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<chapter id="driverbasics">
  <title>Writing FreeBSD Device Drivers</title>

  <para>This chapter was written by &a.murray; with selections from a
    variety of sources including the intro(4) man page by
    &a.joerg;.</para>

  <sect1>
    <title>Introduction</title>
    <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
      behaviour 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>

    <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 file system hierarchy.  Until
      devfs is fully integrated into FreeBSD, each device node must be
      created statically and independent of the existence of the
      associated device driver.  Most device nodes on the system are
      created by running <command>MAKEDEV</command>.</para>

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

  </sect1>

  <sect1>
    <title>Dynamic Kernel Linker Facility - KLD</title> 

    <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>

    <para>The kld interface is used through the following
      privileged commands:

    <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 the currently loaded
	modules</simpara></listitem>
    </itemizedlist>
    </para>

    <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 = EINVAL;
    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>FreeBSD provides a makefile include that you can use to
	quickly compile your kernel addition.</para>

      <programlisting>SRCS=skeleton.c
KMOD=skeleton

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

      <para>Simply running <command>make</command> with this makefile
        will create a file <filename>skeleton.ko</filename> that can
        be loaded into your system by typing:
<screen>&prompt.root; kldload -v ./skeleton.ko</screen>
      </para>
    </sect2>
  </sect1>

  <sect1>
    <title>Accessing a device driver</title>

    <para>Unix provides a common set of system calls for user
      applications to use.  The upper layers of the kernel dispatch
      these calls to the corresponding device driver when a user
      accesses a device node.  The <command>/dev/MAKEDEV</command>
      script makes most of the device nodes for your system but if you
      are doing your own driver development it may be necessary to
      create your own device nodes with <command>mknod</command>.
    </para>

    <sect2>
      <title>Creating static device nodes</title>

      <para>The <command>mknod</command> command requires four
	arguments to create a device node.  You must specify the name
	of the device node, the type of device, the major number of
	the device, and the minor number of the device.</para>
    </sect2>

    <sect2>
      <title>Dynamic device nodes</title>

      <para>The device filesystem, or devfs, provides access to the
	kernel's device namespace in the global filesystem namespace.
	This eliminates the problems of potentially having a device
	driver without a static device node, or a device node without
	an installed device driver.  Devfs is still a work in
	progress, but it is already working quite nicely.</para>
    </sect2>

  </sect1>

  <sect1>
    <title>Character Devices</title>

    <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
      you write to it and can then supply them back to you when you
      read from it.</para>

    <programlisting>/*
 * Simple `echo' pseudo-device KLD
 *
 * Murray Stokely
 */

#define MIN(a,b) (((a) < (b)) ? (a) : (b))

#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 */
#include &lt;sys/conf.h&gt;   /* cdevsw struct */
#include &lt;sys/uio.h&gt;    /* uio struct */
#include &lt;sys/malloc.h&gt;

#define BUFFERSIZE 256

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

/* Character device entry points */
static struct cdevsw echo_cdevsw = {
  echo_open,
  echo_close,
  echo_read,
  echo_write,
  noioctl,
  nopoll,
  nommap,
  nostrategy,
  "echo",
  33,                   /* reserved for lkms - /usr/src/sys/conf/majors */
  nodump,
  nopsize,
  D_TTY,
  -1
};

typedef struct s_echo {
  char msg[BUFFERSIZE];
  int len;
} t_echo;

/* vars */
static dev_t sdev;
static int len;
static int count;
static t_echo *echomsg;

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

/*
 * This function acts 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, int what, void *arg)
{
  int err = 0;
  
  switch (what) {
  case MOD_LOAD:                /* kldload */
    sdev = make_dev(<literal>&</literal>echo_cdevsw,
		    0,
		    UID_ROOT,
		    GID_WHEEL,
		    0600,
		    "echo");
    /* kmalloc memory for use by this driver */
    /*    malloc(256,M_ECHOBUF,M_WAITOK); */
    MALLOC(echomsg, t_echo *, sizeof(t_echo), M_ECHOBUF, M_WAITOK);
    printf("Echo device loaded.\n");
    break;
  case MOD_UNLOAD:
    destroy_dev(sdev);
    FREE(echomsg,M_ECHOBUF);
    printf("Echo device unloaded.\n");
    break;
  default:
    err = EINVAL;
    break;
  }
  return(err);
}

int 
echo_open(dev_t dev, int oflags, int devtype, struct proc *p)
{
  int err = 0;
  
  uprintf("Opened device \"echo\" successfully.\n");
  return(err);
}

int 
echo_close(dev_t dev, int fflag, int devtype, struct proc *p)
{
  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) 
 */

int
echo_read(dev_t dev, struct uio *uio, int ioflag)
{
  int err = 0;
  int amt;

  /* How big is this read operation?  Either as big as the user wants,
     or as big as the remaining data */
  amt = MIN(uio->uio_resid, (echomsg->len - uio->uio_offset > 0) ? echomsg->len - uio->uio_offset : 0);
  if ((err = uiomove(echomsg->msg + uio->uio_offset,amt,uio)) != 0) {
    uprintf("uiomove failed!\n");
  }

  return err;
}

/*
 * echo_write takes in a character string and saves it
 * to buf for later accessing.
 */

int
echo_write(dev_t dev, struct uio *uio, int ioflag)
{
  int err = 0;

  /* Copy the string in from user memory to kernel memory */
  err = copyin(uio->uio_iov->iov_base, echomsg->msg, MIN(uio->uio_iov->iov_len,BUFFERSIZE));

  /* Now we need to null terminate */
  *(echomsg->msg + MIN(uio->uio_iov->iov_len,BUFFERSIZE)) = 0;
  /* Record the length */
  echomsg->len = MIN(uio->uio_iov->iov_len,BUFFERSIZE);

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

  count++;
  return(err);
}

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

    <para>To install this driver you will first need to make a node on
      your filesystem with a command such as:</para>

    <screen>&prompt.root; mknod /dev/echo c 33 0</screen>

    <para>With this driver loaded you should now be able to type
      something like:</para>

    <screen>&prompt.root; echo -n "Test Data" > /dev/echo
&prompt.root; cat /dev/echo
Test Data</screen>

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

    <para>Additional Resources
    <itemizedlist>
      <listitem><simpara><ulink
	url="http://www.daemonnews.org/200010/blueprints.html">Dynamic
	Kernel Linker (KLD) Facility Programming Tutorial</ulink> -
	<ulink url="http://www.daemonnews.org/">Daemonnews</ulink> October 2000</simpara></listitem>
      <listitem><simpara><ulink
	url="http://www.daemonnews.org/200007/newbus-intro.html">How
	to Write Kernel Drivers with NEWBUS</ulink> - <ulink
	url="http://www.daemonnews.org/">Daemonnews</ulink> July
	2000</simpara></listitem>
    </itemizedlist>
    </para>
  </sect1>

  <sect1>
    <title>Network Drivers</title>

    <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>man ifnet(), loopback device, Bill Paul's drivers,
      etc..</para>

  </sect1>

</chapter>

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