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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1	S390 Debug Feature
2	==================
3	
4	files: arch/s390/kernel/debug.c
5	       arch/s390/include/asm/debug.h
6	
7	Description:
8	------------
9	The goal of this feature is to provide a kernel debug logging API 
10	where log records can be stored efficiently in memory, where each component 
11	(e.g. device drivers) can have one separate debug log.
12	One purpose of this is to inspect the debug logs after a production system crash
13	in order to analyze the reason for the crash.
14	If the system still runs but only a subcomponent which uses dbf fails,
15	it is possible to look at the debug logs on a live system via the Linux
16	debugfs filesystem.
17	The debug feature may also very useful for kernel and driver development.
18	
19	Design:
20	-------
21	Kernel components (e.g. device drivers) can register themselves at the debug 
22	feature with the function call debug_register(). This function initializes a 
23	debug log for the caller. For each debug log exists a number of debug areas 
24	where exactly one is active at one time.  Each debug area consists of contiguous
25	pages in memory. In the debug areas there are stored debug entries (log records)
26	which are written by event- and exception-calls. 
27	
28	An event-call writes the specified debug entry to the active debug
29	area and updates the log pointer for the active area. If the end 
30	of the active debug area is reached, a wrap around is done (ring buffer) 
31	and the next debug entry will be written at the beginning of the active 
32	debug area.
33	
34	An exception-call writes the specified debug entry to the log and
35	switches to the next debug area. This is done in order to be sure
36	that the records which describe the origin of the exception are not
37	overwritten when a wrap around for the current area occurs.
38	
39	The debug areas themselves are also ordered in form of a ring buffer.
40	When an exception is thrown in the last debug area, the following debug 
41	entries are then written again in the very first area.
42	
43	There are three versions for the event- and exception-calls: One for
44	logging raw data, one for text and one for numbers.
45	
46	Each 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	
54	The debug logs can be inspected in a live system through entries in
55	the debugfs-filesystem. Under the toplevel directory "s390dbf" there is
56	a directory for each registered component, which is named like the
57	corresponding 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	
61	The content of the directories are files which represent different views
62	to the debug log. Each component can decide which views should be
63	used through registering them with the function debug_register_view().
64	Predefined views for hex/ascii, sprintf and raw binary data are provided.
65	It is also possible to define other views. The content of
66	a view can be inspected simply by reading the corresponding debugfs file.
67	
68	All debug logs have an actual debug level (range from 0 to 6).
69	The default level is 3. Event and Exception functions have a 'level'
70	parameter. Only debug entries with a level that is lower or equal
71	than the actual level are written to the log. This means, when
72	writing events, high priority log entries should have a low level
73	value whereas low priority entries should have a high one.
74	The actual debug level can be changed with the help of the debugfs-filesystem
75	through writing a number string "x" to the 'level' debugfs file which is
76	provided for every debug log. Debugging can be switched off completely
77	by using "-" on the 'level' debugfs file.
78	
79	Example:
80	
81	> echo "-" > /sys/kernel/debug/s390dbf/dasd/level
82	
83	It is also possible to deactivate the debug feature globally for every
84	debug log. You can change the behavior using  2 sysctl parameters in
85	/proc/sys/s390dbf:
86	There are currently 2 possible triggers, which stop the debug feature
87	globally. The first possibility is to use the "debug_active" sysctl. If
88	set to 1 the debug feature is running. If "debug_active" is set to 0 the
89	debug feature is turned off.
90	The second trigger which stops the debug feature is a kernel oops.
91	That prevents the debug feature from overwriting debug information that
92	happened before the oops. After an oops you can reactivate the debug feature
93	by piping 1 to /proc/sys/s390dbf/debug_active. Nevertheless, its not
94	suggested to use an oopsed kernel in a production environment.
95	If you want to disallow the deactivation of the debug feature, you can use
96	the "debug_stoppable" sysctl. If you set "debug_stoppable" to 0 the debug
97	feature cannot be stopped. If the debug feature is already stopped, it
98	will stay deactivated.
