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Based on kernel version 3.9. Page generated on 2013-05-02 23:13 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     
147	               Must not be called within an interrupt handler 
148	
149	---------------------------------------------------------------------------
150	void debug_set_level (debug_info_t * id, int new_level);
151	
152	Parameter:     id:        handle for debug log  
153	               new_level: new debug level 
154	
155	Return Value:  none 
156	
157	Description:   Sets new actual debug level if new_level is valid. 
158	
159	---------------------------------------------------------------------------
160	void debug_stop_all(void);
161	
162	Parameter:     none
163	
164	Return Value:  none
165	
166	Description:   stops the debug feature if stopping is allowed. Currently
167	               used in case of a kernel oops.
168	
169	---------------------------------------------------------------------------
170	debug_entry_t* debug_event (debug_info_t* id, int level, void* data, 
171	                            int length);
172	
173	Parameter:     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	
178	Return Value:  Address of written debug entry 
179	
180	Description:   writes debug entry to active debug area (if level <= actual 
181	               debug level)    
182	
183	---------------------------------------------------------------------------
184	debug_entry_t* debug_int_event (debug_info_t * id, int level, 
185	                                unsigned int data);
186	debug_entry_t* debug_long_event(debug_info_t * id, int level,
187	                                unsigned long data);
188	
189	Parameter:     id:     handle for debug log  
190	               level:  debug level           
191	               data:   integer value for debug entry           
192	
193	Return Value:  Address of written debug entry 
194	
195	Description:   writes debug entry to active debug area (if level <= actual 
196	               debug level)    
197	
198	---------------------------------------------------------------------------
199	debug_entry_t* debug_text_event (debug_info_t * id, int level, 
200	                                 const char* data);
201	
202	Parameter:     id:     handle for debug log  
203	               level:  debug level           
204	               data:   string for debug entry  
205	
206	Return Value:  Address of written debug entry 
207	
208	Description:   writes debug entry in ascii format to active debug area 
209	               (if level <= actual debug level)     
210	
211	---------------------------------------------------------------------------
212	debug_entry_t* debug_sprintf_event (debug_info_t * id, int level, 
213	                                    char* string,...);
214	
215	Parameter:     id:    handle for debug log 
216	               level: debug level
217	               string: format string for debug entry 
218	               ...: varargs used as in sprintf()
219	
220	Return Value:  Address of written debug entry
221	
222	Description:   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	
228	debug_entry_t* debug_exception (debug_info_t* id, int level, void* data, 
229	                                int length);
230	
231	Parameter:     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	
236	Return Value:  Address of written debug entry 
237	
238	Description:   writes debug entry to active debug area (if level <= actual 
239	               debug level) and switches to next debug area  
240	
241	---------------------------------------------------------------------------
242	debug_entry_t* debug_int_exception (debug_info_t * id, int level, 
243	                                    unsigned int data);
244	debug_entry_t* debug_long_exception(debug_info_t * id, int level,
245	                                    unsigned long data);
246	
247	Parameter:     id:     handle for debug log  
248	               level:  debug level           
249	               data:   integer value for debug entry           
250	
251	Return Value:  Address of written debug entry 
252	
253	Description:   writes debug entry to active debug area (if level <= actual 
254	               debug level) and switches to next debug area  
255	
256	---------------------------------------------------------------------------
257	debug_entry_t* debug_text_exception (debug_info_t * id, int level, 
258	                                     const char* data);
259	
260	Parameter:     id:     handle for debug log  
261	               level:  debug level           
262	               data:   string for debug entry  
263	
264	Return Value:  Address of written debug entry 
265	
266	Description:   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	---------------------------------------------------------------------------
271	debug_entry_t* debug_sprintf_exception (debug_info_t * id, int level,
272	                                        char* string,...);
273	
274	Parameter:     id:    handle for debug log  
275	               level: debug level  
