasis></title>

	<programlisting>

	  /*

	   * 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;

	</programlisting>

      </figure>

    </sect2>

  </sect1>

</chapter>