linux/Documentation/s390/s390dbf.txt
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   1S390 Debug Feature
   2==================
   3
   4files: arch/s390/kernel/debug.c
   5       arch/s390/include/asm/debug.h
   6
   7Description:
   8------------
   9The goal of this feature is to provide a kernel debug logging API 
  10where log records can be stored efficiently in memory, where each component 
  11(e.g. device drivers) can have one separate debug log.
  12One purpose of this is to inspect the debug logs after a production system crash
  13in order to analyze the reason for the crash.
  14If the system still runs but only a subcomponent which uses dbf fails,
  15it is possible to look at the debug logs on a live system via the Linux
  16debugfs filesystem.
  17The debug feature may also very useful for kernel and driver development.
  18
  19Design:
  20-------
  21Kernel components (e.g. device drivers) can register themselves at the debug 
  22feature with the function call debug_register(). This function initializes a 
  23debug log for the caller. For each debug log exists a number of debug areas 
  24where exactly one is active at one time.  Each debug area consists of contiguous
  25pages in memory. In the debug areas there are stored debug entries (log records)
  26which are written by event- and exception-calls. 
  27
  28An event-call writes the specified debug entry to the active debug
  29area and updates the log pointer for the active area. If the end 
  30of the active debug area is reached, a wrap around is done (ring buffer) 
  31and the next debug entry will be written at the beginning of the active 
  32debug area.
  33
  34An exception-call writes the specified debug entry to the log and
  35switches to the next debug area. This is done in order to be sure
  36that the records which describe the origin of the exception are not
  37overwritten when a wrap around for the current area occurs.
  38
  39The debug areas themselves are also ordered in form of a ring buffer.
  40When an exception is thrown in the last debug area, the following debug 
  41entries are then written again in the very first area.
  42
  43There are three versions for the event- and exception-calls: One for
  44logging raw data, one for text and one for numbers.
  45
  46Each debug entry contains the following data:
  47
  48- Timestamp
  49- Cpu-Number of calling task
  50- Level of debug entry (0...6)
  51- Return Address to caller
  52- Flag, if entry is an exception or not
  53
  54The debug logs can be inspected in a live system through entries in
  55the debugfs-filesystem. Under the toplevel directory "s390dbf" there is
  56a directory for each registered component, which is named like the
  57corresponding component. The debugfs normally should be mounted to
  58/sys/kernel/debug therefore the debug feature can be accessed under
  59/sys/kernel/debug/s390dbf.
  60
  61The content of the directories are files which represent different views
  62to the debug log. Each component can decide which views should be
  63used through registering them with the function debug_register_view().
  64Predefined views for hex/ascii, sprintf and raw binary data are provided.
  65It is also possible to define other views. The content of
  66a view can be inspected simply by reading the corresponding debugfs file.
  67
  68All debug logs have an actual debug level (range from 0 to 6).
  69The default level is 3. Event and Exception functions have a 'level'
  70parameter. Only debug entries with a level that is lower or equal
  71than the actual level are written to the log. This means, when
  72writing events, high priority log entries should have a low level
  73value whereas low priority entries should have a high one.
  74The actual debug level can be changed with the help of the debugfs-filesystem
  75through writing a number string "x" to the 'level' debugfs file which is
  76provided for every debug log. Debugging can be switched off completely
  77by using "-" on the 'level' debugfs file.
  78
  79Example:
  80
  81> echo "-" > /sys/kernel/debug/s390dbf/dasd/level
  82
  83It is also possible to deactivate the debug feature globally for every
  84debug log. You can change the behavior using  2 sysctl parameters in
  85/proc/sys/s390dbf:
  86There are currently 2 possible triggers, which stop the debug feature
  87globally. The first possibility is to use the "debug_active" sysctl. If
  88set to 1 the debug feature is running. If "debug_active" is set to 0 the
  89debug feature is turned off.
  90The second trigger which stops the debug feature is a kernel oops.
  91That prevents the debug feature from overwriting debug information that
  92happened before the oops. After an oops you can reactivate the debug feature
  93by piping 1 to /proc/sys/s390dbf/debug_active. Nevertheless, its not
  94suggested to use an oopsed kernel in a production environment.
