kernel/printk.c

   1 /*
   2  *  linux/kernel/printk.c
   3  *
   4  *  Copyright (C) 1991, 1992  Linus Torvalds
   5  *
   6  * Modified to make sys_syslog() more flexible: added commands to
   7  * return the last 4k of kernel messages, regardless of whether
   8  * they've been read or not.  Added option to suppress kernel printk's
   9  * to the console.  Added hook for sending the console messages
  10  * elsewhere, in preparation for a serial line console (someday).
  11  * Ted Ts'o, 2/11/93.
  12  * Modified for sysctl support, 1/8/97, Chris Horn.
  13  * Fixed SMP synchronization, 08/08/99, Manfred Spraul
  14  *     manfreds@colorfullife.com
  15  * Rewrote bits to get rid of console_lock
  16  *      01Mar01 Andrew Morton <andrewm@uow.edu.au>
  17  */
  18
  19 #include <linux/kernel.h>
  20 #include <linux/mm.h>
  21 #include <linux/tty.h>
  22 #include <linux/tty_driver.h>
  23 #include <linux/smp_lock.h>
  24 #include <linux/console.h>
  25 #include <linux/init.h>
  26 #include <linux/module.h>
  27 #include <linux/interrupt.h>                    /* For in_interrupt() */
  28 #include <linux/config.h>
  29 #include <linux/delay.h>
  30 #include <linux/smp.h>
  31 #include <linux/security.h>
  32 #include <linux/bootmem.h>
  33 #include <linux/syscalls.h>
  34
  35 #include <asm/uaccess.h>
  36
  37 #define __LOG_BUF_LEN   (1 << CONFIG_LOG_BUF_SHIFT)
  38
  39 #ifdef        CONFIG_DEBUG_LL
  40 extern void printascii(char *);
  41 #endif
  42
  43 /* printk's without a loglevel use this.. */
  44 #define DEFAULT_MESSAGE_LOGLEVEL 4 /* KERN_WARNING */
  45
  46 /* We show everything that is MORE important than this.. */
  47 #define MINIMUM_CONSOLE_LOGLEVEL 1 /* Minimum loglevel we let people use */
  48 #define DEFAULT_CONSOLE_LOGLEVEL 7 /* anything MORE serious than KERN_DEBUG */
  49
  50 DECLARE_WAIT_QUEUE_HEAD(log_wait);
  51
  52 int console_printk[4] = {
  53         DEFAULT_CONSOLE_LOGLEVEL,       /* console_loglevel */
  54         DEFAULT_MESSAGE_LOGLEVEL,       /* default_message_loglevel */
  55         MINIMUM_CONSOLE_LOGLEVEL,       /* minimum_console_loglevel */
  56         DEFAULT_CONSOLE_LOGLEVEL,       /* default_console_loglevel */
  57 };
  58
  59 EXPORT_SYMBOL(console_printk);
  60
  61 /*
  62  * Low lever drivers may need that to know if they can schedule in
  63  * their unblank() callback or not. So let's export it.
  64  */
  65 int oops_in_progress;
  66 EXPORT_SYMBOL(oops_in_progress);
  67
  68 /*
  69  * console_sem protects the console_drivers list, and also
  70  * provides serialisation for access to the entire console
  71  * driver system.
  72  */
  73 static DECLARE_MUTEX(console_sem);
  74 struct console *console_drivers;
  75 /*
  76  * This is used for debugging the mess that is the VT code by
  77  * keeping track if we have the console semaphore held. It's
  78  * definitely not the perfect debug tool (we don't know if _WE_
  79  * hold it are racing, but it helps tracking those weird code
  80  * path in the console code where we end up in places I want
  81  * locked without the console sempahore held
  82  */
  83 static int console_locked;
  84
  85 /*
  86  * logbuf_lock protects log_buf, log_start, log_end, con_start and logged_chars
  87  * It is also used in interesting ways to provide interlocking in
  88  * release_console_sem().
