StackOverflow2013

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copied!<p>The read() and write() callback functions in our ‘cmosram.c’ device-driver only transfer a single byte of data for each time called, so it takes 128 system-calls to read all of the RTC storage-locations!</p> <p>Can you improve this driver’s efficiency, by modifying its read() and write() functions, so they’ll transfer as many valid bytes as the supplied buffer’s space could hold?</p> <p>code is as follows</p> <pre><code>char modname[] = "cmosram"; // name of this kernel module char devname[] = "cmos"; // name for the device's file int my_major = 70; // major ID-number for driver int cmos_size = 128; // total bytes of cmos memory int write_max = 9; // largest 'writable' address ssize_t my_read( struct file *file, char *buf, size_t len, loff_t *pos ) { unsigned char datum; if ( *pos >= cmos_size ) return 0; outb( *pos, 0x70 ); datum = inb( 0x71 ); if ( put_user( datum, buf ) ) return -EFAULT; *pos += 1; return 1; } ssize_t my_write( struct file *file, const char *buf, size_t len, loff_t *pos ) { unsigned char datum; if ( *pos >= cmos_size ) return 0; if ( *pos > write_max ) return -EPERM; if ( get_user( datum, buf ) ) return -EFAULT; outb( *pos, 0x70 ); outb( datum, 0x71 ); *pos += 1; return 1; } loff_t my_llseek( struct file *file, loff_t pos, int whence ) { loff_t newpos = -1; switch ( whence ) { case 0: newpos = pos; break; // SEEK_SET case 1: newpos = file->f_pos + pos; break; // SEEK_CUR case 2: newpos = cmos_size + pos; break; // SEEK_END } if (( newpos < 0 )||( newpos > cmos_size )) return -EINVAL; file->f_pos = newpos; return newpos; } struct file_operations my_fops = { owner: THIS_MODULE, llseek: my_llseek, write: my_write, read: my_read, }; static int __init my_init( void ) { printk( "<1>\nInstalling \'%s\' module ", devname ); printk( "(major=%d) \n", my_major ); return register_chrdev( my_major, devname, &my_fops ); } static void __exit my_exit(void ) { unregister_chrdev( my_major, devname ); printk( "<1>Removing \'%s\' module\n", devname ); } module_init( my_init ); module_exit( my_exit ); MODULE_LICENSE("GPL"); </code></pre>

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