276	               string: format string for debug entry  
277	               ...: varargs used as in sprintf()
278	
279	Return Value:  Address of written debug entry 
280	
281	Description:   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	
288	int debug_register_view (debug_info_t * id, struct debug_view *view);
289	
290	Parameter:     id:    handle for debug log  
291	               view:  pointer to debug view struct 
292	
293	Return Value:  0  : ok 
294	               < 0: Error 
295	
296	Description:   registers new debug view and creates debugfs dir entry
297	
298	---------------------------------------------------------------------------
299	int debug_unregister_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:   unregisters debug view and removes debugfs dir entry
308	
309	
310	
311	Predefined views:
312	-----------------
313	
314	extern struct debug_view debug_hex_ascii_view;
315	extern struct debug_view debug_raw_view;
316	extern struct debug_view debug_sprintf_view;
317	
318	Examples
319	--------
320	
321	/*
322	 * hex_ascii- + raw-view Example
323	 */
324	
325	#include <linux/init.h>
326	#include <asm/debug.h>
327	
328	static debug_info_t* debug_info;
329	
330	static 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	
345	static void cleanup(void)
346	{
347	    debug_unregister (debug_info);
348	}
349	
350	module_init(init);
351	module_exit(cleanup);
352	
353	---------------------------------------------------------------------------
354	
355	/*
356	 * sprintf-view Example
357	 */
358	
359	#include <linux/init.h>
360	#include <asm/debug.h>
361	
362	static debug_info_t* debug_info;
363	
364	static 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	
378	static void cleanup(void)
379	{
380	    debug_unregister (debug_info);
381	}
382	
383	module_init(init);
384	module_exit(cleanup);
385	
386	
387	
388	Debugfs Interface
389	----------------
390	Views to the debug logs can be investigated through reading the corresponding 
391	debugfs-files:
392	
393	Example:
394	
395	> ls /sys/kernel/debug/s390dbf/dasd
396	flush  hex_ascii  level pages raw
397	> cat /sys/kernel/debug/s390dbf/dasd/hex_ascii | sort +1
398	00 00974733272:680099 2 - 02 0006ad7e  07 ea 4a 90 | ....
399	00 00974733272:682210 2 - 02 0006ade6  46 52 45 45 | FREE
400	00 00974733272:682213 2 - 02 0006adf6  07 ea 4a 90 | ....
401	00 00974733272:682281 1 * 02 0006ab08  41 4c 4c 43 | EXCP 
402	01 00974733272:682284 2 - 02 0006ab16  45 43 4b 44 | ECKD
403	01 00974733272:682287 2 - 02 0006ab28  00 00 00 04 | ....
404	01 00974733272:682289 2 - 02 0006ab3e  00 00 00 20 | ... 
405	01 00974733272:682297 2 - 02 0006ad7e  07 ea 4a 90 | ....
406	01 00974733272:684384 2 - 00 0006ade6  46 52 45 45 | FREE
407	01 00974733272:684388 2 - 00 0006adf6  07 ea 4a 90 | ....
408	
409	See section about predefined views for explanation of the above output!
410	
411	Changing the debug level
412	------------------------
413	
414	Example:
415	
416	
417	> cat /sys/kernel/debug/s390dbf/dasd/level
418	3
419	> echo "5" > /sys/kernel/debug/s390dbf/dasd/level
420	> cat /sys/kernel/debug/s390dbf/dasd/level
421	5
422	
423	Flushing debug areas
424	--------------------
425	Debug areas can be flushed with piping the number of the desired
426	area (0...n) to the debugfs file "flush". When using "-" all debug areas
427	are flushed.
428	
429	Examples:
430	
431	1. Flush debug area 0:
432	> echo "0" > /sys/kernel/debug/s390dbf/dasd/flush
433	
434	2. Flush all debug areas:
435	> echo "-" > /sys/kernel/debug/s390dbf/dasd/flush
436	
437	Changing the size of debug areas
438	------------------------------------
439	It is possible the change the size of debug areas through piping
440	the number of pages to the debugfs file "pages". The resize request will
441	also flush the debug areas.
442	
443	Example:
444	
445	Define 4 pages for the debug areas of debug feature "dasd":
446	> echo "4" > /sys/kernel/debug/s390dbf/dasd/pages
447	
448	Stooping the debug feature
449	--------------------------
450	Example:
451	
452	1. Check if stopping is allowed
453	> cat /proc/sys/s390dbf/debug_stoppable
454	2. Stop debug feature
455	> echo 0 > /proc/sys/s390dbf/debug_active
456	
457	lcrash Interface
458	----------------
459	It 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 
461	to investigate the debug logs on a live system and with a memory dump after 
462	a system crash.
463	
464	Investigating raw memory
465	------------------------
466	One last possibility to investigate the debug logs at a live
467	system and after a system crash is to look at the raw memory
468	under VM or at the Service Element.
469	It is possible to find the anker of the debug-logs through
470	the 'debug_area_first' symbol in the System map. Then one has
471	to follow the correct pointers of the data-structures defined
472	in debug.h and find the debug-areas in memory.
473	Normally modules which use the debug feature will also have
474	a global variable with the pointer to the debug-logs. Following
475	this pointer it will also be possible to find the debug logs in
476	memory.