  89  */
  90 static DEFINE_SPINLOCK(logbuf_lock);
  91
  92 #define LOG_BUF_MASK    (log_buf_len-1)
  93 #define LOG_BUF(idx) (log_buf[(idx) & LOG_BUF_MASK])
  94
  95 /*
  96  * The indices into log_buf are not constrained to log_buf_len - they
  97  * must be masked before subscripting
  98  */
  99 static unsigned long log_start; /* Index into log_buf: next char to be read by syslog() */
 100 static unsigned long con_start; /* Index into log_buf: next char to be sent to consoles */
 101 static unsigned long log_end;   /* Index into log_buf: most-recently-written-char + 1 */
 102
 103 /*
 104  *      Array of consoles built from command line options (console=)
 105  */
 106 struct console_cmdline
 107 {
 108         char    name[8];                        /* Name of the driver       */
 109         int     index;                          /* Minor dev. to use        */
 110         char    *options;                       /* Options for the driver   */
 111 };
 112
 113 #define MAX_CMDLINECONSOLES 8
 114
 115 static struct console_cmdline console_cmdline[MAX_CMDLINECONSOLES];
 116 static int selected_console = -1;
 117 static int preferred_console = -1;
 118
 119 /* Flag: console code may call schedule() */
 120 static int console_may_schedule;
 121
 122 #ifdef CONFIG_PRINTK
 123
 124 static char __log_buf[__LOG_BUF_LEN];
 125 static char *log_buf = __log_buf;
 126 static int log_buf_len = __LOG_BUF_LEN;
 127 static unsigned long logged_chars; /* Number of chars produced since last read+clear operation */
 128
 129 /*
 130  *      Setup a list of consoles. Called from init/main.c
 131  */
 132 static int __init console_setup(char *str)
 133 {
 134         char name[sizeof(console_cmdline[0].name)];
 135         char *s, *options;
 136         int idx;
 137
 138         /*
 139          *      Decode str into name, index, options.
 140          */
 141         if (str[0] >= '0' && str[0] <= '9') {
 142                 strcpy(name, "ttyS");
 143                 strncpy(name + 4, str, sizeof(name) - 5);
 144         } else
 145                 strncpy(name, str, sizeof(name) - 1);
 146         name[sizeof(name) - 1] = 0;
 147         if ((options = strchr(str, ',')) != NULL)
 148                 *(options++) = 0;
 149 #ifdef __sparc__
 150         if (!strcmp(str, "ttya"))
 151                 strcpy(name, "ttyS0");
 152         if (!strcmp(str, "ttyb"))
 153                 strcpy(name, "ttyS1");
 154 #endif
 155         for(s = name; *s; s++)
 156                 if ((*s >= '0' && *s <= '9') || *s == ',')
 157                         break;
 158         idx = simple_strtoul(s, NULL, 10);
 159         *s = 0;
 160
 161         add_preferred_console(name, idx, options);
 162         return 1;
 163 }
 164
 165 __setup("console=", console_setup);
 166
 167 static int __init log_buf_len_setup(char *str)
 168 {
 169         unsigned long size = memparse(str, &str);
 170         unsigned long flags;
 171
 172         if (size)
 173                 size = roundup_pow_of_two(size);
 174         if (size > log_buf_len) {
 175                 unsigned long start, dest_idx, offset;
 176                 char * new_log_buf;
 177
 178                 new_log_buf = alloc_bootmem(size);
 179                 if (!new_log_buf) {
 180                         printk("log_buf_len: allocation failed\n");
 181                         goto out;
 182                 }
 183
 184                 spin_lock_irqsave(&logbuf_lock, flags);
 185                 log_buf_len = size;
 186                 log_buf = new_log_buf;
 187
 188                 offset = start = min(con_start, log_start);
 189                 dest_idx = 0;
 190                 while (start != log_end) {
 191                         log_buf[dest_idx] = __log_buf[start & (__LOG_BUF_LEN - 1)];
 192                         start++;
 193                         dest_idx++;
 194                 }
 195                 log_start -= offset;
 196                 con_start -= offset;
 197                 log_end -= offset;
 198                 spin_unlock_irqrestore(&logbuf_lock, flags);
 199
 200                 printk("log_buf_len: %d\n", log_buf_len);
 201         }
 202 out:
 203
 204         return 1;
 205 }
 206
 207 __setup("log_buf_len=", log_buf_len_setup);
 208
 209 /*
 210  * Commands to do_syslog:
 211  *
 212  *      0 -- Close the log.  Currently a NOP.
 213  *      1 -- Open the log. Currently a NOP.
 214  *      2 -- Read from the log.
 215  *      3 -- Read all messages remaining in the ring buffer.
 216  *      4 -- Read and clear all messages remaining in the ring buffer
 217  *      5 -- Clear ring buffer.
