path: root/en_US.ISO8859-1/books/arch-handbook/driverbasics/chapter.xml

<?xml version="1.0" encoding="iso-8859-1"?>
<!--
     The FreeBSD Documentation Project

     $FreeBSD$
-->

<chapter xmlns="http://docbook.org/ns/docbook"
  xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0"
  xml:id="driverbasics">

  <info>
    <title>Writing FreeBSD Device Drivers</title>

    <authorgroup>
      <author>
	<personname>
	  <firstname>Murray</firstname>
	  <surname>Stokely</surname>
	</personname>

	<contrib>Written by </contrib>
      </author>
    </authorgroup>

    <authorgroup>
      <author>
	<personname>
	  <firstname>J&ouml;rg</firstname>
	  <surname>Wunsch</surname>
	</personname>

	<contrib>Based on intro(4) manual page by </contrib>
      </author>
    </authorgroup>
  </info>

  <sect1 xml:id="driverbasics-intro">
    <title>Introduction</title>

    <indexterm><primary>device driver</primary></indexterm>
    <indexterm><primary>pseudo-device</primary></indexterm>

    <para>This chapter provides a brief introduction to writing device
      drivers for FreeBSD.  A device in this context is a term used
      mostly for hardware-related stuff that belongs to the system,
      like disks, printers, or a graphics display with its keyboard.
      A device driver is the software component of the operating
      system that controls a specific device.  There are also
      so-called pseudo-devices where a device driver emulates the
      behavior of a device in software without any particular
      underlying hardware.  Device drivers can be compiled into the
      system statically or loaded on demand through the dynamic kernel
      linker facility `kld'.</para>

    <indexterm><primary>device nodes</primary></indexterm>

    <para>Most devices in a &unix;-like operating system are accessed
      through device-nodes, sometimes also called special files.
      These files are usually located under the directory
      <filename>/dev</filename> in the filesystem hierarchy.</para>

    <para>Device drivers can roughly be broken down into two
      categories; character and network device drivers.</para>

  </sect1>

  <sect1 xml:id="driverbasics-kld">
    <title>Dynamic Kernel Linker Facility - KLD</title>

    <indexterm>
      <primary>kernel linking</primary>
      <secondary>dynamic</secondary>
    </indexterm>
    <indexterm>
      <primary>kernel loadable modules (KLD)</primary>
    </indexterm>

    <para>The kld interface allows system administrators to
      dynamically add and remove functionality from a running system.
      This allows device driver writers to load their new changes into
      a running kernel without constantly rebooting to test
      changes.</para>

    <indexterm>
      <primary>kernel modules</primary>
      <secondary>loading</secondary>
    </indexterm>
    <indexterm>
      <primary>kernel modules</primary>
      <secondary>unloading</secondary>
    </indexterm>
    <indexterm>
      <primary>kernel modules</primary>
      <secondary>listing</secondary>
    </indexterm>

    <para>The kld interface is used through:</para>

    <itemizedlist>
      <listitem>
	<simpara><command>kldload</command> - loads a new kernel
	  module</simpara></listitem>
      <listitem>
	<simpara><command>kldunload</command> - unloads a kernel
	  module</simpara></listitem>
      <listitem>
	<simpara><command>kldstat</command> - lists loaded
	  modules</simpara></listitem>
    </itemizedlist>

    <para>Skeleton Layout of a kernel module</para>

    <programlisting>/*
 * KLD Skeleton
 * Inspired by Andrew Reiter's Daemonnews article
 */

#include &lt;sys/types.h&gt;
#include &lt;sys/module.h&gt;
#include &lt;sys/systm.h&gt;  /* uprintf */
#include &lt;sys/errno.h&gt;
#include &lt;sys/param.h&gt;  /* defines used in kernel.h */
#include &lt;sys/kernel.h&gt; /* types used in module initialization */

/*
 * Load handler that deals with the loading and unloading of a KLD.
 */

static int
skel_loader(struct module *m, int what, void *arg)
{
  int err = 0;

  switch (what) {
  case MOD_LOAD:                /* kldload */
    uprintf("Skeleton KLD loaded.\n");
    break;
  case MOD_UNLOAD:
    uprintf("Skeleton KLD unloaded.\n");
    break;
  default:
    err = EOPNOTSUPP;
    break;
  }
  return(err);
}

