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	<META NAME="AUTHOR" CONTENT="Georg Potthast">
	<META NAME="CREATED" CONTENT="20170105;19172240">
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			<H2 CLASS="western" STYLE="font-style: normal">Writing kernel mode
			drivers for ELKS</H2>
			<P STYLE="margin-bottom: 0.5cm"><FONT FACE="Arial, sans-serif">Like
			most alternative operating systems, ELKS is missing drivers for
			many peripherals. Therefore this text describes to write
			additional drivers.</FONT></P>
			<P STYLE="margin-bottom: 0.5cm"><FONT FACE="Arial, sans-serif">As
			a proof of concept, the 'hellodev' driver will be implemented now:</FONT></P>
			<PRE CLASS="western">/*
 * hello driver for ELKS kernel
 */

#include &lt;linuxmt/kernel.h&gt; /* printk() */
#include &lt;linuxmt/fs.h&gt;     /* file_operations struct */
#include &lt;string.h&gt;

#define HELLO_DEVICE_NAME       &quot;hellodev&quot;
#define HELLO_MAJOR     11

static char message[256];
char *msgptr = message;

static int hello_write(struct inode *inode, struct file *file, char* buf, int len)
{
    memcpy_fromfs(message,buf,len);
    printk(&quot;hellodev: hello_write() called\n&quot;);
    return 0;
}

static int hello_read(struct inode *inode, struct file *file, char* buf, int len)
{
    memcpy_tofs(buf, message, len);
    printk(&quot;hellodev: hello_read() called\n&quot;);
    return len;
}

static int hello_open(struct inode *inode, struct file *file)
{
    printk(&quot;hellodev: hello_open() called\n&quot;);
    return 0;
}

static void hello_release(struct inode *inode, struct file *file)
{
    printk(&quot;hellodev: hello_release() called\n&quot;);
    return;
}

static struct file_operations hello_fops = {
    NULL,                       /* lseek */
    hello_read,                 /* read */
    hello_write,                /* write */
    NULL,                       /* readdir */
    NULL,                       /* select */
    NULL,                       /* ioctl */
    hello_open,                 /* open */
    hello_release               /* release */
    
#ifdef BLOAT_FS
        ,
    NULL,                       /* fsync */
    NULL,                       /* check_media_type */
    NULL                        /* revalidate */
#endif
};

void hello_init(void)
{
        printk(&quot;hellodev: hello_init() called\n&quot;);

    /* register device */
    if (register_chrdev(HELLO_MAJOR, HELLO_DEVICE_NAME, &amp;hello_fops))
        printk(&quot;hellodev: unable to register\n&quot;);
}</PRE><P STYLE="margin-bottom: 0.5cm">
			<FONT FACE="Arial, sans-serif">This driver just implements the
			functions hello_init(), hello_open(), hello_read(), hello_write()
			and hello_release(). When these functions are called it outputs a
			message using the printk() statement. This is the equivalent for
			printf() in kernel code. Also the functions memcpy_fromfs() and
			memcpy_tofs() are used. The kernel will for security reasons not
			accept pointers from userspace which may be faulty. Therefore you
			have to use these functions. For single chars there are equivalent
			functions available called get_user_char() and put_user_char().
			You will also not be able to use inportb() and outportb()
			functions, use the inb() and inb_p() or out() and out_p()
			functions instead. The &bdquo;_p&ldquo; versions insert a small
			pause or delay before returning to support slower devices.</FONT></P>
			<P STYLE="margin-bottom: 0.5cm"><FONT FACE="Arial, sans-serif">When
			the driver is loaded the kernel will call hello_init() and the
			hellodev driver will execute the register_chrdev() function. This
			function contains the device name which will appear in the /dev
			directory and a pointer to the file_operations structure called
			hello_ops. In there the kernel will find the addresses of the
			functions defined in the driver.</FONT></P>
			<P STYLE="margin-bottom: 0.5cm"><FONT FACE="Arial, sans-serif">The
			kernel identifies the driver by its unique the major number. In
			our demo driver we have included these macros in the code: </FONT>
			</P>
			<PRE CLASS="western">#define HELLO_DEVICE_NAME       &quot;hellodev&quot;
#define HELLO_MAJOR     11</PRE><P STYLE="margin-bottom: 0.5cm">
			<FONT FACE="Arial, sans-serif">Usually in ELKS major numbers are defined
			in the elks/include/linuxmt/major.h file which the drivers
			include. Refer to the major numbers in that file to determine the next
			available major number. That file also needs to be changed. Increase
			the maximum number of char devices by 1:</FONT></P>
			<P STYLE="margin-bottom: 0.5cm"><FONT FACE="Arial, sans-serif">#define
			MAX_CHRDEV 12</FONT></P>
			<P STYLE="margin-bottom: 0.5cm"><FONT FACE="Arial, sans-serif">Put
			the code above as the hellodev.c file into the
			elks/arch/i86/drivers/char/ directory and add it to the Makefile
			in this directory:</FONT></P>
			<PRE CLASS="western">
OBJS  = init.o mem.o ntty.o meta.o tcpdev.o pty.o lp.o <B>hellodev.o</B>
			</PRE><P STYLE="margin-bottom: 0.5cm">
			<FONT FACE="Arial, sans-serif">Also edit the init.c file:</FONT></P>
			<PRE CLASS="western">void chr_dev_init(void)
{
#if 1
hello_init();
#endif
#ifdef CONFIG_CHAR_DEV_RS
rs_init();</PRE><P STYLE="margin-bottom: 0.5cm">
			<FONT FACE="Arial, sans-serif"><FONT SIZE=3>Normally, you would
			define a macro in the elks/arch/i86/defconfig file which can be
			modified with &bdquo;menuconfig&ldquo; and modify the files
			elks/arch/i86/drivers/char/config.in plus
			Documentation/Configure.help.</FONT></FONT></P>
			<P STYLE="margin-bottom: 0.5cm"><FONT FACE="Arial, sans-serif"><FONT SIZE=3>Further,
			the hello_init() function has to be declared in the
			elks/include/linuxmt/init.h file:</FONT></FONT></P>
			<PRE CLASS="western">extern void xtk_init(void);
extern void hello_init(void);