 218  *      6 -- Disable printk's to console
 219  *      7 -- Enable printk's to console
 220  *      8 -- Set level of messages printed to console
 221  *      9 -- Return number of unread characters in the log buffer
 222  *     10 -- Return size of the log buffer
 223  */
 224 int do_syslog(int type, char __user * buf, int len)
 225 {
 226         unsigned long i, j, limit, count;
 227         int do_clear = 0;
 228         char c;
 229         int error = 0;
 230
 231         error = security_syslog(type);
 232         if (error)
 233                 return error;
 234
 235         switch (type) {
 236         case 0:         /* Close log */
 237                 break;
 238         case 1:         /* Open log */
 239                 break;
 240         case 2:         /* Read from log */
 241                 error = -EINVAL;
 242                 if (!buf || len < 0)
 243                         goto out;
 244                 error = 0;
 245                 if (!len)
 246                         goto out;
 247                 if (!access_ok(VERIFY_WRITE, buf, len)) {
 248                         error = -EFAULT;
 249                         goto out;
 250                 }
 251                 error = wait_event_interruptible(log_wait, (log_start - log_end));
 252                 if (error)
 253                         goto out;
 254                 i = 0;
 255                 spin_lock_irq(&logbuf_lock);
 256                 while (!error && (log_start != log_end) && i < len) {
 257                         c = LOG_BUF(log_start);
 258                         log_start++;
 259                         spin_unlock_irq(&logbuf_lock);
 260                         error = __put_user(c,buf);
 261                         buf++;
 262                         i++;
 263                         cond_resched();
 264                         spin_lock_irq(&logbuf_lock);
 265                 }
 266                 spin_unlock_irq(&logbuf_lock);
 267                 if (!error)
 268                         error = i;
 269                 break;
 270         case 4:         /* Read/clear last kernel messages */
 271                 do_clear = 1;
 272                 /* FALL THRU */
 273         case 3:         /* Read last kernel messages */
 274                 error = -EINVAL;
 275                 if (!buf || len < 0)
 276                         goto out;
 277                 error = 0;
 278                 if (!len)
 279                         goto out;
 280                 if (!access_ok(VERIFY_WRITE, buf, len)) {
 281                         error = -EFAULT;
 282                         goto out;
 283                 }
 284                 count = len;
 285                 if (count > log_buf_len)
 286                         count = log_buf_len;
 287                 spin_lock_irq(&logbuf_lock);
 288                 if (count > logged_chars)
 289                         count = logged_chars;
 290                 if (do_clear)
 291                         logged_chars = 0;
 292                 limit = log_end;
 293                 /*
 294                  * __put_user() could sleep, and while we sleep
 295                  * printk() could overwrite the messages
 296                  * we try to copy to user space. Therefore
 297                  * the messages are copied in reverse. <manfreds>
 298                  */
 299                 for(i = 0; i < count && !error; i++) {
 300                         j = limit-1-i;
 301                         if (j + log_buf_len < log_end)
 302                                 break;
 303                         c = LOG_BUF(j);
 304                         spin_unlock_irq(&logbuf_lock);
 305                         error = __put_user(c,&buf[count-1-i]);
 306                         cond_resched();
 307                         spin_lock_irq(&logbuf_lock);
 308                 }
 309                 spin_unlock_irq(&logbuf_lock);
 310                 if (error)
 311                         break;
 312                 error = i;
 313                 if(i != count) {
 314                         int offset = count-error;
 315                         /* buffer overflow during copy, correct user buffer. */
 316                         for(i=0;i<error;i++) {
 317                                 if (__get_user(c,&buf[i+offset]) ||
 318                                     __put_user(c,&buf[i])) {
 319                                         error = -EFAULT;
 320                                         break;
 321                                 }
 322                                 cond_resched();
 323                         }
 324                 }
 325                 break;
 326         case 5:         /* Clear ring buffer */
 327                 logged_chars = 0;
 328                 break;
 329         case 6:         /* Disable logging to console */
 330                 console_loglevel = minimum_console_loglevel;
 331                 break;
 332         case 7:         /* Enable logging to console */
 333                 console_loglevel = default_console_loglevel;
 334                 break;
 335         case 8:         /* Set level of messages printed to console */
 336                 error = -EINVAL;
 337                 if (len < 1 || len > 8)
 338                         goto out;
 339                 if (len < minimum_console_loglevel)
 340                         len = minimum_console_loglevel;
 341                 console_loglevel = len;
 342                 error = 0;
 343                 break;
 344         case 9:         /* Number of chars in the log buffer */
 345                 error = log_end - log_start;
 346                 break;
 347         case 10:        /* Size of the log buffer */
 348                 error = log_buf_len;
 349                 break;
 350         default:
 351                 error = -EINVAL;
 352                 break;
 353         }
 354 out:
 355         return error;
 356 }
 357
 358 asmlinkage long sys_syslog(int type, char __user * buf, int len)
 359 {
 360         return do_syslog(type, buf, len);
 361 }
 362
 363 /*
 364  * Call the console drivers on a range of log_buf
 365  */
 366 static void __call_console_drivers(unsigned long start, unsigned long end)
 367 {
 368         struct console *con;
 369
 370         for (con = console_drivers; con; con = con->next) {
 371                 if ((con->flags & CON_ENABLED) && con->write)
 372                         con->write(con, &LOG_BUF(start), end - start);
 373         }
 374 }
 375
 376 /*
 377  * Write out chars from start to end - 1 inclusive
 378  */
 379 static void _call_console_drivers(unsigned long start,
 380                                 unsigned long end, int msg_log_level)
 381 {
 382         if (msg_log_level < console_loglevel &&
 383                         console_drivers && start != end) {
 384                 if ((start & LOG_BUF_MASK) > (end & LOG_BUF_MASK)) {
 385                         /* wrapped write */
 386                         __call_console_drivers(start & LOG_BUF_MASK,
 387                                                 log_buf_len);
 388                         __call_console_drivers(0, end & LOG_BUF_MASK);
 389                 } else {
 390                         __call_console_drivers(start, end);
 391                 }
 392         }
 393 }
 394
 395 /*
 396  * Call the console drivers, asking them to write out
 397  * log_buf[start] to log_buf[end - 1].