/* Declare this module to the rest of the kernel */

static moduledata_t skel_mod = {
  "skel",
  skel_loader,
  NULL
};

DECLARE_MODULE(skeleton, skel_mod, SI_SUB_KLD, SI_ORDER_ANY);</programlisting>


    <sect2>
      <title>Makefile</title>

      <para>&os; provides a system makefile to simplify compiling a
	kernel module.</para>

      <programlisting>SRCS=skeleton.c
KMOD=skeleton

.include &lt;bsd.kmod.mk&gt;</programlisting>

      <para>Running <command>make</command> with this makefile
	will create a file <filename>skeleton.ko</filename> that can
	be loaded into the kernel by typing:</para>

      <screen>&prompt.root; <userinput>kldload -v ./skeleton.ko</userinput></screen>
    </sect2>
  </sect1>

  <sect1 xml:id="driverbasics-char">
    <title>Character Devices</title>

    <indexterm>
      <primary>character devices</primary>
    </indexterm>
    <para>A character device driver is one that transfers data
      directly to and from a user process.  This is the most common
      type of device driver and there are plenty of simple examples in
      the source tree.</para>

    <para>This simple example pseudo-device remembers whatever values
      are written to it and can then echo them back when
      read.</para>

    <example>
      <title>Example of a Sample Echo Pseudo-Device Driver for
	&os;&nbsp;10.X - 12.X</title>

      <programlisting>/*
 * Simple Echo pseudo-device KLD
 *
 * Murray Stokely
 * Søren (Xride) Straarup
 * Eitan Adler
 */

#include &lt;sys/types.h&gt;
#include &lt;sys/module.h&gt;
#include &lt;sys/systm.h&gt;  /* uprintf */
#include &lt;sys/param.h&gt;  /* defines used in kernel.h */
#include &lt;sys/kernel.h&gt; /* types used in module initialization */
#include &lt;sys/conf.h&gt;   /* cdevsw struct */
#include &lt;sys/uio.h&gt;    /* uio struct */
#include &lt;sys/malloc.h&gt;

#define BUFFERSIZE 255

/* Function prototypes */
static d_open_t      echo_open;
static d_close_t     echo_close;
static d_read_t      echo_read;
static d_write_t     echo_write;

/* Character device entry points */
static struct cdevsw echo_cdevsw = {
	.d_version = D_VERSION,
	.d_open = echo_open,
	.d_close = echo_close,
	.d_read = echo_read,
	.d_write = echo_write,
	.d_name = "echo",
};

struct s_echo {
	char msg[BUFFERSIZE + 1];
	int len;
};

/* vars */
static struct cdev *echo_dev;
static struct s_echo *echomsg;

MALLOC_DECLARE(M_ECHOBUF);
MALLOC_DEFINE(M_ECHOBUF, "echobuffer", "buffer for echo module");

/*
 * This function is called by the kld[un]load(2) system calls to
 * determine what actions to take when a module is loaded or unloaded.
 */
static int
echo_loader(struct module *m __unused, int what, void *arg __unused)
{
	int error = 0;

	switch (what) {
	case MOD_LOAD:                /* kldload */
		error = make_dev_p(MAKEDEV_CHECKNAME | MAKEDEV_WAITOK,
		    &amp;echo_dev,
		    &amp;echo_cdevsw,
		    0,
		    UID_ROOT,
		    GID_WHEEL,
		    0600,
		    "echo");
		if (error != 0)
			break;

		echomsg = malloc(sizeof(*echomsg), M_ECHOBUF, M_WAITOK |
		    M_ZERO);
		printf("Echo device loaded.\n");
		break;
	case MOD_UNLOAD:
		destroy_dev(echo_dev);
		free(echomsg, M_ECHOBUF);
		printf("Echo device unloaded.\n");
		break;
	default:
		error = EOPNOTSUPP;
		break;
	}
	return (error);
}

static int
echo_open(struct cdev *dev __unused, int oflags __unused, int devtype __unused,
    struct thread *td __unused)
{
	int error = 0;

	uprintf("Opened device \"echo\" successfully.\n");
	return (error);
}

static int
echo_close(struct cdev *dev __unused, int fflag __unused, int devtype __unused,
    struct thread *td __unused)
{

	uprintf("Closing device \"echo\".\n");
	return (0);
}

/*
 * The read function just takes the buf that was saved via
 * echo_write() and returns it to userland for accessing.
 * uio(9)
 */
static int
echo_read(struct cdev *dev __unused, struct uio *uio, int ioflag __unused)
{
	size_t amt;
	int error;