  95If you want to disallow the deactivation of the debug feature, you can use
  96the "debug_stoppable" sysctl. If you set "debug_stoppable" to 0 the debug
  97feature cannot be stopped. If the debug feature is already stopped, it
  98will stay deactivated.
  99
 100Kernel Interfaces:
 101------------------
 102
 103----------------------------------------------------------------------------
 104debug_info_t *debug_register(char *name, int pages, int nr_areas,
 105                             int buf_size);
 106
 107Parameter:    name:        Name of debug log (e.g. used for debugfs entry)
 108              pages:       number of pages, which will be allocated per area
 109              nr_areas:    number of debug areas
 110              buf_size:    size of data area in each debug entry
 111
 112Return Value: Handle for generated debug area   
 113              NULL if register failed 
 114
 115Description:  Allocates memory for a debug log     
 116              Must not be called within an interrupt handler 
 117
 118----------------------------------------------------------------------------
 119debug_info_t *debug_register_mode(char *name, int pages, int nr_areas,
 120                                  int buf_size, mode_t mode, uid_t uid,
 121                                  gid_t gid);
 122
 123Parameter:    name:        Name of debug log (e.g. used for debugfs entry)
 124              pages:       Number of pages, which will be allocated per area
 125              nr_areas:    Number of debug areas
 126              buf_size:    Size of data area in each debug entry
 127              mode:        File mode for debugfs files. E.g. S_IRWXUGO
 128              uid:         User ID for debugfs files. Currently only 0 is
 129                           supported.
 130              gid:         Group ID for debugfs files. Currently only 0 is
 131                           supported.
 132
 133Return Value: Handle for generated debug area
 134              NULL if register failed
 135
 136Description:  Allocates memory for a debug log
 137              Must not be called within an interrupt handler
 138
 139---------------------------------------------------------------------------
 140void debug_unregister (debug_info_t * id);
 141
 142Parameter:     id:   handle for debug log  
 143
 144Return Value:  none 
 145
 146Description:   frees memory for a debug log     
 147               Must not be called within an interrupt handler 
 148
 149---------------------------------------------------------------------------
 150void debug_set_level (debug_info_t * id, int new_level);
 151
 152Parameter:     id:        handle for debug log  
 153               new_level: new debug level 
 154
 155Return Value:  none 
 156
 157Description:   Sets new actual debug level if new_level is valid. 
 158
 159---------------------------------------------------------------------------
 160void debug_stop_all(void);
 161
 162Parameter:     none
 163
 164Return Value:  none
 165
 166Description:   stops the debug feature if stopping is allowed. Currently
 167               used in case of a kernel oops.
 168
 169---------------------------------------------------------------------------
 170debug_entry_t* debug_event (debug_info_t* id, int level, void* data, 
 171                            int length);
 172
 173Parameter:     id:     handle for debug log  
 174               level:  debug level           
 175               data:   pointer to data for debug entry  
 176               length: length of data in bytes       
 177
 178Return Value:  Address of written debug entry 
 179
 180Description:   writes debug entry to active debug area (if level <= actual 
 181               debug level)    
 182
 183---------------------------------------------------------------------------
 184debug_entry_t* debug_int_event (debug_info_t * id, int level, 
 185                                unsigned int data);
 186debug_entry_t* debug_long_event(debug_info_t * id, int level,
 187                                unsigned long data);
 188
 189Parameter:     id:     handle for debug log  
 190               level:  debug level           
 191               data:   integer value for debug entry           
 192
 193Return Value:  Address of written debug entry 
 194
 195Description:   writes debug entry to active debug area (if level <= actual 
 196               debug level)    
 197
 198---------------------------------------------------------------------------
 199debug_entry_t* debug_text_event (debug_info_t * id, int level, 
 200                                 const char* data);
 201
 202Parameter:     id:     handle for debug log  
 203               level:  debug level           
 204               data:   string for debug entry  
 205
 206Return Value:  Address of written debug entry 
 207
 208Description:   writes debug entry in ascii format to active debug area 
 209               (if level <= actual debug level)     
 210
 211---------------------------------------------------------------------------
 212debug_entry_t* debug_sprintf_event (debug_info_t * id, int level, 
 213                                    char* string,...);
 214
 215Parameter:     id:    handle for debug log 
 216               level: debug level
 217               string: format string for debug entry 
 218               ...: varargs used as in sprintf()
 219
 220Return Value:  Address of written debug entry
 221
 222Description:   writes debug entry with format string and varargs (longs) to 
 223               active debug area (if level $<=$ actual debug level). 