 398  * The console_sem must be held.
 399  */
 400 static void call_console_drivers(unsigned long start, unsigned long end)
 401 {
 402         unsigned long cur_index, start_print;
 403         static int msg_level = -1;
 404
 405         if (((long)(start - end)) > 0)
 406                 BUG();
 407
 408         cur_index = start;
 409         start_print = start;
 410         while (cur_index != end) {
 411                 if (    msg_level < 0 &&
 412                         ((end - cur_index) > 2) &&
 413                         LOG_BUF(cur_index + 0) == '<' &&
 414                         LOG_BUF(cur_index + 1) >= '0' &&
 415                         LOG_BUF(cur_index + 1) <= '7' &&
 416                         LOG_BUF(cur_index + 2) == '>')
 417                 {
 418                         msg_level = LOG_BUF(cur_index + 1) - '0';
 419                         cur_index += 3;
 420                         start_print = cur_index;
 421                 }
 422                 while (cur_index != end) {
 423                         char c = LOG_BUF(cur_index);
 424                         cur_index++;
 425
 426                         if (c == '\n') {
 427                                 if (msg_level < 0) {
 428                                         /*
 429                                          * printk() has already given us loglevel tags in
 430                                          * the buffer.  This code is here in case the
 431                                          * log buffer has wrapped right round and scribbled
 432                                          * on those tags
 433                                          */
 434                                         msg_level = default_message_loglevel;
 435                                 }
 436                                 _call_console_drivers(start_print, cur_index, msg_level);
 437                                 msg_level = -1;
 438                                 start_print = cur_index;
 439                                 break;
 440                         }
 441                 }
 442         }
 443         _call_console_drivers(start_print, end, msg_level);
 444 }
 445
 446 static void emit_log_char(char c)
 447 {
 448         LOG_BUF(log_end) = c;
 449         log_end++;
 450         if (log_end - log_start > log_buf_len)
 451                 log_start = log_end - log_buf_len;
 452         if (log_end - con_start > log_buf_len)
 453                 con_start = log_end - log_buf_len;
 454         if (logged_chars < log_buf_len)
 455                 logged_chars++;
 456 }
 457
 458 /*
 459  * Zap console related locks when oopsing. Only zap at most once
 460  * every 10 seconds, to leave time for slow consoles to print a
 461  * full oops.
 462  */
 463 static void zap_locks(void)
 464 {
 465         static unsigned long oops_timestamp;
 466
 467         if (time_after_eq(jiffies, oops_timestamp) &&
 468                         !time_after(jiffies, oops_timestamp + 30*HZ))
 469                 return;
 470
 471         oops_timestamp = jiffies;
 472
 473         /* If a crash is occurring, make sure we can't deadlock */
 474         spin_lock_init(&logbuf_lock);
 475         /* And make sure that we print immediately */
 476         init_MUTEX(&console_sem);
 477 }
 478
 479 #if defined(CONFIG_PRINTK_TIME)
 480 static int printk_time = 1;
 481 #else
 482 static int printk_time = 0;
 483 #endif
 484
 485 static int __init printk_time_setup(char *str)
 486 {
 487         if (*str)
 488                 return 0;
 489         printk_time = 1;
 490         return 1;
 491 }
 492
 493 __setup("time", printk_time_setup);
 494
 495 /*
 496  * This is printk.  It can be called from any context.  We want it to work.
 497  *
 498  * We try to grab the console_sem.  If we succeed, it's easy - we log the output and
 499  * call the console drivers.  If we fail to get the semaphore we place the output
 500  * into the log buffer and return.  The current holder of the console_sem will
 501  * notice the new output in release_console_sem() and will send it to the
 502  * consoles before releasing the semaphore.