	/*
	 * How big is this read operation?  Either as big as the user wants,
	 * or as big as the remaining data.  Note that the 'len' does not
	 * include the trailing null character.
	 */
	amt = MIN(uio-&gt;uio_resid, uio-&gt;uio_offset &gt;= echomsg-&gt;len + 1 ? 0 :
	    echomsg-&gt;len + 1 - uio-&gt;uio_offset);

	if ((error = uiomove(echomsg-&gt;msg, amt, uio)) != 0)
		uprintf("uiomove failed!\n");

	return (error);
}

/*
 * echo_write takes in a character string and saves it
 * to buf for later accessing.
 */
static int
echo_write(struct cdev *dev __unused, struct uio *uio, int ioflag __unused)
{
	size_t amt;
	int error;

	/*
	 * We either write from the beginning or are appending -- do
	 * not allow random access.
	 */
	if (uio-&gt;uio_offset != 0 &amp;&amp; (uio-&gt;uio_offset != echomsg-&gt;len))
		return (EINVAL);

	/* This is a new message, reset length */
	if (uio-&gt;uio_offset == 0)
		echomsg-&gt;len = 0;

	/* Copy the string in from user memory to kernel memory */
	amt = MIN(uio-&gt;uio_resid, (BUFFERSIZE - echomsg-&gt;len));

	error = uiomove(echomsg-&gt;msg + uio-&gt;uio_offset, amt, uio);

	/* Now we need to null terminate and record the length */
	echomsg-&gt;len = uio-&gt;uio_offset;
	echomsg-&gt;msg[echomsg-&gt;len] = 0;

	if (error != 0)
		uprintf("Write failed: bad address!\n");
	return (error);
}

DEV_MODULE(echo, echo_loader, NULL);</programlisting>
    </example>

    <para>With this driver loaded try:</para>

    <screen>&prompt.root; <userinput>echo -n "Test Data" &gt; /dev/echo</userinput>
&prompt.root; <userinput>cat /dev/echo</userinput>
Opened device "echo" successfully.
Test Data
Closing device "echo".</screen>

    <para>Real hardware devices are described in the next
      chapter.</para>
  </sect1>

  <sect1 xml:id="driverbasics-block">
    <title>Block Devices (Are Gone)</title>

    <indexterm><primary>block devices</primary></indexterm>

    <para>Other &unix; systems may support a second type of disk
      device known as block devices.  Block devices are disk devices
      for which the kernel provides caching.  This caching makes
      block-devices almost unusable, or at least dangerously
      unreliable.  The caching will reorder the sequence of write
      operations, depriving the application of the ability to know the
      exact disk contents at any one instant in time.</para>

    <para>This makes predictable and reliable crash recovery of
      on-disk data structures (filesystems, databases, etc.)
      impossible.  Since writes may be delayed, there is no way
      the kernel can report to the application which particular
      write operation encountered a write error, this further
      compounds the consistency problem.</para>

    <para>For this reason, no serious applications rely on block
      devices, and in fact, almost all applications which access
      disks directly take great pains to specify that character
      (or <quote>raw</quote>) devices should always be used.  As
      the implementation of the aliasing of each disk (partition) to
      two devices with different semantics significantly complicated
      the relevant kernel code, &os; dropped support for cached disk
      devices as part of the modernization of the disk I/O
      infrastructure.</para>
  </sect1>

  <sect1 xml:id="driverbasics-net">
    <title>Network Drivers</title>

    <indexterm>
      <primary>network devices</primary>
    </indexterm>
    <para>Drivers for network devices do not use device nodes in order
      to be accessed.  Their selection is based on other decisions
      made inside the kernel and instead of calling open(), use of a
      network device is generally introduced by using the system call
      socket(2).</para>

    <para>For more information see ifnet(9), the source of the
      loopback device, and Bill Paul's network drivers.</para>
  </sect1>
</chapter>