 224               floats and long long datatypes cannot be used as varargs.
 225
 226---------------------------------------------------------------------------
 227
 228debug_entry_t* debug_exception (debug_info_t* id, int level, void* data, 
 229                                int length);
 230
 231Parameter:     id:     handle for debug log  
 232               level:  debug level           
 233               data:   pointer to data for debug entry  
 234               length: length of data in bytes       
 235
 236Return Value:  Address of written debug entry 
 237
 238Description:   writes debug entry to active debug area (if level <= actual 
 239               debug level) and switches to next debug area  
 240
 241---------------------------------------------------------------------------
 242debug_entry_t* debug_int_exception (debug_info_t * id, int level, 
 243                                    unsigned int data);
 244debug_entry_t* debug_long_exception(debug_info_t * id, int level,
 245                                    unsigned long data);
 246
 247Parameter:     id:     handle for debug log  
 248               level:  debug level           
 249               data:   integer value for debug entry           
 250
 251Return Value:  Address of written debug entry 
 252
 253Description:   writes debug entry to active debug area (if level <= actual 
 254               debug level) and switches to next debug area  
 255
 256---------------------------------------------------------------------------
 257debug_entry_t* debug_text_exception (debug_info_t * id, int level, 
 258                                     const char* data);
 259
 260Parameter:     id:     handle for debug log  
 261               level:  debug level           
 262               data:   string for debug entry  
 263
 264Return Value:  Address of written debug entry 
 265
 266Description:   writes debug entry in ascii format to active debug area 
 267               (if level <= actual debug level) and switches to next debug 
 268               area  
 269
 270---------------------------------------------------------------------------
 271debug_entry_t* debug_sprintf_exception (debug_info_t * id, int level,
 272                                        char* string,...);
 273
 274Parameter:     id:    handle for debug log  
 275               level: debug level  
 276               string: format string for debug entry  
 277               ...: varargs used as in sprintf()
 278
 279Return Value:  Address of written debug entry 
 280
 281Description:   writes debug entry with format string and varargs (longs) to 
 282               active debug area (if level $<=$ actual debug level) and
 283               switches to next debug area. 
 284               floats and long long datatypes cannot be used as varargs.
 285
 286---------------------------------------------------------------------------
 287
 288int debug_register_view (debug_info_t * id, struct debug_view *view);
 289
 290Parameter:     id:    handle for debug log  
 291               view:  pointer to debug view struct 
 292
 293Return Value:  0  : ok 
 294               < 0: Error 
 295
 296Description:   registers new debug view and creates debugfs dir entry
 297
 298---------------------------------------------------------------------------
 299int debug_unregister_view (debug_info_t * id, struct debug_view *view); 
 300
 301Parameter:     id:    handle for debug log  
 302               view:  pointer to debug view struct 
 303
 304Return Value:  0  : ok 
 305               < 0: Error 
 306
 307Description:   unregisters debug view and removes debugfs dir entry
 308
 309
 310
 311Predefined views:
 312-----------------
 313
 314extern struct debug_view debug_hex_ascii_view;