 503  *
 504  * One effect of this deferred printing is that code which calls printk() and
 505  * then changes console_loglevel may break. This is because console_loglevel
 506  * is inspected when the actual printing occurs.
 507  */
 508
 509 asmlinkage int printk(const char *fmt, ...)
 510 {
 511         va_list args;
 512         int r;
 513
 514         va_start(args, fmt);
 515         r = vprintk(fmt, args);
 516         va_end(args);
 517
 518         return r;
 519 }
 520
 521 /* cpu currently holding logbuf_lock */
 522 static volatile unsigned int printk_cpu = UINT_MAX;
 523
 524 asmlinkage int vprintk(const char *fmt, va_list args)
 525 {
 526         unsigned long flags;
 527         int printed_len;
 528         char *p;
 529         static char printk_buf[1024];
 530         static int log_level_unknown = 1;
 531
 532         preempt_disable();
 533         if (unlikely(oops_in_progress) && printk_cpu == smp_processor_id())
 534                 /* If a crash is occurring during printk() on this CPU,
 535                  * make sure we can't deadlock */
 536                 zap_locks();
 537
 538         /* This stops the holder of console_sem just where we want him */
 539         spin_lock_irqsave(&logbuf_lock, flags);
 540         printk_cpu = smp_processor_id();
 541
 542         /* Emit the output into the temporary buffer */
 543         printed_len = vscnprintf(printk_buf, sizeof(printk_buf), fmt, args);
 544
 545 #ifdef  CONFIG_DEBUG_LL
 546         printascii(printk_buf);
 547 #endif
 548
 549         /*
 550          * Copy the output into log_buf.  If the caller didn't provide
 551          * appropriate log level tags, we insert them here
 552          */
 553         for (p = printk_buf; *p; p++) {
 554                 if (log_level_unknown) {
 555                         /* log_level_unknown signals the start of a new line */
 556                         if (printk_time) {
 557                                 int loglev_char;
 558                                 char tbuf[50], *tp;
 559                                 unsigned tlen;
 560                                 unsigned long long t;
 561                                 unsigned long nanosec_rem;
 562
 563                                 /*
 564                                  * force the log level token to be
 565                                  * before the time output.
 566                                  */
 567                                 if (p[0] == '<' && p[1] >='0' &&
 568                                    p[1] <= '7' && p[2] == '>') {
 569                                         loglev_char = p[1];
 570                                         p += 3;
 571                                         printed_len += 3;
 572                                 } else {
 573                                         loglev_char = default_message_loglevel
 574                                                 + '0';
 575                                 }
 576                                 t = sched_clock();
 577                                 nanosec_rem = do_div(t, 1000000000);
 578                                 tlen = sprintf(tbuf,
 579                                                 "<%c>[%5lu.%06lu] ",
 580                                                 loglev_char,
 581                                                 (unsigned long)t,
 582                                                 nanosec_rem/1000);
 583
 584                                 for (tp = tbuf; tp < tbuf + tlen; tp++)
 585                                         emit_log_char(*tp);
 586                                 printed_len += tlen - 3;
 587                         } else {
 588                                 if (p[0] != '<' || p[1] < '0' ||
 589                                    p[1] > '7' || p[2] != '>') {
 590                                         emit_log_char('<');
 591                                         emit_log_char(default_message_loglevel
 592                                                 + '0');
 593                                         emit_log_char('>');
 594                                 }
 595                                 printed_len += 3;
 596                         }
 597                         log_level_unknown = 0;
 598                         if (!*p)
 599                                 break;
 600                 }
 601                 emit_log_char(*p);
 602                 if (*p == '\n')
 603                         log_level_unknown = 1;
 604         }
 605
 606         if (!cpu_online(smp_processor_id())) {
 607                 /*
 608                  * Some console drivers may assume that per-cpu resources have
 609                  * been allocated.  So don't allow them to be called by this
 610                  * CPU until it is officially up.  We shouldn't be calling into
 611                  * random console drivers on a CPU which doesn't exist yet..
 612                  */
 613                 printk_cpu = UINT_MAX;
 614                 spin_unlock_irqrestore(&logbuf_lock, flags);
 615                 goto out;
 616         }
 617         if (!down_trylock(&console_sem)) {
 618                 console_locked = 1;
 619                 /*
 620                  * We own the drivers.  We can drop the spinlock and let
 621                  * release_console_sem() print the text
 622                  */
 623                 printk_cpu = UINT_MAX;
 624                 spin_unlock_irqrestore(&logbuf_lock, flags);
 625                 console_may_schedule = 0;
 626                 release_console_sem();
 627         } else {
 628                 /*
 629                  * Someone else owns the drivers.  We drop the spinlock, which
 630                  * allows the semaphore holder to proceed and to call the
 631                  * console drivers with the output which we just produced.