99	
100	Kernel Interfaces:
101	------------------
102	
103	----------------------------------------------------------------------------
104	debug_info_t *debug_register(char *name, int pages, int nr_areas,
105	                             int buf_size);
106	
107	Parameter:    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	
112	Return Value: Handle for generated debug area   
113	              NULL if register failed 
114	
115	Description:  Allocates memory for a debug log     
116	              Must not be called within an interrupt handler 
117	
118	----------------------------------------------------------------------------
119	debug_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	
123	Parameter:    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	
133	Return Value: Handle for generated debug area
134		      NULL if register failed
135	
136	Description:  Allocates memory for a debug log
137		      Must not be called within an interrupt handler
138	
139	---------------------------------------------------------------------------
140	void debug_unregister (debug_info_t * id);
141	
142	Parameter:     id:   handle for debug log  
143	
144	Return Value:  none 
145	
146	Description:   frees memory for a debug log and removes all registered debug
147		       views.
148	               Must not be called within an interrupt handler 
149	
150	---------------------------------------------------------------------------
151	void debug_set_level (debug_info_t * id, int new_level);
152	
153	Parameter:     id:        handle for debug log  
154	               new_level: new debug level 
155	
156	Return Value:  none 
157	
158	Description:   Sets new actual debug level if new_level is valid. 
159	
160	---------------------------------------------------------------------------
161	bool debug_level_enabled (debug_info_t * id, int level);
162	
163	Parameter:    id:	  handle for debug log
164		      level:	  debug level
165	
166	Return Value: True if level is less or equal to the current debug level.
167	
168	Description:  Returns true if debug events for the specified level would be
169		      logged. Otherwise returns false.
170	---------------------------------------------------------------------------
171	void debug_stop_all(void);
172	
173	Parameter:     none
174	
175	Return Value:  none
176	
177	Description:   stops the debug feature if stopping is allowed. Currently
178	               used in case of a kernel oops.
179	
180	---------------------------------------------------------------------------
181	debug_entry_t* debug_event (debug_info_t* id, int level, void* data, 
182	                            int length);
183	
184	Parameter:     id:     handle for debug log  
185	               level:  debug level           
186	               data:   pointer to data for debug entry  
187	               length: length of data in bytes       
188	
189	Return Value:  Address of written debug entry 
190	
191	Description:   writes debug entry to active debug area (if level <= actual 
192	               debug level)    
193	
194	---------------------------------------------------------------------------
195	debug_entry_t* debug_int_event (debug_info_t * id, int level, 
196	                                unsigned int data);
197	debug_entry_t* debug_long_event(debug_info_t * id, int level,
198	                                unsigned long data);
199	
200	Parameter:     id:     handle for debug log  
201	               level:  debug level           
202	               data:   integer value for debug entry           
203	
204	Return Value:  Address of written debug entry 
205	
206	Description:   writes debug entry to active debug area (if level <= actual 
207	               debug level)    
208	
209	---------------------------------------------------------------------------
210	debug_entry_t* debug_text_event (debug_info_t * id, int level, 
211	                                 const char* data);
212	
213	Parameter:     id:     handle for debug log  
214	               level:  debug level           
215	               data:   string for debug entry  
216	
217	Return Value:  Address of written debug entry 
218	
219	Description:   writes debug entry in ascii format to active debug area 
220	               (if level <= actual debug level)     
221	
222	---------------------------------------------------------------------------
223	debug_entry_t* debug_sprintf_event (debug_info_t * id, int level, 
224	                                    char* string,...);
225	
226	Parameter:     id:    handle for debug log 
227	               level: debug level
228	               string: format string for debug entry 
229	               ...: varargs used as in sprintf()
230	
231	Return Value:  Address of written debug entry
232	
233	Description:   writes debug entry with format string and varargs (longs) to 
234	               active debug area (if level $<=$ actual debug level). 
235	               floats and long long datatypes cannot be used as varargs.