 315extern struct debug_view debug_raw_view;
 316extern struct debug_view debug_sprintf_view;
 317
 318Examples
 319--------
 320
 321/*
 322 * hex_ascii- + raw-view Example
 323 */
 324
 325#include <linux/init.h>
 326#include <asm/debug.h>
 327
 328static debug_info_t* debug_info;
 329
 330static int init(void)
 331{
 332    /* register 4 debug areas with one page each and 4 byte data field */
 333
 334    debug_info = debug_register ("test", 1, 4, 4 );
 335    debug_register_view(debug_info,&debug_hex_ascii_view);
 336    debug_register_view(debug_info,&debug_raw_view);
 337
 338    debug_text_event(debug_info, 4 , "one ");
 339    debug_int_exception(debug_info, 4, 4711);
 340    debug_event(debug_info, 3, &debug_info, 4);
 341
 342    return 0;
 343}
 344
 345static void cleanup(void)
 346{
 347    debug_unregister (debug_info);
 348}
 349
 350module_init(init);
 351module_exit(cleanup);
 352
 353---------------------------------------------------------------------------
 354
 355/*
 356 * sprintf-view Example
 357 */
 358
 359#include <linux/init.h>
 360#include <asm/debug.h>
 361
 362static debug_info_t* debug_info;
 363
 364static int init(void)
 365{
 366    /* register 4 debug areas with one page each and data field for */
 367    /* format string pointer + 2 varargs (= 3 * sizeof(long))       */
 368
 369    debug_info = debug_register ("test", 1, 4, sizeof(long) * 3);
 370    debug_register_view(debug_info,&debug_sprintf_view);
 371
 372    debug_sprintf_event(debug_info, 2 , "first event in %s:%i\n",__FILE__,__LINE__);
 373    debug_sprintf_exception(debug_info, 1, "pointer to debug info: %p\n",&debug_info);
 374
 375    return 0;
 376}
 377
 378static void cleanup(void)
 379{
 380    debug_unregister (debug_info);
 381}
 382
 383module_init(init);
 384module_exit(cleanup);
 385
 386
 387
 388Debugfs Interface
 389----------------
 390Views to the debug logs can be investigated through reading the corresponding 
 391debugfs-files:
 392
 393Example:
 394
 395> ls /sys/kernel/debug/s390dbf/dasd
 396flush  hex_ascii  level pages raw
 397> cat /sys/kernel/debug/s390dbf/dasd/hex_ascii | sort +1
 39800 00974733272:680099 2 - 02 0006ad7e  07 ea 4a 90 | ....
 39900 00974733272:682210 2 - 02 0006ade6  46 52 45 45 | FREE
 40000 00974733272:682213 2 - 02 0006adf6  07 ea 4a 90 | ....
 40100 00974733272:682281 1 * 02 0006ab08  41 4c 4c 43 | EXCP 
 40201 00974733272:682284 2 - 02 0006ab16  45 43 4b 44 | ECKD
 40301 00974733272:682287 2 - 02 0006ab28  00 00 00 04 | ....
 40401 00974733272:682289 2 - 02 0006ab3e  00 00 00 20 | ... 
 40501 00974733272:682297 2 - 02 0006ad7e  07 ea 4a 90 | ....
 40601 00974733272:684384 2 - 00 0006ade6  46 52 45 45 | FREE
 40701 00974733272:684388 2 - 00 0006adf6  07 ea 4a 90 | ....
 408
 409See section about predefined views for explanation of the above output!
 410
 411Changing the debug level
 412------------------------
 413
 414Example:
 415
 416
 417> cat /sys/kernel/debug/s390dbf/dasd/level
 4183
 419> echo "5" > /sys/kernel/debug/s390dbf/dasd/level
 420> cat /sys/kernel/debug/s390dbf/dasd/level
 4215
 422
 423Flushing debug areas
 424--------------------
 425Debug areas can be flushed with piping the number of the desired
 426area (0...n) to the debugfs file "flush". When using "-" all debug areas
 427are flushed.
 428
 429Examples:
 430
 4311. Flush debug area 0:
 432> echo "0" > /sys/kernel/debug/s390dbf/dasd/flush
 433
 4342. Flush all debug areas:
 435> echo "-" > /sys/kernel/debug/s390dbf/dasd/flush
 436
 437Changing the size of debug areas
 438------------------------------------
 439It is possible the change the size of debug areas through piping
 440the number of pages to the debugfs file "pages". The resize request will
 441also flush the debug areas.
