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