236	
237	---------------------------------------------------------------------------
238	
239	debug_entry_t* debug_exception (debug_info_t* id, int level, void* data, 
240	                                int length);
241	
242	Parameter:     id:     handle for debug log  
243	               level:  debug level           
244	               data:   pointer to data for debug entry  
245	               length: length of data in bytes       
246	
247	Return Value:  Address of written debug entry 
248	
249	Description:   writes debug entry to active debug area (if level <= actual 
250	               debug level) and switches to next debug area  
251	
252	---------------------------------------------------------------------------
253	debug_entry_t* debug_int_exception (debug_info_t * id, int level, 
254	                                    unsigned int data);
255	debug_entry_t* debug_long_exception(debug_info_t * id, int level,
256	                                    unsigned long data);
257	
258	Parameter:     id:     handle for debug log  
259	               level:  debug level           
260	               data:   integer value for debug entry           
261	
262	Return Value:  Address of written debug entry 
263	
264	Description:   writes debug entry to active debug area (if level <= actual 
265	               debug level) and switches to next debug area  
266	
267	---------------------------------------------------------------------------
268	debug_entry_t* debug_text_exception (debug_info_t * id, int level, 
269	                                     const char* data);
270	
271	Parameter:     id:     handle for debug log  
272	               level:  debug level           
273	               data:   string for debug entry  
274	
275	Return Value:  Address of written debug entry 
276	
277	Description:   writes debug entry in ascii format to active debug area 
278	               (if level <= actual debug level) and switches to next debug 
279	               area  
280	
281	---------------------------------------------------------------------------
282	debug_entry_t* debug_sprintf_exception (debug_info_t * id, int level,
283	                                        char* string,...);
284	
285	Parameter:     id:    handle for debug log  
286	               level: debug level  
287	               string: format string for debug entry  
288	               ...: varargs used as in sprintf()
289	
290	Return Value:  Address of written debug entry 
291	
292	Description:   writes debug entry with format string and varargs (longs) to 
293	               active debug area (if level $<=$ actual debug level) and
294	               switches to next debug area. 
295	               floats and long long datatypes cannot be used as varargs.
296	
297	---------------------------------------------------------------------------
298	
299	int debug_register_view (debug_info_t * id, struct debug_view *view);
300	
301	Parameter:     id:    handle for debug log  
302	               view:  pointer to debug view struct 
303	
304	Return Value:  0  : ok 
305	               < 0: Error 
306	
307	Description:   registers new debug view and creates debugfs dir entry
308	
309	---------------------------------------------------------------------------
310	int debug_unregister_view (debug_info_t * id, struct debug_view *view); 
311	
312	Parameter:     id:    handle for debug log  
313	               view:  pointer to debug view struct 
314	
315	Return Value:  0  : ok 
316	               < 0: Error 
317	
318	Description:   unregisters debug view and removes debugfs dir entry
319	
320	
321	
322	Predefined views:
323	-----------------
324	
325	extern struct debug_view debug_hex_ascii_view;
326	extern struct debug_view debug_raw_view;
327	extern struct debug_view debug_sprintf_view;
328	
329	Examples
330	--------
331	
332	/*
333	 * hex_ascii- + raw-view Example
334	 */
335	
336	#include <linux/init.h>
337	#include <asm/debug.h>
338	
339	static debug_info_t* debug_info;
340	
341	static int init(void)
342	{
343	    /* register 4 debug areas with one page each and 4 byte data field */