 632                  */
 633                 printk_cpu = UINT_MAX;
 634                 spin_unlock_irqrestore(&logbuf_lock, flags);
 635         }
 636 out:
 637         preempt_enable();
 638         return printed_len;
 639 }
 640 EXPORT_SYMBOL(printk);
 641 EXPORT_SYMBOL(vprintk);
 642
 643 #else
 644
 645 asmlinkage long sys_syslog(int type, char __user * buf, int len)
 646 {
 647         return 0;
 648 }
 649
 650 int do_syslog(int type, char __user * buf, int len) { return 0; }
 651 static void call_console_drivers(unsigned long start, unsigned long end) {}
 652
 653 #endif
 654
 655 /**
 656  * add_preferred_console - add a device to the list of preferred consoles.
 657  *
 658  * The last preferred console added will be used for kernel messages
 659  * and stdin/out/err for init.  Normally this is used by console_setup
 660  * above to handle user-supplied console arguments; however it can also
 661  * be used by arch-specific code either to override the user or more
 662  * commonly to provide a default console (ie from PROM variables) when
 663  * the user has not supplied one.
 664  */
 665 int __init add_preferred_console(char *name, int idx, char *options)
 666 {
 667         struct console_cmdline *c;
 668         int i;
 669
 670         /*
 671          *      See if this tty is not yet registered, and
 672          *      if we have a slot free.
 673          */
 674         for(i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0]; i++)
 675                 if (strcmp(console_cmdline[i].name, name) == 0 &&
 676                           console_cmdline[i].index == idx) {
 677                                 selected_console = i;
 678                                 return 0;
 679                 }
 680         if (i == MAX_CMDLINECONSOLES)
 681                 return -E2BIG;
 682         selected_console = i;
 683         c = &console_cmdline[i];
 684         memcpy(c->name, name, sizeof(c->name));
 685         c->name[sizeof(c->name) - 1] = 0;
 686         c->options = options;
 687         c->index = idx;
 688         return 0;
 689 }
 690
 691 /**
 692  * acquire_console_sem - lock the console system for exclusive use.
 693  *
 694  * Acquires a semaphore which guarantees that the caller has
 695  * exclusive access to the console system and the console_drivers list.
 696  *
 697  * Can sleep, returns nothing.
 698  */
 699 void acquire_console_sem(void)
 700 {
 701         if (in_interrupt())
 702                 BUG();
 703         down(&console_sem);
 704         console_locked = 1;
 705         console_may_schedule = 1;
 706 }
 707 EXPORT_SYMBOL(acquire_console_sem);
 708
 709 int try_acquire_console_sem(void)
 710 {
 711         if (down_trylock(&console_sem))
 712                 return -1;
 713         console_locked = 1;
 714         console_may_schedule = 0;
 715         return 0;
 716 }
 717 EXPORT_SYMBOL(try_acquire_console_sem);
 718
 719 int is_console_locked(void)
 720 {
 721         return console_locked;
 722 }
 723 EXPORT_SYMBOL(is_console_locked);
 724
 725 /**
 726  * release_console_sem - unlock the console system
 727  *
 728  * Releases the semaphore which the caller holds on the console system
 729  * and the console driver list.
 730  *
 731  * While the semaphore was held, console output may have been buffered
 732  * by printk().  If this is the case, release_console_sem() emits
 733  * the output prior to releasing the semaphore.
 734  *
 735  * If there is output waiting for klogd, we wake it up.
 736  *
 737  * release_console_sem() may be called from any context.
 738  */
 739 void release_console_sem(void)
 740 {
 741         unsigned long flags;
 742         unsigned long _con_start, _log_end;
 743         unsigned long wake_klogd = 0;
 744
 745         for ( ; ; ) {
 746                 spin_lock_irqsave(&logbuf_lock, flags);
 747                 wake_klogd |= log_start - log_end;
 748                 if (con_start == log_end)
 749                         break;                  /* Nothing to print */
 750                 _con_start = con_start;
 751                 _log_end = log_end;
 752                 con_start = log_end;            /* Flush */
 753                 spin_unlock(&logbuf_lock);
 754                 call_console_drivers(_con_start, _log_end);
 755                 local_irq_restore(flags);
 756         }
 757         console_locked = 0;
 758         console_may_schedule = 0;
 759         up(&console_sem);
 760         spin_unlock_irqrestore(&logbuf_lock, flags);
 761         if (wake_klogd && !oops_in_progress && waitqueue_active(&log_wait))
 762                 wake_up_interruptible(&log_wait);
 763 }
 764 EXPORT_SYMBOL(release_console_sem);
 765
 766 /** console_conditional_schedule - yield the CPU if required
 767  *
 768  * If the console code is currently allowed to sleep, and
 769  * if this CPU should yield the CPU to another task, do
 770  * so here.