 442
 443Example:
 444
 445Define 4 pages for the debug areas of debug feature "dasd":
 446> echo "4" > /sys/kernel/debug/s390dbf/dasd/pages
 447
 448Stooping the debug feature
 449--------------------------
 450Example:
 451
 4521. Check if stopping is allowed
 453> cat /proc/sys/s390dbf/debug_stoppable
 4542. Stop debug feature
 455> echo 0 > /proc/sys/s390dbf/debug_active
 456
 457lcrash Interface
 458----------------
 459It is planned that the dump analysis tool lcrash gets an additional command
 460's390dbf' to display all the debug logs. With this tool it will be possible 
 461to investigate the debug logs on a live system and with a memory dump after 
 462a system crash.
 463
 464Investigating raw memory
 465------------------------
 466One last possibility to investigate the debug logs at a live
 467system and after a system crash is to look at the raw memory
 468under VM or at the Service Element.
 469It is possible to find the anker of the debug-logs through
 470the 'debug_area_first' symbol in the System map. Then one has
 471to follow the correct pointers of the data-structures defined
 472in debug.h and find the debug-areas in memory.
 473Normally modules which use the debug feature will also have
 474a global variable with the pointer to the debug-logs. Following
 475this pointer it will also be possible to find the debug logs in
 476memory.
 477
 478For this method it is recommended to use '16 * x + 4' byte (x = 0..n)
 479for the length of the data field in debug_register() in
 480order to see the debug entries well formatted.
 481
 482
 483Predefined Views
 484----------------
 485
 486There are three predefined views: hex_ascii, raw and sprintf. 
 487The hex_ascii view shows the data field in hex and ascii representation 
 488(e.g. '45 43 4b 44 | ECKD'). 
 489The raw view returns a bytestream as the debug areas are stored in memory.
 490
 491The sprintf view formats the debug entries in the same way as the sprintf
 492function would do. The sprintf event/exception functions write to the
 493debug entry a pointer to the format string (size = sizeof(long)) 
 494and for each vararg a long value. So e.g. for a debug entry with a format 
 495string plus two varargs one would need to allocate a (3 * sizeof(long)) 
 496byte data area in the debug_register() function.
 497
 498IMPORTANT: Using "%s" in sprintf event functions is dangerous. You can only
 499use "%s" in the sprintf event functions, if the memory for the passed string is
 500available as long as the debug feature exists. The reason behind this is that
 501due to performance considerations only a pointer to the string is stored in
 502the debug feature. If you log a string that is freed afterwards, you will get
 503an OOPS when inspecting the debug feature, because then the debug feature will
 504access the already freed memory.
 505
 506NOTE: If using the sprintf view do NOT use other event/exception functions
 507than the sprintf-event and -exception functions.
 508
 509The format of the hex_ascii and sprintf view is as follows:
 510- Number of area
 511- Timestamp (formatted as seconds and microseconds since 00:00:00 Coordinated 
 512  Universal Time (UTC), January 1, 1970)
 513- level of debug entry
 514- Exception flag (* = Exception)
 515- Cpu-Number of calling task
 516- Return Address to caller
 517- data field
 518
 519The format of the raw view is:
 520- Header as described in debug.h
 521- datafield 
 522
 523A typical line of the hex_ascii view will look like the following (first line 
 524is only for explanation and will not be displayed when 'cating' the view):
 525
 526area  time           level exception cpu caller    data (hex + ascii)
 527--------------------------------------------------------------------------
 52800    00964419409:440690 1 -         00  88023fe   
 529
 530
 531Defining views
 532--------------
 533
 534Views are specified with the 'debug_view' structure. There are defined
 535callback functions which are used for reading and writing the debugfs files:
 536
 537struct debug_view {
 538        char name[DEBUG_MAX_PROCF_LEN];  
 539        debug_prolog_proc_t* prolog_proc; 
 540        debug_header_proc_t* header_proc;
 541        debug_format_proc_t* format_proc;
 542        debug_input_proc_t*  input_proc;
 543        void*                private_data;
 544};
 545
 546where
 547
 548typedef int (debug_header_proc_t) (debug_info_t* id,
 549                                   struct debug_view* view,
 550                                   int area,
 551                                   debug_entry_t* entry,
 552                                   char* out_buf);
 553
 554typedef int (debug_format_proc_t) (debug_info_t* id,
 555                                   struct debug_view* view, char* out_buf,
 556                                   const char* in_buf);
 557typedef int (debug_prolog_proc_t) (debug_info_t* id,
 558                                   struct debug_view* view,
 559                                   char* out_buf);
 560typedef int (debug_input_proc_t) (debug_info_t* id,
 561                                  struct debug_view* view,
 562                                  struct file* file, const char* user_buf,
 563                                  size_t in_buf_size, loff_t* offset);
 564
 565
 566The "private_data" member can be used as pointer to view specific data.