344	
345	    debug_info = debug_register ("test", 1, 4, 4 );
346	    debug_register_view(debug_info,&debug_hex_ascii_view);
347	    debug_register_view(debug_info,&debug_raw_view);
348	
349	    debug_text_event(debug_info, 4 , "one ");
350	    debug_int_exception(debug_info, 4, 4711);
351	    debug_event(debug_info, 3, &debug_info, 4);
352	
353	    return 0;
354	}
355	
356	static void cleanup(void)
357	{
358	    debug_unregister (debug_info);
359	}
360	
361	module_init(init);
362	module_exit(cleanup);
363	
364	---------------------------------------------------------------------------
365	
366	/*
367	 * sprintf-view Example
368	 */
369	
370	#include <linux/init.h>
371	#include <asm/debug.h>
372	
373	static debug_info_t* debug_info;
374	
375	static int init(void)
376	{
377	    /* register 4 debug areas with one page each and data field for */
378	    /* format string pointer + 2 varargs (= 3 * sizeof(long))       */
379	
380	    debug_info = debug_register ("test", 1, 4, sizeof(long) * 3);
381	    debug_register_view(debug_info,&debug_sprintf_view);
382	
383	    debug_sprintf_event(debug_info, 2 , "first event in %s:%i\n",__FILE__,__LINE__);
384	    debug_sprintf_exception(debug_info, 1, "pointer to debug info: %p\n",&debug_info);
385	
386	    return 0;
387	}
388	
389	static void cleanup(void)
390	{
391	    debug_unregister (debug_info);
392	}
393	
394	module_init(init);
395	module_exit(cleanup);
396	
397	
398	
399	Debugfs Interface
400	----------------
401	Views to the debug logs can be investigated through reading the corresponding 
402	debugfs-files:
403	
404	Example:
405	
406	> ls /sys/kernel/debug/s390dbf/dasd
407	flush  hex_ascii  level pages raw
408	> cat /sys/kernel/debug/s390dbf/dasd/hex_ascii | sort +1
409	00 00974733272:680099 2 - 02 0006ad7e  07 ea 4a 90 | ....
410	00 00974733272:682210 2 - 02 0006ade6  46 52 45 45 | FREE
411	00 00974733272:682213 2 - 02 0006adf6  07 ea 4a 90 | ....
412	00 00974733272:682281 1 * 02 0006ab08  41 4c 4c 43 | EXCP 
413	01 00974733272:682284 2 - 02 0006ab16  45 43 4b 44 | ECKD
414	01 00974733272:682287 2 - 02 0006ab28  00 00 00 04 | ....
415	01 00974733272:682289 2 - 02 0006ab3e  00 00 00 20 | ... 
416	01 00974733272:682297 2 - 02 0006ad7e  07 ea 4a 90 | ....
417	01 00974733272:684384 2 - 00 0006ade6  46 52 45 45 | FREE
418	01 00974733272:684388 2 - 00 0006adf6  07 ea 4a 90 | ....
419	
420	See section about predefined views for explanation of the above output!
421	
422	Changing the debug level
423	------------------------
424	
425	Example:
426	
427	
428	> cat /sys/kernel/debug/s390dbf/dasd/level
429	3
430	> echo "5" > /sys/kernel/debug/s390dbf/dasd/level
431	> cat /sys/kernel/debug/s390dbf/dasd/level
432	5
433	
434	Flushing debug areas
435	--------------------
436	Debug areas can be flushed with piping the number of the desired
437	area (0...n) to the debugfs file "flush". When using "-" all debug areas
438	are flushed.
439	
440	Examples:
441	
442	1. Flush debug area 0:
443	> echo "0" > /sys/kernel/debug/s390dbf/dasd/flush
444	
445	2. Flush all debug areas:
446	> echo "-" > /sys/kernel/debug/s390dbf/dasd/flush
447	
448	Changing the size of debug areas
449	------------------------------------
450	It is possible the change the size of debug areas through piping
451	the number of pages to the debugfs file "pages". The resize request will
452	also flush the debug areas.
453	
454	Example:
455	
456	Define 4 pages for the debug areas of debug feature "dasd":
457	> echo "4" > /sys/kernel/debug/s390dbf/dasd/pages
458	
459	Stooping the debug feature
460	--------------------------
461	Example:
462	
463	1. Check if stopping is allowed
464	> cat /proc/sys/s390dbf/debug_stoppable
465	2. Stop debug feature
466	> echo 0 > /proc/sys/s390dbf/debug_active
467	
468	lcrash Interface
469	----------------
470	It is planned that the dump analysis tool lcrash gets an additional command
471	's390dbf' to display all the debug logs. With this tool it will be possible 
472	to investigate the debug logs on a live system and with a memory dump after 
473	a system crash.