 771  *
 772  * Must be called within acquire_console_sem().
 773  */
 774 void __sched console_conditional_schedule(void)
 775 {
 776         if (console_may_schedule)
 777                 cond_resched();
 778 }
 779 EXPORT_SYMBOL(console_conditional_schedule);
 780
 781 void console_print(const char *s)
 782 {
 783         printk(KERN_EMERG "%s", s);
 784 }
 785 EXPORT_SYMBOL(console_print);
 786
 787 void console_unblank(void)
 788 {
 789         struct console *c;
 790
 791         /*
 792          * console_unblank can no longer be called in interrupt context unless
 793          * oops_in_progress is set to 1..
 794          */
 795         if (oops_in_progress) {
 796                 if (down_trylock(&console_sem) != 0)
 797                         return;
 798         } else
 799                 acquire_console_sem();
 800
 801         console_locked = 1;
 802         console_may_schedule = 0;
 803         for (c = console_drivers; c != NULL; c = c->next)
 804                 if ((c->flags & CON_ENABLED) && c->unblank)
 805                         c->unblank();
 806         release_console_sem();
 807 }
 808 EXPORT_SYMBOL(console_unblank);
 809
 810 /*
 811  * Return the console tty driver structure and its associated index
 812  */
 813 struct tty_driver *console_device(int *index)
 814 {
 815         struct console *c;
 816         struct tty_driver *driver = NULL;
 817
 818         acquire_console_sem();
 819         for (c = console_drivers; c != NULL; c = c->next) {
 820                 if (!c->device)
 821                         continue;
 822                 driver = c->device(c, index);
 823                 if (driver)
 824                         break;
 825         }
 826         release_console_sem();
 827         return driver;
 828 }
 829
 830 /*
 831  * Prevent further output on the passed console device so that (for example)
 832  * serial drivers can disable console output before suspending a port, and can
 833  * re-enable output afterwards.
 834  */
 835 void console_stop(struct console *console)
 836 {
 837         acquire_console_sem();
 838         console->flags &= ~CON_ENABLED;
 839         release_console_sem();
 840 }
 841 EXPORT_SYMBOL(console_stop);
 842
 843 void console_start(struct console *console)
 844 {
 845         acquire_console_sem();
 846         console->flags |= CON_ENABLED;
 847         release_console_sem();
 848 }
 849 EXPORT_SYMBOL(console_start);
 850
 851 /*
 852  * The console driver calls this routine during kernel initialization
 853  * to register the console printing procedure with printk() and to
 854  * print any messages that were printed by the kernel before the
 855  * console driver was initialized.
 856  */
 857 void register_console(struct console * console)
 858 {
 859         int     i;
 860         unsigned long flags;
 861
 862         if (preferred_console < 0)
 863                 preferred_console = selected_console;
 864
 865         /*
 866          *      See if we want to use this console driver. If we
 867          *      didn't select a console we take the first one
 868          *      that registers here.
 869          */
 870         if (preferred_console < 0) {
 871                 if (console->index < 0)
 872                         console->index = 0;
 873                 if (console->setup == NULL ||
 874                     console->setup(console, NULL) == 0) {
 875                         console->flags |= CON_ENABLED | CON_CONSDEV;
 876                         preferred_console = 0;
 877                 }
 878         }
 879
 880         /*
 881          *      See if this console matches one we selected on
 882          *      the command line.
 883          */
 884         for(i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0]; i++) {
 885                 if (strcmp(console_cmdline[i].name, console->name) != 0)
 886                         continue;
 887                 if (console->index >= 0 &&
 888                     console->index != console_cmdline[i].index)
 889                         continue;
 890                 if (console->index < 0)
 891                         console->index = console_cmdline[i].index;
 892                 if (console->setup &&
 893                     console->setup(console, console_cmdline[i].options) != 0)
 894                         break;
 895                 console->flags |= CON_ENABLED;
 896                 console->index = console_cmdline[i].index;
 897                 if (i == selected_console) {
 898                         console->flags |= CON_CONSDEV;
 899                         preferred_console = selected_console;
 900                 }
 901                 break;
 902         }
 903
 904         if (!(console->flags & CON_ENABLED))
 905                 return;
 906
 907         if (console_drivers && (console_drivers->flags & CON_BOOT)) {
 908                 unregister_console(console_drivers);
 909                 console->flags &= ~CON_PRINTBUFFER;
 910         }
 911
 912         /*
 913          *      Put this console in the list - keep the
 914          *      preferred driver at the head of the list.