477	
478	For this method it is recommended to use '16 * x + 4' byte (x = 0..n)
479	for the length of the data field in debug_register() in
480	order to see the debug entries well formatted.
481	
482	
483	Predefined Views
484	----------------
485	
486	There are three predefined views: hex_ascii, raw and sprintf. 
487	The hex_ascii view shows the data field in hex and ascii representation 
488	(e.g. '45 43 4b 44 | ECKD'). 
489	The raw view returns a bytestream as the debug areas are stored in memory.
490	
491	The sprintf view formats the debug entries in the same way as the sprintf
492	function would do. The sprintf event/exception functions write to the
493	debug entry a pointer to the format string (size = sizeof(long)) 
494	and for each vararg a long value. So e.g. for a debug entry with a format 
495	string plus two varargs one would need to allocate a (3 * sizeof(long)) 
496	byte data area in the debug_register() function.
497	
498	IMPORTANT: Using "%s" in sprintf event functions is dangerous. You can only
499	use "%s" in the sprintf event functions, if the memory for the passed string is
500	available as long as the debug feature exists. The reason behind this is that
501	due to performance considerations only a pointer to the string is stored in
502	the debug feature. If you log a string that is freed afterwards, you will get
503	an OOPS when inspecting the debug feature, because then the debug feature will
504	access the already freed memory.
505	
506	NOTE: If using the sprintf view do NOT use other event/exception functions
507	than the sprintf-event and -exception functions.
508	
509	The 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	
519	The format of the raw view is:
520	- Header as described in debug.h
521	- datafield 
522	
523	A typical line of the hex_ascii view will look like the following (first line 
524	is only for explanation and will not be displayed when 'cating' the view):
525	
526	area  time           level exception cpu caller    data (hex + ascii)
527	--------------------------------------------------------------------------
528	00    00964419409:440690 1 -         00  88023fe   
529	
530	
531	Defining views
532	--------------
533	
534	Views are specified with the 'debug_view' structure. There are defined
535	callback functions which are used for reading and writing the debugfs files:
536	
537	struct 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	
546	where
547	
548	typedef 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	
554	typedef int (debug_format_proc_t) (debug_info_t* id,
555	                                   struct debug_view* view, char* out_buf,
556	                                   const char* in_buf);
557	typedef int (debug_prolog_proc_t) (debug_info_t* id,
558	                                   struct debug_view* view,
559	                                   char* out_buf);
560	typedef 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	
566	The "private_data" member can be used as pointer to view specific data.
567	It is not used by the debug feature itself.
568	
569	The 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	
578	When a view is read from the debugfs, the Debug Feature calls the
579	'prolog_proc' once for writing the prolog.
580	Then 'header_proc' and 'format_proc' are called for each 
581	existing debug entry.
582	
583	The input_proc can be used to implement functionality when it is written to 
584	the view (e.g. like with 'echo "0" > /sys/kernel/debug/s390dbf/dasd/level).
585	
586	For header_proc there can be used the default function
587	debug_dflt_header_fn() which is defined in debug.h.
588	and which produces the same header output as the predefined views.
589	E.g:
590	00 00964419409:440761 2 - 00 88023ec
591	
592	In order to see how to use the callback functions check the implementation
593	of the default views!
594	
595	Example
596	
597	#include <asm/debug.h>
598	
599	#define UNKNOWNSTR "data: %08x"
600	
601	const 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	
610	static 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	
628	struct 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	=====
638	test:
639	=====
640	debug_info_t *debug_info;
641	...
642	debug_info = debug_register ("test", 0, 4, 4 ));
643	debug_register_view(debug_info, &debug_test_view);
644	for(i = 0; i < 10; i ++) debug_int_event(debug_info, 1, i);
645	
646	> cat /sys/kernel/debug/s390dbf/test/myview
647	00 00964419734:611402 1 - 00 88042ca   This error...........
648	00 00964419734:611405 1 - 00 88042ca   That error...........
649	00 00964419734:611408 1 - 00 88042ca   Problem..............
650	00 00964419734:611411 1 - 00 88042ca   Something went wrong.
651	00 00964419734:611414 1 - 00 88042ca   Everything ok........
652	00 00964419734:611417 1 - 00 88042ca   data: 00000005
653	00 00964419734:611419 1 - 00 88042ca   data: 00000006
654	00 00964419734:611422 1 - 00 88042ca   data: 00000007
655	00 00964419734:611425 1 - 00 88042ca   data: 00000008
656	00 00964419734:611428 1 - 00 88042ca   data: 00000009
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