474	
475	Investigating raw memory
476	------------------------
477	One last possibility to investigate the debug logs at a live
478	system and after a system crash is to look at the raw memory
479	under VM or at the Service Element.
480	It is possible to find the anker of the debug-logs through
481	the 'debug_area_first' symbol in the System map. Then one has
482	to follow the correct pointers of the data-structures defined
483	in debug.h and find the debug-areas in memory.
484	Normally modules which use the debug feature will also have
485	a global variable with the pointer to the debug-logs. Following
486	this pointer it will also be possible to find the debug logs in
487	memory.
488	
489	For this method it is recommended to use '16 * x + 4' byte (x = 0..n)
490	for the length of the data field in debug_register() in
491	order to see the debug entries well formatted.
492	
493	
494	Predefined Views
495	----------------
496	
497	There are three predefined views: hex_ascii, raw and sprintf. 
498	The hex_ascii view shows the data field in hex and ascii representation 
499	(e.g. '45 43 4b 44 | ECKD'). 
500	The raw view returns a bytestream as the debug areas are stored in memory.
501	
502	The sprintf view formats the debug entries in the same way as the sprintf
503	function would do. The sprintf event/exception functions write to the
504	debug entry a pointer to the format string (size = sizeof(long)) 
505	and for each vararg a long value. So e.g. for a debug entry with a format 
506	string plus two varargs one would need to allocate a (3 * sizeof(long)) 
507	byte data area in the debug_register() function.
508	
509	IMPORTANT: Using "%s" in sprintf event functions is dangerous. You can only
510	use "%s" in the sprintf event functions, if the memory for the passed string is
511	available as long as the debug feature exists. The reason behind this is that
512	due to performance considerations only a pointer to the string is stored in
513	the debug feature. If you log a string that is freed afterwards, you will get
514	an OOPS when inspecting the debug feature, because then the debug feature will
515	access the already freed memory.
516	
517	NOTE: If using the sprintf view do NOT use other event/exception functions
518	than the sprintf-event and -exception functions.
519	
520	The format of the hex_ascii and sprintf view is as follows:
521	- Number of area
522	- Timestamp (formatted as seconds and microseconds since 00:00:00 Coordinated 
523	  Universal Time (UTC), January 1, 1970)
524	- level of debug entry
525	- Exception flag (* = Exception)
526	- Cpu-Number of calling task
527	- Return Address to caller
528	- data field
529	
530	The format of the raw view is:
531	- Header as described in debug.h
532	- datafield 
533	
534	A typical line of the hex_ascii view will look like the following (first line 
535	is only for explanation and will not be displayed when 'cating' the view):
536	
537	area  time           level exception cpu caller    data (hex + ascii)
538	--------------------------------------------------------------------------
539	00    00964419409:440690 1 -         00  88023fe   
540	
541	
542	Defining views
543	--------------
544	
545	Views are specified with the 'debug_view' structure. There are defined
546	callback functions which are used for reading and writing the debugfs files:
547	
548	struct debug_view {
549	        char name[DEBUG_MAX_PROCF_LEN];  
550	        debug_prolog_proc_t* prolog_proc; 
551	        debug_header_proc_t* header_proc;
552	        debug_format_proc_t* format_proc;
553	        debug_input_proc_t*  input_proc;
554		void*                private_data;
555	};
556	
557	where
558	
559	typedef int (debug_header_proc_t) (debug_info_t* id,
560	                                   struct debug_view* view,
561	                                   int area,
562	                                   debug_entry_t* entry,
563	                                   char* out_buf);
564	
565	typedef int (debug_format_proc_t) (debug_info_t* id,
566	                                   struct debug_view* view, char* out_buf,
567	                                   const char* in_buf);
568	typedef int (debug_prolog_proc_t) (debug_info_t* id,
569	                                   struct debug_view* view,
570	                                   char* out_buf);
571	typedef int (debug_input_proc_t) (debug_info_t* id,
572	                                  struct debug_view* view,
573	                                  struct file* file, const char* user_buf,
574	                                  size_t in_buf_size, loff_t* offset);