 915          */
 916         acquire_console_sem();
 917         if ((console->flags & CON_CONSDEV) || console_drivers == NULL) {
 918                 console->next = console_drivers;
 919                 console_drivers = console;
 920                 if (console->next)
 921                         console->next->flags &= ~CON_CONSDEV;
 922         } else {
 923                 console->next = console_drivers->next;
 924                 console_drivers->next = console;
 925         }
 926         if (console->flags & CON_PRINTBUFFER) {
 927                 /*
 928                  * release_console_sem() will print out the buffered messages
 929                  * for us.
 930                  */
 931                 spin_lock_irqsave(&logbuf_lock, flags);
 932                 con_start = log_start;
 933                 spin_unlock_irqrestore(&logbuf_lock, flags);
 934         }
 935         release_console_sem();
 936 }
 937 EXPORT_SYMBOL(register_console);
 938
 939 int unregister_console(struct console * console)
 940 {
 941         struct console *a,*b;
 942         int res = 1;
 943
 944         acquire_console_sem();
 945         if (console_drivers == console) {
 946                 console_drivers=console->next;
 947                 res = 0;
 948         } else {
 949                 for (a=console_drivers->next, b=console_drivers ;
 950                      a; b=a, a=b->next) {
 951                         if (a == console) {
 952                                 b->next = a->next;
 953                                 res = 0;
 954                                 break;
 955                         }
 956                 }
 957         }
 958
 959         /* If last console is removed, we re-enable picking the first
 960          * one that gets registered. Without that, pmac early boot console
 961          * would prevent fbcon from taking over.
 962          *
 963          * If this isn't the last console and it has CON_CONSDEV set, we
 964          * need to set it on the next preferred console.
 965          */
 966         if (console_drivers == NULL)
 967                 preferred_console = selected_console;
 968         else if (console->flags & CON_CONSDEV)
 969                 console_drivers->flags |= CON_CONSDEV;
 970
 971         release_console_sem();
 972         return res;
 973 }
 974 EXPORT_SYMBOL(unregister_console);
 975
 976 /**
 977  * tty_write_message - write a message to a certain tty, not just the console.
 978  *
 979  * This is used for messages that need to be redirected to a specific tty.
 980  * We don't put it into the syslog queue right now maybe in the future if
 981  * really needed.
 982  */
 983 void tty_write_message(struct tty_struct *tty, char *msg)
 984 {
 985         if (tty && tty->driver->write)
 986                 tty->driver->write(tty, msg, strlen(msg));
 987         return;
 988 }
 989
 990 /*
 991  * printk rate limiting, lifted from the networking subsystem.
 992  *
 993  * This enforces a rate limit: not more than one kernel message
 994  * every printk_ratelimit_jiffies to make a denial-of-service
 995  * attack impossible.
 996  */
 997 int __printk_ratelimit(int ratelimit_jiffies, int ratelimit_burst)
 998 {
 999         static DEFINE_SPINLOCK(ratelimit_lock);
1000         static unsigned long toks = 10*5*HZ;
1001         static unsigned long last_msg;
1002         static int missed;
1003         unsigned long flags;
1004         unsigned long now = jiffies;
1005
1006         spin_lock_irqsave(&ratelimit_lock, flags);
1007         toks += now - last_msg;
1008         last_msg = now;
1009         if (toks > (ratelimit_burst * ratelimit_jiffies))
1010                 toks = ratelimit_burst * ratelimit_jiffies;
1011         if (toks >= ratelimit_jiffies) {
1012                 int lost = missed;
1013                 missed = 0;
1014                 toks -= ratelimit_jiffies;
1015                 spin_unlock_irqrestore(&ratelimit_lock, flags);
1016                 if (lost)
1017                         printk(KERN_WARNING "printk: %d messages suppressed.\n", lost);
1018                 return 1;
1019         }
1020         missed++;
1021         spin_unlock_irqrestore(&ratelimit_lock, flags);
1022         return 0;
1023 }
1024 EXPORT_SYMBOL(__printk_ratelimit);
1025
1026 /* minimum time in jiffies between messages */
1027 int printk_ratelimit_jiffies = 5*HZ;
1028
1029 /* number of messages we send before ratelimiting */
1030 int printk_ratelimit_burst = 10;
1031
1032 int printk_ratelimit(void)
1033 {
1034         return __printk_ratelimit(printk_ratelimit_jiffies,
1035                                 printk_ratelimit_burst);
1036 }
1037 EXPORT_SYMBOL(printk_ratelimit);