 567It is not used by the debug feature itself.
 568
 569The output when reading a debugfs file is structured like this:
 570
 571"prolog_proc output"
 572
 573"header_proc output 1"  "format_proc output 1"
 574"header_proc output 2"  "format_proc output 2"
 575"header_proc output 3"  "format_proc output 3"
 576...
 577
 578When a view is read from the debugfs, the Debug Feature calls the
 579'prolog_proc' once for writing the prolog.
 580Then 'header_proc' and 'format_proc' are called for each 
 581existing debug entry.
 582
 583The input_proc can be used to implement functionality when it is written to 
 584the view (e.g. like with 'echo "0" > /sys/kernel/debug/s390dbf/dasd/level).
 585
 586For header_proc there can be used the default function
 587debug_dflt_header_fn() which is defined in debug.h.
 588and which produces the same header output as the predefined views.
 589E.g:
 59000 00964419409:440761 2 - 00 88023ec
 591
 592In order to see how to use the callback functions check the implementation
 593of the default views!
 594
 595Example
 596
 597#include <asm/debug.h>
 598
 599#define UNKNOWNSTR "data: %08x"
 600
 601const char* messages[] =
 602{"This error...........\n",
 603 "That error...........\n",
 604 "Problem..............\n",
 605 "Something went wrong.\n",
 606 "Everything ok........\n",
 607 NULL
 608};
 609
 610static int debug_test_format_fn(
 611   debug_info_t * id, struct debug_view *view, 
 612   char *out_buf, const char *in_buf
 613)
 614{
 615  int i, rc = 0;
 616
 617  if(id->buf_size >= 4) {
 618     int msg_nr = *((int*)in_buf);
 619     if(msg_nr < sizeof(messages)/sizeof(char*) - 1)
 620        rc += sprintf(out_buf, "%s", messages[msg_nr]); 
 621     else
 622        rc += sprintf(out_buf, UNKNOWNSTR, msg_nr);
 623  }
 624 out:
 625   return rc;
 626}
 627
 628struct debug_view debug_test_view = {
 629  "myview",                 /* name of view */
 630  NULL,                     /* no prolog */
 631  &debug_dflt_header_fn,    /* default header for each entry */
 632  &debug_test_format_fn,    /* our own format function */
 633  NULL,                     /* no input function */
 634  NULL                      /* no private data */
 635};
 636
 637=====
 638test:
 639=====
 640debug_info_t *debug_info;
 641...
 642debug_info = debug_register ("test", 0, 4, 4 ));
 643debug_register_view(debug_info, &debug_test_view);
 644for(i = 0; i < 10; i ++) debug_int_event(debug_info, 1, i);
 645
 646> cat /sys/kernel/debug/s390dbf/test/myview
 64700 00964419734:611402 1 - 00 88042ca   This error...........
 64800 00964419734:611405 1 - 00 88042ca   That error...........
 64900 00964419734:611408 1 - 00 88042ca   Problem..............
 65000 00964419734:611411 1 - 00 88042ca   Something went wrong.
 65100 00964419734:611414 1 - 00 88042ca   Everything ok........
 65200 00964419734:611417 1 - 00 88042ca   data: 00000005
 65300 00964419734:611419 1 - 00 88042ca   data: 00000006
 65400 00964419734:611422 1 - 00 88042ca   data: 00000007
 65500 00964419734:611425 1 - 00 88042ca   data: 00000008
 65600 00964419734:611428 1 - 00 88042ca   data: 00000009
 657
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