575	
576	
577	The "private_data" member can be used as pointer to view specific data.
578	It is not used by the debug feature itself.
579	
580	The output when reading a debugfs file is structured like this:
581	
582	"prolog_proc output"
583	
584	"header_proc output 1"  "format_proc output 1"
585	"header_proc output 2"  "format_proc output 2"
586	"header_proc output 3"  "format_proc output 3"
587	...
588	
589	When a view is read from the debugfs, the Debug Feature calls the
590	'prolog_proc' once for writing the prolog.
591	Then 'header_proc' and 'format_proc' are called for each 
592	existing debug entry.
593	
594	The input_proc can be used to implement functionality when it is written to 
595	the view (e.g. like with 'echo "0" > /sys/kernel/debug/s390dbf/dasd/level).
596	
597	For header_proc there can be used the default function
598	debug_dflt_header_fn() which is defined in debug.h.
599	and which produces the same header output as the predefined views.
600	E.g:
601	00 00964419409:440761 2 - 00 88023ec
602	
603	In order to see how to use the callback functions check the implementation
604	of the default views!
605	
606	Example
607	
608	#include <asm/debug.h>
609	
610	#define UNKNOWNSTR "data: %08x"
611	
612	const char* messages[] =
613	{"This error...........\n",
614	 "That error...........\n",
615	 "Problem..............\n",
616	 "Something went wrong.\n",
617	 "Everything ok........\n",
618	 NULL
619	};
620	
621	static int debug_test_format_fn(
622	   debug_info_t * id, struct debug_view *view, 
623	   char *out_buf, const char *in_buf
624	)
625	{
626	  int i, rc = 0;
627	
628	  if(id->buf_size >= 4) {
629	     int msg_nr = *((int*)in_buf);
630	     if(msg_nr < sizeof(messages)/sizeof(char*) - 1)
631	        rc += sprintf(out_buf, "%s", messages[msg_nr]);	
632	     else
633	        rc += sprintf(out_buf, UNKNOWNSTR, msg_nr);
634	  }
635	 out:
636	   return rc;
637	}
638	
639	struct debug_view debug_test_view = {
640	  "myview",                 /* name of view */
641	  NULL,                     /* no prolog */
642	  &debug_dflt_header_fn,    /* default header for each entry */
643	  &debug_test_format_fn,    /* our own format function */
644	  NULL,                     /* no input function */
645	  NULL                      /* no private data */
646	};
647	
648	=====
649	test:
650	=====
651	debug_info_t *debug_info;
652	...
653	debug_info = debug_register ("test", 0, 4, 4 ));
654	debug_register_view(debug_info, &debug_test_view);
655	for(i = 0; i < 10; i ++) debug_int_event(debug_info, 1, i);
656	
657	> cat /sys/kernel/debug/s390dbf/test/myview
658	00 00964419734:611402 1 - 00 88042ca   This error...........
659	00 00964419734:611405 1 - 00 88042ca   That error...........
660	00 00964419734:611408 1 - 00 88042ca   Problem..............
661	00 00964419734:611411 1 - 00 88042ca   Something went wrong.
662	00 00964419734:611414 1 - 00 88042ca   Everything ok........
663	00 00964419734:611417 1 - 00 88042ca   data: 00000005
664	00 00964419734:611419 1 - 00 88042ca   data: 00000006
665	00 00964419734:611422 1 - 00 88042ca   data: 00000007
666	00 00964419734:611425 1 - 00 88042ca   data: 00000008
667	00 00964419734:611428 1 - 00 88042ca   data: 00000009
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