extern void init_console(void);</PRE><P STYLE="margin-bottom: 0.5cm">
			<FONT FACE="Arial, sans-serif"><FONT SIZE=3>To load the driver
			simply add it after the last character device in the
			image/Make.devices script:</FONT></FONT></P>
			<PRE CLASS="western">
# CGATEXT
ifdef CONFIG_CHAR_DEV_CGATEXT
	$(MKDEV) /dev/cgatext	c 10 0
endif
	$(MKDEV) /dev/hellodev	c 11 0
        	</PRE><P STYLE="margin-bottom: 0.5cm">
			<FONT FACE="Arial, sans-serif"><FONT SIZE=3>The letter &bdquo;c&ldquo;
			stands for character driver while the number 11 is the major number
			and zero the minor number. Be careful to use the major number we defined
			above.</FONT></FONT>
			</P>
			<P STYLE="margin-bottom: 0.5cm"><FONT FACE="Arial, sans-serif"><FONT SIZE=3>To
			sum it up, you added hellodev.c into the
			elks/arch/i86/drivers/char directory and modified init.c and the
			Makefile in there. Also you modified major.h and init.h in the
			elks/include/linuxmt directory and the file Make.devices in the
			image directory.</FONT></FONT></P>
			<P STYLE="margin-bottom: 0.5cm"><FONT FACE="Arial, sans-serif">Now
			you can compile ELKS again and it will include the hellodev driver
			and load it. If you enter &bdquo;ls /dev&ldquo; you will see a
			device node called hellodev.</FONT></P>
			<P STYLE="margin-bottom: 0.5cm"><FONT FACE="Arial, sans-serif">To
			test this driver use this program which you may call drvtest.c:</FONT></P>
			<PRE CLASS="western">#include&lt;stdio.h&gt;
#include&lt;stdlib.h&gt;
#include&lt;errno.h&gt;
#include&lt;fcntl.h&gt;
 
#define BUFFER_LENGTH 256               
static char receive[BUFFER_LENGTH];     
 
int main(){
   int ret, fd;
   char stringToSend[BUFFER_LENGTH];

   printf(&quot;Starting device test code example...\n&quot;);

   fd = open(&quot;/dev/hellodev&quot;, O_RDWR);  

   if (fd &lt; 0){
      perror(&quot;Failed to open the device...&quot;);
      return errno;
   }

   printf(&quot;Type in a short string to send to the kernel module:\n&quot;);
   scanf(&quot;%s&quot;, stringToSend);                // Read in a string
   printf(&quot;Writing message to the device [%s].\n&quot;, stringToSend);

   ret = write(fd, stringToSend, strlen(stringToSend)); 
   if (ret &lt; 0){
      perror(&quot;Failed to write the message to the device.&quot;);
      return errno;
   }
 
   printf(&quot;Now reading the string back from the device...\n&quot;);
   ret = read(fd, &amp;receive[0], BUFFER_LENGTH);        
   if (ret &lt; 0){
      perror(&quot;Failed to read the message from the device.&quot;);
      return errno;
   }

   printf(&quot;The received message is: [%s]\n&quot;, receive);

   return 0;
}</PRE><P STYLE="margin-bottom: 0.5cm">
			<FONT FACE="Arial, sans-serif">To compile this program enter &bdquo;
			ia16-elf-gcc drvtest.c -o drvtest -melks-libc -mcmodel=small&ldquo;
			and e.g. copy the executable drvtest to the image file using a loop
			device.</FONT></P>
			<P STYLE="margin-bottom: 0.5cm"><FONT FACE="Arial, sans-serif">Here
			are some additional notes how ELKS drivers work describing the
			concepts used above taken from the fs.txt file:<BR><BR>The
			inode-&gt;i_op pointer is initialised in the function which reads
			in an inode, e.g. V1_minix_read_inode, and its contents depend on
			the type of file. If the node is a character or block device, i_op
			points to chrdev_inode_operations or blkdev_inode_operations
			respectively. These tables are declared in devices.c and contain
			pointers just to the respective &quot;open&quot; functions,
			chrdev_open and blkdev_open. <BR><BR>When the user opens one of
			these inodes, the kernel calls the open function and passes in a
			pointer to the relevant &quot;struct file&quot; record. The fops
			pointer in this record is copied from chrdevs[major].fops or
			blkdevs[major].fops, which was set up when the device was
			initialised and registered itself.<BR><BR>So, in the case of the
			console for example, the file record defined as dircon_ops (see
			drivers/char/dircon.c) has pointers to the functions to perform
			I/O to the console. <BR><BR>In the case of block devices, the
			read() and write() functions *don't* point to device-specific read
			and write functions; rather, they point to the generic functions
			block_read() and block_write() in block_dev.c. These take care of
			caching blocks, part-block reads and writes etc.</FONT></P>
			<P STYLE="margin-bottom: 0.5cm"><BR><BR>
			</P>
			<P STYLE="margin-bottom: 0.5cm"><FONT FACE="Arial, sans-serif">4<SUP>th</SUP>
			of March 2017 Georg Potthast</FONT></P>
			<P><BR>
			</P>
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